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• Published: 30 April 2020

Contributions of recycled wastewater to clean water and sanitation Sustainable Development Goals

• Cecilia Tortajada 1  

npj Clean Water volume  3 , Article number:  22 ( 2020 ) Cite this article

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Water resources are essential for every development activity, not only in terms of available quantity but also in terms of quality. Population growth and urbanisation are increasing the number of users and uses of water, making water resources scarcer and more polluted. Changes in rainfall patterns threaten to worsen these effects in many areas. Water scarcity, due to physical lack or pollution, has become one of the most pressing issues globally, a matter of human, economic and environmental insecurity. Wastewater, whose value had not been appreciated until recently, is increasingly recognised as a potential ‘new’ source of clean water for potable and non-potable uses, resulting in social, environmental and economic benefits. This paper discusses the potential of recycled wastewater (also known as reused water) to become a significant source of safe water for drinking purposes and improved sanitation in support of the Sustainable Development Goals.

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Introduction.

The Sustainable Development Goals (SDGs) are the most recent attempt by the international community to mobilise government, private and non-governmental actors at national, regional and local levels to improve the quality of life of billions of people in the developed and developing worlds. The goals are an ambitious, challenging and much-needed action plan for “people, planet and prosperity” until the year 2030 1 .

Of the 17 SDGs, the sixth goal is to “ensure availability and sustainable management of water and sanitation for all”. The achievement of this goal, even if partially, would greatly benefit humankind, given the importance of clean water for overall socio-economic development and quality of life, including health and environmental protection.

In 2000, the Millennium Development Goals (MDGs) aimed at reducing by half the proportion of the population without sustainable access to safe drinking water and sanitation by 2015. This objective, however, did not take into consideration water quality or wastewater management aspects, which represented a main limitation for its achievement 2 . This omission has been rectified in the Sustainable Development Goals (SDGs), where one of the goals (SDG 6) calls for clean water and sanitation for all people by ensuring “availability and sustainable management of water and sanitation for all”. Among other aspects, it considers improvement of water quality by reducing by half the amount of wastewater that is not treated, and increasing recycling and safe reuse globally. This will result in the availability of more clean water for all uses, and on an enormous progress on sanitation and wastewater management. This target unequivocally indicates the close interrelation between clean water, sanitation and wastewater management, giving these two last aspects the importance they deserve. No government of any human settlement irrespective of its size, be it a megacity, mid-size city or large or small town, can provide clean water without concurrently considering sanitation and wastewater management. Clean water is not, and will never be possible, if wastewater is not collected, treated and disposed properly for the intended uses.

Constraints for the provision of clean water and sanitation for all are complex, and depend on decisions of actors at all levels of government, private sector, non-governmental organisations and the public. They are also determined by broad development policies that may or may not prioritise provision of these services over the long-term, national and local action plans that, even when properly formulated, are often not adequately implemented due to short-term planning, lack of managerial, financial and/or man-power capacity and water needs of other sectors such as the energy or agriculture sectors on which the water sector has limited say or control. The most damaging limitation is often political will that is not sustained and that depends on political interests and electoral cycles. These aspects as well as many others that hampered the progress of the MDGs and represent serious constraints for the SDGs include discrepancy between global goals and national and local limitations, lack of continuity in decisions, policies and investments from one administration to the other, poor or inexistent data that inform decision-making or disadvantaged populations that do not have access to appropriate water and sanitation services 3 .

In most developing countries, provision of clean water and, to a certain degree, also sanitation services, are prioritised over other services. Nevertheless, this prioritisation is not always accompanied by sustained support, resources, or interest. Regarding wastewater management, this is simply left behind. There does not seem to be appreciation of the numerous negative impacts wastewater and related pollution have for provision of clean water, and how much they adversely affect human health and the environment.

It is a fact that water resources globally are under pressure from economic development, population growth, urbanisation, and more recently, climate variability and change; however, it is also pollution to a large extent what is restricting the availability of water for all people for all uses in quantity and quality. It is difficult to find a solution because, as discussed earlier, this depends on numerous technical and non-technical decisions that are taken without analysing their implications on water availability. The situations are further exacerbated by legal and regulatory frameworks that are not implementable, absence of long-term planning, inadequate management and governance, government capability, neglect of demand-side practices (pricing and non-pricing measures), disregard of awareness building including attitudes and behaviour, and poor intersectoral collaboration. Adequate consideration of these aspects depends on economic, social, environmental, cultural and political contexts and institutional capabilities of the places where they are implemented. Properly pursuing SDGs in general, and SDG 6 in particular, have the potential to improve not only access to water and sanitation and quality of life of billions of people, but also contributing towards better capacities of national and local governments.

SDGs main targets of reducing by half the amount of wastewater that is not treated, and increasing recycling and safe reuse present the distinct possibility of producing ‘new’ sources of clean water for all uses that would not be available otherwise. It would further mean that wastewater discharged to water bodies would be cleaner and safer than what it is at present, and that source water for communities downstream would be of much better quality. It would further contribute to improvements in aquatic environments.

Potable water reuse is not new. However, what has made it more relevant at local and also at national levels such as in Singapore, and now potentially in United States, is growing water scarcity and pollution that is reducing water resources available for larger populations and more uses.

The rest of the paper presents the poor status of water quality globally, and discusses the distinct potential wastewater treatment and reuse have to produce new sources of clean water, as well as to improve sanitation and wastewater management, supporting the UN’s development goal of clean water and sanitation for all. This would also contribute, at least partially, to the progress of several others non-water related SDGs such as poverty alleviation, good health and well-being, and improved education and gender equality. Examples of projects that produce reused water for potable purposes are presented including their benefits, as well as the views of the local populations. Finally, challenges to implement potable water reuse more extensively are discussed.

Results and Discussion

Water pollution and impacts on human health and environment.

Worsening water pollution affects both developed and developing countries. In developing countries, it is mostly due to rapid population growth and urbanisation, increased industrial and other economic activities, and intensification and expansion of agriculture, coupled with lack of local and national legal and institutional capacities (managerial, technical, financial, enforcement, etc.) and political and public apathy to improve and maintain water and wastewater management processes in the long-term. Much attention is given to sanitation, specially to construction of toilets and wastewater treatment plants, but their construction alone will not improve water quality over medium- and long-terms unless commensurate attention is given to significantly improving institutional capacity for planning, management, and implementation 4 .

Water pollution has increased significantly in most rivers in Africa, Asia and Latin America since 1990. Pathogenic and organic pollution has worsened in more than half of river stretches, severely limiting their use. These findings are based on measurements of parameters that indicate pathogen pollution (faecal coliform bacteria), organic pollution (biochemical oxygen demand), and salinity (total dissolved solids) 5 . Although sanitation coverage and wastewater treatment have improved in some countries, they have not been enough to reduce the faecal coliform pollution reaching surface waters 6 . This does not include contamination due to industrial and agricultural wastewater which discharges contain hazardous chemicals, heavy metals, and other inorganic pollutants. Consequently, an estimated 2 billion people use drinking water sources that are contaminated, making millions sick.

According to the Global Burden of Disease studies 7 , between 1990 and 2017, the worst deterioration of water quality was in Southeast Asia, East Asia, and Oceania (86% increase in the parameters measured), North Africa and the Middle East (58% increase), and South Asia (56% increase). Parameters used to estimate unsafe water sources include proportion of individuals globally with access to different water sources (unimproved, improved except for piped supply, or piped water supply), and who have reported use of household water treatment methods such as boiling, filtering, chlorinating or solar filtering (or none of these). For unsafe sanitation, the parameters used are the proportion of individuals with access to different sanitation facilities (unimproved, improved except sewer, or sewer connection).

In developed countries, people’s access to safe sources of water and to sanitation and wastewater services has improved. However, these services still lag behind for people in poor urban, peri-urban, and rural areas, showing inequality among and within communities and regions, with the poorest people generally being in the most difficult situations 8 . Water quality has also improved in general, but pollutants have multiplied and diversified, putting pressure on governments and utilities to improve treatment processes for both drinking water and wastewater 9 .

United States, for example, acknowledges new and long-standing problems. These include a combination of point sources of pollution (such as toxic substances discharged from factories or wastewater treatment plants) and non-point sources (such as runoff from city streets and agricultural sources like farms and ranches). Another problem has been insufficient financial support for municipal wastewater treatment plants 10 . In 2009, according to data reported by the EPA (2009) 11 and the states, 44% of river and stream miles assessed, and 64% of lake acres assessed, did not meet applicable water quality standards and were not apt for one or more intended uses. In 2019, an assessment of lakes at the national level found that ~20% of them had high levels of phosphorus and nitrogen 12 . Although more work is necessary, the United States has the advantage of robust legal and institutional frameworks that have fostered progress in improving quality in drinking water and bodies of water.

Europe is not without problems. According to the European Environment Agency 13 , good chemical status has been achieved for only 38% of surface waters and 74% of groundwater in the EU member states. Surface water bodies are affected mostly by hydromorphological pressures (40%), non-point sources of pollution (38%, mostly agricultural), atmospheric deposition (38%, mainly mercury), point sources of pollution (18%) and water abstraction (7%). In England, only 14% of rivers meet the minimum good status standard; France, Germany, and Greece have been fined by the European Court of Justice for violating regulatory limits on nitrates, with almost a third of monitoring stations in Germany showing levels of nitrates exceeding EU limits.

Risks posed by emerging contaminants such as pharmaceuticals and microplastics are still poorly understood, and thus cannot be adequately incorporated in planning and management of potable water supply. Current and future research on emerging contaminants and their impacts is necessary to fully understand the best management and treatment processes.

Safe reuse for additional sources of safe water

Safe reuse of water resources (using them more than once) is a radical contribution to the old paradigm of water resources management, which seldom considered the value of recycled wastewater and its reuse for potable uses. Larger populations that require more water and produce more wastewater that is not always treated properly, current and projected water scarcity and degradation and water-related health and environmental concerns have led a growing number of cities to treat municipal wastewater to higher quality, and either reusing it for potable and non-potable purposes or discharging it (now cleaner) to the environment. Appropriate regulations, improved technology, more reliable monitoring and control systems, and considerations of public views have made it a feasible alternative to increase the amount of clean water available for potable purposes 14 .

Augmentation of water resources for potable purposes with reused water can be done either directly or indirectly. Terminology varies, but generally, in indirect potable reuse (IPR), reused water is introduced into an environmental buffer (reservoir, river, lake or aquifer) and then treated again as part of the standard supply process. In direct potable reuse (DPR), reused water is sent to a drinking water treatment plant for direct distribution without going through an environmental buffer.

Potable water reuse projects have been implemented in cities in the United States, Namibia, Australia, Belgium, United Kingdom and South Africa, as well as in Singapore 15 . The common denomination in all cases for project development has been water scarcity. All projects have prioritised public health and the environment and risk management. Because water reuse diversifies the water resources available, its value has become more evident during droughts, when surface and groundwater are more limited for all uses.

Local experiences considered successful

This section refers to potable water reuse in several cities, with emphasis on United States because of its current progress in this area.

United States has developed the largest number of water reuse projects of any country, supported by policies and regulations that promote safe reuse of water from recycled wastewater (in 2017, 14 states had policies to address indirect potable reuse and three to address direct potable reuse, compared with eight and none, respectively, in 2012). Measures have been taken to improve use and management of freshwater resources, developing water management tools and drought preparedness plans, conservation actions, addressing dependence on expensive inter-basin water transfers, assessing climate change, and revising water reuse from the knowledge, management, technological, financial, and public-opinion viewpoints.

In US, there are no specific federal regulations for potable water reuse; however, all potable water should meet federal and state water quality regulations, such as the Safe Drinking Water Act and the Clean Water Act. In parallel to these Acts, several states have developed their own regulations or guidelines governing indirect potable reuse, while direct potable reuse facilities are currently considered on a case-by-case basis. In Big Spring and Wichita Falls, Texas, direct potable reuse has been implemented as the most effective, or the only feasible way to provide clean water 16 .

California is the most progressive state regarding indirect potable water reuse, with the most developed regulatory frameworks. For more than 50 years, several cities have implemented planned replenishment of groundwater basins with reused water. Regulations were adopted in 1978 and revised in 2014. In 2018, indirect potable reuse regulations of surface water augmentation were adopted. They allow reused water to be added to surface water reservoirs that are used as sources of drinking water 17 . No project has been implemented yet but the first two (in San Diego County) are expected to be completed by 2022.

The state does not have regulations for direct potable reuse at present. However, the State Water Board is working on a Proposed Framework for Regulating Direct Potable Reuse to develop uniform water recycling criteria that will protect public health, and avoid “discontinuities” in the risk assessment/risk management approach as progressively more difficult conditions are addressed 18 .

The best-known potable reuse project in California, in the country, and internationally, is the Orange Country Groundwater Replenishment System. Indirect potable reuse has been the long-term response of the district (as has been for the state) to provide clean water for growing human and environmental needs. The system supplies potable reused water for ~850,000 people. Reused water is for recharging the groundwater basin to protect it from seawater intrusion. A final expansion project will increase the system’s treatment capacity, enabling the district to continue protecting the groundwater basin and providing clean water to its growing population 19 . The project is considered a precursor and benchmark for subsequent water reuse projects in El Paso, Texas, the West Basin Water Recycling Plant in California and the Scottsdale Water Campus in Arizona.

A recent initiative of the EPA, the National Water Reuse Action Plan, has the potential to implement water reuse at the national level. This Action Plan, announced in February 2020, has the objective to secure the country’s water future for all uses by improving security, sustainability, and resilience of water resources through water reuse and identify types of collaboration between governmental and nongovernmental organisations to make this possible. The plan also aims to address policy, programmatic issues, and science and technology gaps to better inform related regulations and policies 20 .

Reused water has also been produced in Windhoek and Singapore. Windhoek is the first example of direct potable reuse globally from 1968, as the best, and only alternative to water scarcity, exacerbated by recurrent droughts 21 . Given its importance for water security, potable reuse has been considered for decades as a strategic component of water resources management. During the very severe drought in 2015–2017, surface water (the main water source) fell to 2% of supply from the normal 75%, putting enormous pressure on the water system and on the domestic, commercial and industrial sectors. Most of the water used to replace the surface water was drawn from the local aquifer, and potable reused water increased to 30% of supply 22 . Potable water reuse additional domestic supplies and domestic water rationing was not necessary. From October 2019 and through the writing of this article in early 2020, Windhoek faced another very severe drought during which potable water reuse also represented an essential source of clean water for potable purposes, until it finally rained.

In Singapore, production of NEWater (as reused water is known) started in 2003 as part of a long-term strategy to diversify water resources and reduce Singapore’s dependence on water imported from Johor, Malaysia, with a goal of resilience and self-sufficiency by 2060. Reused water meets ~40% of Singapore’s daily water needs and will cover ~55% by 2060. During dry months, NEWater is added to the reservoirs to blend with raw water before undergoing treatment and being supplied for potable use 23 . While water reuse was not a new concept in 2003, what has been significant in this case is its large-scale implementation and the wide public acceptance of indirect potable and direct non-potable reuse due comprehensive education and communication strategies 24 . These emphasise the water-scarcity reality in the city-state and the importance of water reuse to produce the water that is needed for overall development.

In Europe, the EU recognises the importance of reducing pressures on the water environment due to water scarcity, and encourages efficient resource use. Its policy on water reuse does not include potable uses, leaving this decision to the member states; it refers only to non-potable uses, with focus on irrigation for agriculture 25 .

Within this framework, the only two projects that have been developed in the region so far are the Langford Recycling Scheme in United Kingdom and Torrelle plant in Belgium. Both produce water to be used indirectly for drinking water supplies. The Langford Recycling Scheme operates only when the flow of the River Chelmer is low, supplying up to 70% of the flow during drought periods. The highest production has been during drought periods in 2005–2006 and 2010–2011 26 . In Belgium, Torrelle plant supplies safe drinking water to nearby communities, ~60,000 people in 2012, and is also used for artificial recharge of the dune aquifer of Saint-André preventing seawater intrusion 27 .

Table 1 presents an overview of the projects mentioned above 28 . In the decades over which these projects have supplied drinking water, no negative health effects have been documented.

Local experiences where challenges remain

The most recent potable reuse projects that have been stopped are in Australia. The country has robust legal and regulatory frameworks to support potable reuse 29 , but so far only one project has been successfully implemented, in Perth, Western Australia 30 . Two potable water reuse projects in Queensland have been halted due to health concerns, poor communication and public opposition in one case (Toowoomba 31 ), and on lack of political support in the other case (Western Corridor Recycled Water Project) 32 . In both cases, decisions were taken even when there were concerns on the impacts of climate change in the region and the possibility that rainfall patterns might not be appropriate for future purposes.

Acceptance of potable water reuse requires robust regulations and advanced technology; however, it also requires serious consideration of the soft-aspects such as education, communication and engagement of politicians, decision-makers and the public, and emotional response and trust 33 . Messages should not be limited to the benefits of the projects. They should also discuss aspects such as water quality and safety, water supply alternatives and their feasibility and costs, risk management, and implications for those who will consume the water 34 .

In the developing world, cities in Brazil, Mexico, Kuwait, and India have constructed or are planning projects, for potable water reuse. Their possibilities to succeed are limited as projects would have to be implemented within regulatory, institutional, governance, management, financial and technological frameworks that are robust and promote innovation, and utilities would have to ensure technical, managerial and financial capacities in the long-term. A serious limitation is that water management in general, and collection and conventional treatment of municipal and industrial wastewater in particular, are still challenging; often water quality standards and monitoring are poorly enforced, and risk assessment frameworks are lacking. Irrespective of how important potable water reuse is for clean water and sanitation goals at local, regional and national levels, challenges remain for its extended implementation.

Knowledge gaps and research needs

Protection of human health and the environment is paramount for any source of drinking water, be it reused water or not. To ensure reused water is safe for potable purposes, it is crucial that it meets standards for pathogens and chemicals (federal, state and local), monitoring is robust, comprehensive and continuous, reporting and independent auditing are performed and knowledge gaps and research needs are addressed 35 .

Overall, types of research needed include further evaluations of common drinking water treatment processes and their inactivation and/or removal efficiency, regulated and unregulated contaminants and their expected presence in reused water, microbial, chemical, radiological and emerging contaminants, monitoring of the influents and effluents of water treatment plants and real-time monitoring of water as it passes through the treatment train. This will facilitate rapid responses, immediately identifying any changes in the water quality due to pathogens or chemical pollutants, detect their types and amounts, and decide on the most appropriate response 36 . General risks can also be reduced through wastewater source control, water source diversification and allocation of risks, so that each party can manage the different risks.

A growing area of concern is the presence of commonly used chemicals and emerging contaminants, their mixture even at low doses, and their effect in human health and ecosystems. This is particularly important if they are detected more often in advanced treated water as they can cause acute or chronic diseases. Better regulations, and improved treatment and monitoring have been identified as key to address the above issues and comply with potable water quality parameters 37 . Web-based data analytics and a system for population water reporting are also important as they will enhance data collection, and increase information accessibility.

To further understand risks of emerging contaminants, major research efforts based on toxicological and epidemiological studies have been carried out. At present, however, health and environmental protection relies in the measurement of chemical and microbiological parameters and the application of formal processes of risk assessment. The objective is that identification, quantification and use of risk information informs decision-making on social and environmental impacts and benefits, as well as on financial costs 38 . Effects on vulnerable groups like infants, elderly, pregnant women, and persons who are already ill, are less understood and thus require additional research.

In direct potable reuse, the absence of an environmental buffer means shorter failure response times, which may affect the ability of plant operators to stop operations if off-specification water is detected. In these cases, supplementary treatment, monitoring, and engineered buffers are expected to provide equivalent protection of public health and response time if water quality specifications are not met 39 .

Table 2 lists benefits and challenges related to potable water reuse. It does not intend to be exhaustive, but to indicate the most relevant issues in both cases.

Potable water reuse schemes are subject to stringent regulations. They follow risk assessment and drinking water safety plans, which include pilot studies, process control considerations, standards, monitoring and auditing of water quality, consideration of stakeholders and public perceptions and risk minimisation, among other factors. Treatment technologies used are advanced and require membrane filtration and ultraviolet disinfection to remove or destroy viruses, bacteria, chemicals, and other constituents of concern as part of the process of converting wastewater into a clean, safe source of municipal drinking water. Reused water is thus cleaner, and safer, than river flows in many cities, especially in the developing world, where improperly treated (or, more commonly, untreated) wastewater is normally discharged.

Potable water reuse and the SDG for water and sanitation

Proper treatment of wastewater and safe reuse are prerequisites if the main targets of Goal 6 are to be reached by 2030. Failure to achieve this goal will mean that health and living conditions of billions of people will suffer, as they have suffered until now, or even more, as populations are growing and water resources are scarcer and more polluted.

Wastewater that is treated and safely reused for potable purposes becomes a new source of water that can be supplied to growing populations. Examples mentioned earlier show that there are thousands of people with access to clean water due to potable water reuse. This is water that would not be available otherwise. Potable water reuse has become more relevant during drought periods when populations with access to reused water have not suffered of water rationing, while people elsewhere without this alternative have not had the same opportunity.

Potable water reuse represents a reliable alternative way to produce safe water, improve the quality of water in receiving water bodies, and mitigate water scarcity for all uses, contributing to the SDG on clean water and sanitation. More broadly, to improve overall quality of life. However, such projects alone cannot enable the achievement of SDG 6, and produce all the safe water the world is running short of at present and will need in the future. As argued earlier, water reuse is part of comprehensive water planning and management strategies.

Water scarcity needs to be approached holistically. At present and looking towards the future, when demands for safe water will be more pressing and water resources will be less available than now, all alternatives for water supply must be considered, potable water reuse included.

The study followed a three-method approach. The first was literature review and analysis to understand the range of issues that determine the extent of the contributions of water reuse towards the realisation of clean water and sanitation Sustainable Development Goals in specific, and to the progress of several other non-water related SDGs positively influencing quality of life. Following the review and analysis, the second approach was the discussion of water reuse projects that have been operational for decades and that have rendered numerous benefits to the population in terms of safe water and sanitation, as well as projects that have been halted due to health concerns and insufficient involvement of the public. Finally, the most recent initiatives to strengthen and diversity the water resources available at the national level, e.g., United States, are presented to emphasise the fundamental role of water reuse towards fulfilment of the SDGs on clean water and sanitation.

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Acknowledgements

This research was funded by the Institute of Water Policy, Lee Kuan Yew School of Public Policy, National University of Singapore. Grant R-603-000-289-490.

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Wastewater treatment: current and future techniques.

1. Introduction

2. summary of the si, author contributions, conflicts of interest.

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Mojiri, A.; Bashir, M.J.K. Wastewater Treatment: Current and Future Techniques. Water 2022 , 14 , 448. https://doi.org/10.3390/w14030448

Mojiri A, Bashir MJK. Wastewater Treatment: Current and Future Techniques. Water . 2022; 14(3):448. https://doi.org/10.3390/w14030448

Mojiri, Amin, and Mohammed J. K. Bashir. 2022. "Wastewater Treatment: Current and Future Techniques" Water 14, no. 3: 448. https://doi.org/10.3390/w14030448

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Articles on Wastewater

Displaying 1 - 20 of 95 articles.

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From The Editor | September 26, 2014

The 10 hottest topics in wastewater—what you need to know.

By Laura Martin

Behind on what's hot in the wastewater industry? Get up-to-date with this list of Water Online articles on the industry trends and challenges that everyone is talking about. Read on and you'll be sure to impress your colleagues.

1) Energy Production And Conservation

Finding the ideal balance between energy and water consumption has always been a challenge. Energy use at a water or wastewater utility can be 30 percent to 50 percent of the municipality’s total electricity consumption. In addition, the energy industry itself requires a significant amount of water to operate. But a water-energy nexus solution is on the horizon, as more energy-efficient technologies and alternative energy production methods are developed. 

Stories On  Energy From Water Online:

Can Co-Locating Utilities Solve The Water-Energy Nexus?  

5 Reasons To Harvest The Power Of Biogas

2) Nutrient Management

Changing regulations and increasingly stringent effluent limits have brought nutrient management to the forefront of the wastewater industry. 

Stories On Nutrient Management From Water Online

'Peecycle' Please: Will Urine Separation For Nutrient Recovery Take Off?

3 Alternative Nutrient-Removal Techniques

What Everyone Should Know About Enhanced Biological Phosphorus Removal

3) Residuals and Biosolids

The management and removal of residuals, sludge, and biosolids, has historically been a burden on wastewater utilities, accounting for nearly 50 percent of treatment costs. But this “waste” may hold the key to additional revenue if reclaimed and sold. 

Stories On   Residuals and Biosolids From Water Online:

Revolutionary Sludge Management Comes To America

Bio-Dredging: Cost-Saving Sludge Digestion For Lagoons

4) Water Reclamation And Reuse

There is a growing trend of reusing treated wastewater effluent for both drinking water and industrial applications. On the drinking water side, water shortages have made direct potable reuse (DPR) and indirect potable reuse applications a necessity in parts of the country. Pressure to use less water on the industrial sector has resulted in innovative reuse applications as well. 

Stories On  Water Reuse From Water Online:

Texas Leads The Way With First Direct Potable Reuse Facilities In U.S.

Fit-for-Purpose Water Reuse And The Road Toward Water Security

New Indirect Potable Reuse Regulations — What To Expect

5) Water Supply And Water Management

In water-scarce areas, managing water supply can be challenging. First, it can be difficult to even determine how much water is available, via groundwater, surface water, reuse, and other sources. Then, there is the challenge of figuring out how water should be allocated between consumers and industrial applications, and how much needs to remain untouched for the sake of the environment. If there isn’t enough to go around, conservation techniques or usage restrictions may have to be considered. 

Stories On  Water Supply And Management From Water Online:

Tackling The Drought: The Relationship Between Water Law And Water Budget

Why Engineers Can't Solve The Water Shortage With Supply-Side Solutions

6) Stormwater, Green Infrastructure, And Wet Weather Management

Stormwater management is a growing focus for the wastewater industry. Heavy wet-weather events often overwhelm wastewater systems — which are often too small for a growing population — and untreated sewage ends up overflowing into local water bodies. Green infrastructure solutions and growing regulation offer solutions. 

Stories On  Stormwater From Water Online:

EPA Stormwater Ruling: How Will It Impact Utilities?

Save The Rain: Preventing Combined Sewer Overflows

7) ‘Flushable’ Wipes And Collection Systems

Recently, collection systems have been in the spotlight. The increased attention is thanks (or no thanks) to “flushables,” non-dispersible cleansing cloths that are wreaking havoc on headworks all over the country. 

Stories On  “Flushables” From Water Online:   

Nondispersibles' Turning Sewers Into Nightmares Nationwide  

Looming In The Sewers: Nonwovens Are Weaving A Tangled Web

8) Industrial Wastewater

Oil and gas, agriculture, pharmaceuticals, mining, food and beverage processing—the list of industries with growing wastewater challenges goes on and on. Water Online has reported on the modeling, design, and operation of industrial wastewater treatment systems, anaerobic and biological industrial treatment processes, regulatory impacts, and more.  

Stories On  Industrial Wastewater From Water Online:

The Importance Of An Industrial Water Treatment Program

Has Fracking Gone ‘Green'?

9) Utility Management

Utility executives and managers have a wide range of challenges to overcome. Their workforce is aging and their budgets are shrinking. Public outreach is more important than ever before, and regulations and government oversight are increasing.  

Stories On  Utility Management From Water Online

New Standard Applies To Every Water Manager, Everywhere

How To Deliver Better Water And Increase Consumer Confidence Simultaneously

10)  Innovative Technology

Change is needed in the wastewater industry. Cutting-edge products and services focused on everything from resource recovery and big data management, to innovative green infrastructure solutions are coming to the forefront.

Stories On Innovation From Water Online:

The Top 12 Water Technology Hotspots In America

Ontario's Water Tech Acceleration Project: Fighting For The Future Of Water

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Comprehensive review of industrial wastewater treatment techniques

• Review Article
• Open access
• Published: 07 August 2024

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• Yasuki Kansha   ORCID: orcid.org/0000-0002-1136-5000 1  

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Water is an indispensable resource for human activity and the environment. Industrial activities generate vast quantities of wastewater that may be heavily polluted or contain toxic contaminants, posing environmental and public health challenges. Different industries generate wastewater with widely varying characteristics, such as the quantity generated, concentration, and pollutant type. It is essential to understand these characteristics to select available treatment techniques for implementation in wastewater treatment facilities to promote sustainable water usage. This review article provides an overview of wastewaters generated by various industries and commonly applied treatment techniques. The characteristics, advantages, and disadvantages of physical, chemical, and biological treatment methods are presented.

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Wastewater Treatment Techniques: An Introduction

Industrial Wastewater: Characteristics, Treatment Techniques and Reclamation of Water

Advantages and disadvantages of techniques used for wastewater treatment

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Introduction

Background of wastewater treatment.

Water is an indispensable resource that sustains ecosystems, supports human life, and drives industry and agriculture. The demand for freshwater has been continuously growing with the global population. Of the water on Earth, 97% is saline and only 3% is freshwater (Oki and Kanae 2006 ). Only 0.5% of this freshwater is available for human use and exists in liquid form in rivers, lakes, ponds, and groundwater (Sangamnere et al. 2023 ). The severity of water scarcity has been underscored by the United Nations World Water Assessment Programme (WWAP); that is, the number of people residing in regions that experience water scarcity for at least 1 month annually is currently 3.6 billion and could increase to 4.8–5.7 billion by 2050 (WWAP 2018 ).

A comprehensive approach is needed to address growing water scarcity that includes water management and wastewater treatment and reuse (Bauer et al. 2020 ). The global increase in freshwater usage and wastewater generation has resulted in persistent water scarcity. The quantity of wastewater generated globally each year is estimated at 380 billion m 3 and is expected to increase by 51% by 2050 (Qadir et al. 2020 ). The discharge of untreated wastewater remains a global issue. According to a WWAP report ( 2017 ), 80% of global wastewater is untreated and discharged, and Jones et al. ( 2021 ) have reported that 48% of global wastewater is untreated. The direct discharge of wastewater leads to the pollution of water bodies and groundwater, which can result in eutrophication (Qadir et al. 2020 ) and endanger the health of plants and animals (Ahmed et al. 2021a ). Natural water bodies have self-purification ability, whereby some pollutants can be removed through natural physical, biological, and chemical means. However, the excess discharge of untreated wastewater surpasses the self-purification ability of natural water bodies (Pratiwi et al. 2023 ).

The implementation of wastewater treatment is a necessary but challenging task to prevent water pollution and meet water demand, especially in low-income regions. In high-income regions, such as North America, Western Europe, and Japan, 74% of the wastewater is treated, whereas 4% of wastewater is treated in low-income regions (Jones et al. 2021 ). High-income countries enforce wastewater quality regulations and possess the technology and infrastructure needed to install wastewater treatment plants (WWTPs), whereas low-income countries lack these resources. Consequently, those living in low-income regions may be exposed to wastewater and have limited access to clean water (WWAP 2017 ). Thus, global efforts are needed to increase the quantity of wastewater treated.

The properties of influent wastewater, such as the pollutant concentration, must be analyzed to determine WWTP specifications. Wastewater is largely classified by its generation source into municipal, agricultural, and industrial wastewater (Samer 2015 ). Industrial wastewater can be further classified into cooling, washing, and process wastewater (Crini and Lichtfouse 2019 ). Wastewater can be discharged from point and nonpoint sources. Point sources discharge wastewater from easily identifiable outlets, such as WWTPs of industrial facilities and municipal WWTPs. Nonpoint sources cannot be easily identified and include multiple sources, such as agricultural and urban runoff. Thus, point sources can be monitored and regulated more easily and are often more concentrated than nonpoint sources (Jones et al. 2021 ). Differences in water usage result in a variety of pollutants in wastewater that may contain toxins or pathogens harmful to human health and the ecosystem. Some common pollutants include organic compounds, inorganic compounds, phosphorus, nitrogen, and heavy metals (Akpor et al. 2014 ).

Overview of wastewater treatment

Typically, WWTPs are designed to treat wastewater cost-effectively while achieving a desired water quality (Crini and Lichtfouse 2019 ). Wastewater treatment often requires the sequential use of various wastewater treatment techniques, each of which is suitable for removing certain contaminants. A wastewater treatment process generally consists of preliminary, primary, secondary, and tertiary stages (Naidoo and Olaniran 2013 ; Quach-Cu et al. 2018 ). The flow of a general wastewater treatment process and corresponding treatment techniques used are shown in Fig.  1 .

The general flow of a wastewater treatment process and the techniques used

Preliminary treatment removes large debris, grit, and solids from wastewater through processes such as screening, comminution, and grit removal. Next, primary treatment removes suspended solids, grit, fats, and oils through processes such as sedimentation and dissolved air flotation (DAF). Secondary treatment consists of using biological techniques to reduce organic matter, nitrogen, and phosphorus in wastewater through biochemical reactions such as conversion into biomass (Samer 2015 ). Tertiary treatment, also known as advanced treatment, is applied to further upgrade the treated wastewater to meet specific standards for water reuse or discharge. Tertiary treatment includes methods such as membrane filtration, adsorption, and chemical oxidation.

Treatment techniques can be classified into physical, biological, and chemical methods. Physical methods include screening, comminution, grit removal, sedimentation, DAF, adsorption, ion exchange, and membrane filtration. Biological treatment can be subdivided into aerobic and anaerobic treatment depending on oxygen availability. Chemical treatment includes precipitation, coagulation, flocculation, evaporation, distillation, membrane distillation, solvent extraction, electrochemical methods, chemical oxidation, and advanced oxidation processes (AOPs). The characteristics, advantages, and disadvantages of each method are detailed in “ Treatment of industrial wastewater ” section.

Industrial wastewater is often more toxic than municipal wastewater (Häder 2018 ). Industrial wastewater contains various contaminants at different concentrations depending on the industry (Ahmed et al. 2021a ). Thus, industrial wastewater may not be adequately treated by municipal WWTPs, which are designed mainly for the removal of biochemical oxygen demand (BOD) (Abu Shmeis 2018 ). Effective treatment of industrial wastewater requires an analysis of the wastewater properties, such as the type and concentration of pollutants, and implementation of a suitable treatment process. Considering the variation in wastewaters across industries, WWTPs are implemented on a case-by-case basis using available treatment techniques. Treatment methods are selected by considering many factors, such as the influent wastewater characteristics, regulatory standards, land availability, technological availability, and economic viability.

Objective of this review

The objective of this review paper is to examine the diverse types of contaminants found in wastewater generated across various industries and to evaluate the range of treatment techniques currently being implemented. An in-depth analysis is presented of the characteristics, advantages, and disadvantages of available physical, chemical, and biological wastewater treatment methods. Although numerous review papers have been written on singular treatments or specific industrial sectors, there is a limited number of holistic overviews covering the wide spectrum of wastewaters generated by multiple industries and corresponding treatment methods. This review bridges this gap by providing a comprehensive description of industrial wastewaters and the strengths and weaknesses of treatment technologies. This information is essential for selecting and integrating suitable treatment options that are cost-effective and sustainable as well as compliant with regulatory standards.

The selection of industries for this study was based on the thorough review of academic literature and reports. The primary criteria for inclusion were the volume of wastewater produced, the diversity and complexity of pollutants, the environmental and health impacts, and regulatory focus.

The reference search was conducted across multiple academic databases, including Google Scholar, PubMed, ScienceDirect, and Web of Science, using keywords related to the specific industry, wastewater, and wastewater treatment. Additionally, the search for references on the wastewater treatment technologies was conducted by using the academic databases and consulting recent reviews in the field and tracing the citations. This methodology offers a detailed and balanced review, highlighting both the pollutants present in the wastewater of the selected industries and the technologies available for their treatment.

Wastewater generated by various industries

Limited information is available on the overall volume of industrial wastewater generated and discharged globally. However, according to reports from the European Union (EU, which consists of developed countries), manufacturing generated the most wastewater among the industrial sectors (WWAP 2017 ). The major point sources of manufacturing industrial wastewater include industries such as petrochemicals, pharmaceuticals, pulp and paper, textiles, iron and steel metal manufacturing, and food.

The use of water for various purposes in industry generates wastewater with different characteristics, such as the types and concentrations of pollutants. Industrial wastewater may be heavily polluted and contain toxic pollutants. Industrial wastewater can be broadly categorized into cooling, washing, and process wastewater (Crini and Lichtfouse 2019 ). Wastewater generated from various industries and uses has diverse characteristics. The pollutants in industrial wastewater include solids, sediments, organic compounds, nutrients, and heavy metals. Considering the diversity and potential toxicity of industrial wastewater, a treatment technique that can sufficiently remove pollutants must be selected and implemented. The pollutants in wastewaters from the cement, chemical, food, iron and steel, pharmaceutical, pulp and paper, and textile industries are reviewed in this section. Wastewater treatment techniques used in each industry are summarized.

Cement, concrete, and ceramics industry

Cement is an important material for constructing buildings and infrastructure, which drive economic development and urbanization. Table 1 shows the three main phases of cement production. Water is used for cooling and washing equipment and, if wet scrubbers are used, for removing particulate matter (PM). In some cases, water is used in the preparation of raw materials for clinker production (Perera et al. 2020 ). The used cooling water is typically recycled and reused in the process (Sharma et al. 2012 ). Cement production generates air pollutants, such as nitrogen oxides (NO x ), particulate matter (PM), carbon dioxide (CO 2 ), and sulfur dioxide (SO 2 ) (Chen et al. 2015 ). Wet scrubbers are used to remove PM from the exhaust gas by trapping particles in water droplets, generating wastewater (Zhu et al. 2022 ). Other methods used to prevent air pollution include the use of simple membrane filters made of fabric, electrostatic precipitators, and bag filters (which are considered the best removal option for PM) (Zhu et al. 2022 ). Cement industry effluents contain suspended solids (such as calcium carbonate), dissolved solids (such as potassium hydroxide), sodium hydroxide, chlorides, and sulfates, BOD (approximately 5 mg/L), chemical oxygen demand (COD, approximately 60 mg/L), nutrients, and heavy metals (such as iron, zinc, and manganese (Meme and Nwadukwe 2016 ; Ipeaiyeda and Obaje 2017 ).

In concrete production, manufactured cement is mixed with water and aggregates (such as sand and gravel). The washing of truck mixers in a ready-mix concrete plant generates wastewater with a high pH that contains dissolved solids, cement, and other pollutants (Ekolu and Dawneerangen 2010 ). Concrete production is characterized by high water usage, where 150 L of freshwater is used to produce a cubic meter of concrete. Thus, the utilization of wastewater has been considered as a possible solution to reduce water consumption and wastewater discharge (Azeem et al. 2023 ).

Ceramics are a broad category of inorganic, nonmetallic materials that are typically hard, brittle, and heat-resistant. Ceramics are used in various applications for properties such as electrical and thermal insulation, corrosion resistance, and decorative appeal. Ceramic products include construction materials (such as tiles and bricks), refractories (such as crucibles and molds), pottery products, and toilets.

Ceramic manufacturing techniques span a wide range from hand-building to advanced industrial techniques. Although the specific manufacturing steps depend on the type of ceramic and the product, several key steps are common to all processes. First, the raw materials, such as clay, alumina, and silica, are prepared through processes such as mixing and grinding. Binders (colloids or polymers, such as polyvinyl alcohol and polyethylene glycol) and plasticizers (such as water, ethylene glycol, and stearic acid) are added during the forming of dry powders and plastics, whereas deflocculants, surfactants, and antifoaming agents are added during slurry processing (U.S. EPA 1996 ). Water is the most used liquid during these processes (U.S. EPA 1996 ). Next, the prepared raw materials are formed into desired shapes using various methods, such as pressing and casting. The formed ceramics are dried carefully and subjected to bisque firing to remove remaining water and impurities. A glaze is optionally applied to the bisque-fired ceramics. The glazed ceramic is fired again using a temperature, duration, pressure, and atmosphere appropriate for the type of clay and glaze. Effluents are generated during various steps of ceramics production, including glaze and slurry preparation, mold preparation, casting, and glazing (de Almeida et al. 2016 ). These effluents have high concentrations of total suspended solids (TSS) (2,000–10,000 mg/L) and dissolved solids (300–1,000 mg/L), moderate COD (500–1200 mg/L), and low concentrations of heavy metals (such as lead, cadmium, iron, copper, and manganese) (Dinçer and Kargı 2000 ). Effluent recycling is one way of avoiding effluent handling and reducing usage of water and raw materials (de Almeida et al. 2016 ).

Physical and chemical treatment

Wastewater from the cement industry has a high pH and turbidity and is treated by neutralization followed by sedimentation (Freeda Gnana Rani et al. 2005 ; Zhu et al. 2022 ). Wastewater from the concrete industry is similarly treated by neutralizing the pH and removing TSS. Coagulation and flocculation may be used to increase the settleability of TSS. De Paula et al. ( 2014 ) proposed a coagulation–flocculation treatment process using aluminum sulfate, Moringa oleifera powder, and floc sedimentation. The process achieved 90% turbidity removal, such that the treated water could be used to wash vehicles and flush toilets. Physical and chemical treatments of wastewater from the ceramic industry include adsorption, screening, sedimentation, filtration, coagulation, flocculation, and filtration (Pujiastuti et al. 2021 ). Pujiastuti et al. ( 2021 ) used a polyaluminum chloride coagulant to treat wastewater from the ceramic industry, achieving removals of up to 99.9% TSS, 98.23% COD, and 99.1% lead.

Biological treatment

Biological treatment is not commonly used for wastewater from the cement industry because the typical pollutants are TSS, dissolved solids, and heavy metals and the pH is high. However, some studies have been conducted on applying biological treatment to wastewater from the cement industry. Ali et al. ( 2021 ) investigated the use of a pilot-scale process consisting of a primary sedimentation tank, integrated fixed-film activated sludge (AS), and a final settling tank. The removals of TSS, COD, BOD, total nitrogen (TN), and total phosphorus (TP) were 94.5%, 87.8%, 90.8%, 75.9%, and 69.4%, respectively. The proposed treatment was found to be effective, satisfying the regulation standards in Egypt to reuse treated wastewater in agriculture. Biological treatment is not commonly used for wastewater from the ceramic industry. However, Dinçer and Kargı ( 2000 ) studied using AS to treat and reduce the BOD and COD of wastewater from the ceramic industry after chemical precipitation, pH adjustment, and nutrient balancing.

Chemical industry

The chemical industry produces a broad range of chemicals, including commodity, specialty, and fine chemicals. These chemicals are used in various sectors, such as agriculture and manufacturing, to make final products. The basic chemicals produced by the petrochemical industry are particularly important because of subsequent use in the manufacturing of plastics, fibers, lubricants, and detergents. Petrochemicals are produced from fossil fuels, such as crude oil, coal, and natural gas or biomass (such as corn and sugarcane). The refining of crude oil provides fuels (such as liquefied petroleum gas, gasoline, and diesel) and chemical products (such as waxes, greases, asphalts, olefins, and aromatics) (Dincer and Zamfirescu 2014 ). The main petrochemical products are olefins (such as ethylene and propylene) and aromatics (such as benzene, toluene, and xylene) (Ren et al. 2009 ; Do et al. 2016 ).

Water usage in refineries for cooling, distillation, hydrotreating, and desalting generates wastewater (Ghimire and Wang 2019 ). Pollutants found in wastewater from the petrochemical industry include aromatics, hydrocarbons, sulfides, ammonia, and heavy metals (such as chromium, iron, nickel, and copper) (Radelyuk et al. 2019 ; Wake 2005 ; Yu et al. 2017 ). These compounds can be harmful to both the environment and human health. Petrochemical wastewater is treated by physical, biological, and chemical methods.

Physical treatment

Oily wastewater is commonly treated by using sedimentation and DAF to separate and remove oil (Abuhasel et al. 2021 ). Adsorption and ion exchange can be used to remove dissolved organics from petrochemical wastewater (Fakhru’l-Razi et al. 2009 ). Membrane separation can be used to remove oils, total organic carbon (TOC), and metal ions from oily wastewater (Yu et al. 2017 ). Some challenges encountered using membrane technology include cost, thermal stability, and corrosion resistance, which may be overcome by developing novel materials and combination with other treatment technologies (Yu et al. 2017 ).

Aerobic and anaerobic treatment is commonly used to remove organics, ammonium, and sulfide from petrochemical wastewater because of advantages such as low cost and a high pollutant removal efficiency (Ghimire and Wang 2019 ). However, refractory organic compounds, such as aromatics, may be difficult to remove by traditional biological methods. Liu et al. ( 2014 ) found that wastewater from a petrochemical complex treated using AS still contained refractory organic compounds, such as alkanes, chloroalkanes, aromatics, and olefins. These compounds could be removed using a ponds-and-wetland system.

Chemical treatment

Coagulation, flocculation, and electrochemical technologies may be used to treat oily wastewater (Abuhasel et al. 2021 ). Advanced oxidation processes, such as heterogeneous photocatalysis, can be used to degrade refractory organic pollutants, such as phenolic compounds, in refinery wastewater (Diya’uddeen et al. 2011 ; Bustillo-Lecompte 2020 ).

Food industry

The industrial manufacture of food is essential for human life and to meet global challenges, such as hunger and environmental sustainability. The food supply chain involves the production of crops and livestock, food processing, and logistics (Sun et al. 2017 ). Water is used for various processing purposes across different food industries. This water is categorized mainly into process water (such as that used as a raw material) and nonprocess water (such as that used for washing, cooling, and heating) (Abdel-Fatah 2023 ). Water usage requirements, such as the water quality and volume, vary across different sectors and processes in the food industry. Consequently, the generated wastewater can have varying characteristics, including the contamination level and volume. The generated wastewater may contain high levels of COD, BOD, TSS, TN, and TP resulting from various processes and nonprocess water usage. Most of the water used in many food industry sectors is employed for washing foodstuff and equipment, generating wastewater containing organics and nutrients.

The food industry can be categorized into several key sectors. These sectors can be ranked in terms of decreasing water consumption as meat, dairy products, other foods, fruits and vegetables, bakery products, grain mill and starch products, edible oils and fats, and fish and shellfish (Mark and Strange 1993 ; Ranken et al. 1997 ; Asgharnejad et al. 2021 ; Eurostat 2023 ). “Other foods” include sugar, coffee, tea, cocoa, chocolate, confectionery, condiments and seasonings, prepared meals, homogenized food, and dietetic food (Eurostat 2023 ). Table 2 summarizes the products, water usage, wastewater characteristics, and wastewater treatment methods for key sectors of the food industry. Sugar, coffee, and tea are included because of being important internationally traded products that require large quantities of water for production (Asgharnejad et al. 2021 ). In 2021, the estimated global production of green coffee, tea (i.e., green, black, and partly fermented tea), and raw cane or beet sugar was 10.50, 6.81, and 176.95 million tons, respectively (FAO 2024 ). Various physicochemical and biological technologies can be used to treat wastewater from the food industry. The treatment method is selected considering factors such as the wastewater characteristics, quality demand for the treated water, cost, and energy requirements. Technologies can be combined to improve the overall treatment efficiency. Considering the high water consumption and wastewater generation of the food industry, direct or indirect use of treated water and resource recovery, such as biogas production, is encouraged to mitigate water scarcity (Shrivastava et al. 2022 ).

Iron and steel industry

Iron and steel are important drivers in modern society and economic development because of their use in infrastructure (such as roads and bridges, buildings, housing, machinery, and equipment in various industries), transportation (such as cars and trains), and consumer goods (such as tools and utensils). However, the production of iron and steel requires extensive usage of water and energy and generates wastewater containing toxic pollutants as well as CO 2 emissions (Garg and Singh 2022 ).

The raw materials used to produce iron and steel include iron ore, coal for making coke, and limestone (Kumar et al. 2023 ). Iron and steel can also be produced from recycled scrap metal using electric arc furnaces (EAFs) (Yang et al. 2014 ). The steel industry uses a large quantity of water for operations, such as cooling, scrubbing, and descaling (Colla et al. 2017 ). Water consumption in a steel plant can range from 1 to 150 m 3 per ton of steel depending on the location, plant configuration, and local regulations (Suvio et al. 2012 ). Either once-through cooling or recirculating cooling is used according to water availability, which depends on the plant location. Once-through cooling is used in coastal locations where seawater is abundant and accounts for approximately 80% on average of the water consumed in steel plants (Suvio et al. 2012 ).

Iron and steel manufacturing involves the processes described below (United States Environmental Protection Agency (EPA) 2008 ; Garg and Singh 2022 ).

Coke production: Metallurgical coke is produced by heating coal in an oxygen-free environment.

Sintering: Fine raw materials, including iron ore, limestone, and coke, are agglomerated at high temperatures. The product (which is called sinter) is fed to a blast furnace (BF).

Ironmaking: Materials containing iron, such as sinter, are reduced in the BF twice using hot gas. Alternatively, direct reduced iron can be produced from iron ore using natural gas, coal gas containing hydrogen, or CO, which does not require the production and use of coke.

Desulfurization of iron: Reagents, such as CaC 2 and CaCO 3 , are injected into the molten iron. The reagents react with sulfur to produce slag, which is removed.

Steelmaking: Steel is produced in basic oxygen furnaces (BOFs), EAFs, or open hearth furnaces (OHFs). In a BOF, oxygen is injected into molten metal to remove impurities. In an EAF, current is run through scrap metal using carbon electrodes, and the scrap metal is melted and refined. An OHF is a shallow basin in which metal scrap and molten metal from the BF are heated, melted, and refined.

Product preparation: Processes, such as pouring the molten steel into ingots, reheating, casting, shaping, and rolling, are used to finish the product.

Water is used in most processes of steel production, including coke production, sintering, the BF and BOF stages, and rolling, for purposes such as cooling, quenching, and gas-cleaning (Suvio et al. 2012 ). In addition to these main steel manufacturing processes, a large quantity of water is used in supporting processes, such as power generation and equipment cooling (Suvio et al. 2012 ). These complex iron and steel production processes generate wastewater containing a large variety of pollutants. The main pollutants in wastewater from the steel industry include COD, NH 3 –N, volatile phenols, cyanide, TSS, heavy metals, and petroleum (Tong et al. 2018 ; Choudhury et al. 2023 ). The largest water consumption occurs in ironmaking and steelmaking, whereas most of the pollution in iron and steel industry wastewater results from coking (Tong et al. 2018 ).

Reducing the water intake and wastewater discharge of the iron and steel industry requires the implementation of wastewater treatment and water recycling or reuse. Wastewater from the iron and steel industry is treated by physical, biological, and chemical methods. A suitable wastewater treatment method can be selected based on the wastewater pollutants, concentration, quantity, and characteristics, which vary across plants and processes, such as coking, ironmaking, and steelmaking (Lawal and Anaun 2022 ). Although conventional primary, secondary, and tertiary treatments are used in industry, emerging processes, such as hybrid biological processes, AOPs, and membrane filtration, have been effectively used for pollutant removal (Rawat et al. 2023 ).

Adsorption is a simple, low-cost, and effective method for removing a variety of pollutants, including heavy metals in wastewater (such as that from the iron and steel industry) (Feng et al. 2022b ). Activated carbon is widely used as an adsorbent because of its effectiveness but can be relatively costly (De Gisi et al. 2016 ). The iron- and steelmaking industry is unique in that byproducts, such as metallurgical slag, can be used as low-cost alternative adsorbents (Manchisi et al. 2020 ). Nguyen et al. ( 2018 ) reported that the byproducts of coal fly ash and blast furnace slag can be used as low-cost adsorbents to effectively remove heavy metals, such as Pb, Cu, Cd, Cr, and Zn. Unmodified raw coal fly ash has been demonstrated as an adsorbent for treating coking wastewater (Wang et al. 2018 ). The raw coal fly ash achieved 90% COD removal and could be regenerated by the Fenton process.

Membrane filtration is sufficiently effective for treating wastewater from the iron and steel industry that the water can be reused at the industrial scale. However, membrane fouling is a major problem that decreases the permeate flux and increases energy usage (Liang et al. 2023 ; Lin et al. 2023 ). The presence of salts in wastewater can cause problems, such as membrane fouling, and recycled water containing salt can cause salt deposition or corrosion of equipment (Colla et al. 2016 ). Lin et al. ( 2023 ) reported that deposits of Fe and Mn ions and oxides in integrated steelwork wastewater may cause fouling of ultrafiltration (UF) membranes and should be removed before UF. Liang et al. ( 2023 ) suggested that further investigation of reverse osmosis (RO) membranes is needed to prevent fouling because these membranes are used for recycling wastewater from the iron and steel industry. Huang et al. ( 2011 ) reported that constructed wetlands are an effective pretreatment before UF and RO for reducing iron and manganese concentrations in wastewater from the iron and steel industry and to improve the quality of the treated water for reuse. The constructed wetland, UF, and RO system achieved 98% desalination. An RO system can effectively reduce the concentration of salts, electrical conductivity, and total dissolved solids (TDS), enabling the treated water to be reused and increasing the equipment lifespan (Colla et al. 2016 ).

Aerobic and anaerobic treatment, such as AS, is used to treat wastewater from the iron and steel industry. Considering that the complexity and toxicity of this wastewater may reduce the performance of conventional biological treatment, hybrid biological processes, such as anoxic–oxic–anoxic–oxic (AOAO), anaerobic–anoxic–oxic (AAO), and anaerobic–anoxic–oxic–oxic (AAOO), have been developed for the efficient removal of pollutants (Rawat et al. 2023 ). Biological treatment plants can use various microorganisms (e.g., bacteria, algae, yeast, and fungi) that have different metabolic pathways to remove organic and inorganic pollutants in wastewater from the iron and steel industry (Kajla et al. 2021 ). Hybrid biological processes employing diverse microbial communities have been used at the industrial scale to meet effluent standards. Ma et al. ( 2015 ) collected sludge from coking WWTPs in China that employ different processes, including anaerobic–oxic (AO), AAO, anaerobic–oxic–oxic (AOO), AAOO, and AOAO. An analysis of the composition of the microbial community in the sludge showed that most sludge contained Thiobacillus, Comamonas, Thauera, Azoarcus, and Rhodoplanes . The key parameters for the biological treatment, such as the operation mode, flow rate, and temperature, were found to affect the makeup of the microbial community and thereby, the pollutant removal performance.

Chemical methods, such as coagulation–flocculation, AOPs, and electrochemical techniques, have been used to treat wastewater from the iron and steel industry (Garg and Singh 2022 ). Coagulation–flocculation has been used in conventional integrated wastewater treatment systems as a primary treatment to remove pollutants, such as oils and heavy metals (Das et al. 2018 ). Coagulation–flocculation can also be used as a pretreatment to prevent filtration membranes from being fouled by effluent (Lin et al. 2023 ). Coking wastewater may be treated by AOPs, such as ozonation (Wang et al. 2019 ), catalytic ozonation (Feng et al. 2022a ), Fenton oxidation (Chu et al. 2012 ; Kwarciak-Kozłowska and Włodarczyk 2020 ), photolysis (Włodarczyk-Makuła et al. 2016 ), and photocatalysis (Sharma and Philip 2016 ), to degrade pollutants and increase the biodegradability of the effluent in subsequent biological treatment. Studies have been performed on using electrochemical methods, such as electrocoagulation and electrochemical oxidation, to treat coking wastewater. Wang et al. ( 2022 ) developed Ti/SnO 2 RuO 2 –Yb electrodes for the electrochemical oxidation of coking wastewater, achieving 85.06% COD removal and 60.59% TOC removal. Ozyonar and Karagozoglu ( 2015 ) studied how pretreated coking wastewater was affected by electrocoagulation and electrochemical peroxidation using a direct pulse current. Electrochemical peroxidation was found to be more effective than electrochemical oxidation in removing COD, TOC, phenol, CN – , and SCN – . Mierzwiński et al. ( 2021 ) investigated the electrocoagulation of coking wastewater using floc characterization, mathematical modeling, and designing an industrial-scale electrocoagulation reactor. Overall, chemical treatment methods can be integrated with physical or biological methods to improve pollutant removal and promote wastewater reuse in the iron and steel industry. Emerging chemical methods are effective but present challenges, such as a high cost and difficulty in scale-up for industrial use.

Pharmaceutical industry

Pharmaceutical products are crucial to society for the prevention, treatment, and management of various medical conditions, enhancing overall public health and well-being. A large quantity of water is used as a raw material, ingredient, and solvent in the industrial production of pharmaceuticals and separation processes, such as extraction and washing (Gadipelly et al. 2014 ). Chemical synthesis and fermentation processes are the major processes that generate wastewater during pharmaceutical production (Gadipelly et al. 2014 ). Pharmaceuticals used in households and agriculture can appear in wastewater. The variety of pharmaceutical products, intermediates, and raw materials results in a diversity of wastewater contaminants. Wastewater from the pharmaceutical industry contains organics that may not be biodegraded, such as antibiotics, anti-inflammatories, steroids, hormones, antidepressants, and spent solvents (Rana et al. 2017 ; Samal et al. 2022 ). Discharge of and exposure to pharmaceuticals may have detrimental effects on plants and animals (Gadipelly et al. 2014 ; Samal et al. 2022 ).

Membrane technologies, such as those based on polymer membranes, are low-energy simple strategies for removing pharmaceutical active compounds from wastewater. However, membrane fouling is an important limitation and has been mitigated by membrane modification (Ratnasari 2023 ). The use of activated carbon for the removal of pharmaceuticals has been studied, but further research needs to be conducted on the cost-effectiveness and application of this technology to real wastewater (Rasras et al. 2021 ).

Aerobic and anaerobic biological treatments, such as AS, membrane bioreactors (MBRs), moving bed biofilm reactors (MBBRs), and constructed wetlands, may be used to remove pharmaceuticals from wastewater (Moghaddam et al. 2023 ). The conventional activated sludge (CAS) treatment often used in municipal WWTPs cannot sufficiently remove persistent micropollutants. However, MBRs are a promising technique for removing these micropollutants because the longer sludge retention time and higher sludge concentration used results in a higher efficiency (Tiwari et al. 2017 ).

Electrochemical coagulation has been studied at the lab scale for the removal of pharmaceuticals from wastewater, but further research needs to be carried out on this technology using real wastewater and performing a cost analysis (Alam et al. 2021 ). Refractory pharmaceuticals can be effectively degraded using AOPs, such as ozone (O 3 ), hydrogen peroxide (H 2 O 2 ), Fenton oxidation, and photocatalysis, which use highly reactive radical species (Gadipelly et al. 2014 ).

Pulp and paper industry

Pulp and paper products, such as newspapers, books, packaging materials, and tissue products, play a vital role in society. Wastewater is generated by the pulp and paper industry through processes such as wood preparation, pulp manufacturing, pulp bleaching, and papermaking (Ashrafi et al. 2015 ). The use of different processes and raw materials results in diverse wastewater characteristics, such as the quantity generated and the pollutant concentration. The quantity of generated wastewater can be as high as 60 m 3 /ton of paper produced (Thompson et al. 2001 ). Preparing wood to produce chips includes harvesting, debarking, chipping, and screening (Amândio et al. 2022 ). The generated wastewater contains TSS, BOD, dirt, grit, and fibers (Pokhrel and Viraraghavan 2004 ). Wood pulp can be manufactured from the prepared wood by mechanical, chemical, or hybrid methods (Toczyłowska-Mamińska 2017 ). Mechanical pulp production involves grinding or refining wood. In hybrid processes, wood pulp is produced by cooking with alkali and milling (Nong et al. 2020 ). Four common chemical pulping processes are kraft, sulfite, neutral sulfite semichemical, and soda (Cheremisinoff and Rosenfeld 2010 ). The kraft process accounts for 90% of chemical pulp production because of a high product quality and low production cost (Argyropoulos et al. 2023 ). During this process, an alkaline solution of sodium hydroxide and sodium sulfide, also known as white liquor, is used to dissolve lignin from cellulose fibers under high temperature and pressure. The dilute spent liquor is concentrated using evaporators to approximately 60%–65% solids (Young et al. 2003 ; Cheremisinoff and Rosenfeld 2010 ). The resulting “black liquor” contains organics, such as lignin and hemicellulose, and inorganics, such as salts and sulfur compounds, which are generated as byproducts (Valderrama et al. 2021 ). The black liquor is burned for energy and to recover chemicals (Young et al. 2003 ). The wood pulp is then bleached with compounds containing chlorine or oxygen (Bajpai 2018 ), which generates bleaching wastewater with a high COD and TSS as well as low biodegradability (Eskelinen et al. 2010 ) and refractory organic compounds, such as adsorbable organic halides (AOX) (Patel et al. 2021 ). In the final papermaking process, additives, such as dyes, may be used to make colored paper, whereby the generated wastewater may contain particulate waste as well as organic and inorganic compounds, such as the dyes used (Patel et al. 2021 ). Wastewater generated from the pulp and paper industry is treated physically, biologically, or chemically to reduce water pollution and recover energy and materials.

Sedimentation and flotation are used to remove TSS in wastewater from pulp and paper mills. Primary clarifiers can effectively remove more than 80% TSS (Thompson et al. 2001 ). Another commonly used technique, DAF, can remove 80%–98% TSS (Miranda et al. 2009 ). Manago et al. ( 2018 ) investigated the removal of fibers using DAF with polyaluminum chloride as a coagulant, where more than 81.7% TSS was removed. Filtration using membranes of various pore sizes can be used to remove pollutants in wastewater from the pulp and paper industry, such as organics, ions, and AOX (Esmaeeli et al. 2023 ). Membranes used for wastewater treatment in the pulp and paper industry must be able to withstand extreme conditions, such as high temperatures and pHs, as well as antifouling measures for organic and inorganic foulants (Esmaeeli et al. 2023 ). Valderrama et al. ( 2021 ) developed nanofiltration (NF) membranes to treat black liquor. These membranes have a high rejection of organics with a TOC removal of 92.5% and high salt removals, such as 88.7% sulfate, 73.21% Na + , and 99.99% Mg 2+ . Adsorption methods, such as the use of activated carbon and zeolite, have been investigated for the treatment of wastewater from the pulp and paper industry by removing heavy metals, such as Cd, Ba, and Cu (Aprianti et al. 2018 ), as well as COD and color (Kapatel et al. 2022 ).

In the pulp and paper industry, aerobic and anaerobic treatment is used to reduce the high organic content of wastewater. Aerobic treatment, such as AS, aerated lagoons, and stabilization ponds, have traditionally been used, whereas anaerobic treatment, such as the upflow anaerobic sludge blanket (UASB) reactor, is promising because of advantages such as lower sludge generation, lower energy consumption, and biogas generation (Patel et al. 2021 ; Esmaeeli et al. 2023 ). Aerobic treatment is more suitable for low- and medium-strength effluents, whereas anaerobic treatment is better for treating high-strength effluents (Buyukkamaci and Koken 2010 ; Esmaeeli et al. 2023 ). Buyukkamaci and Koken ( 2010 ) performed a cost analysis of 96 treatment plants with 12 flow schemes, including physical, chemical, and biological treatment, and reported that biological processes are most economical for treating wastewater from the pulp and paper industry.

Chemical methods for treating wastewater from the pulp and industry wastewater include chemical precipitation, coagulation and flocculation, and AOPs, such as ozonation, ozone/H 2 O 2 , photocatalysis, and Fenton oxidation (Ashrafi et al. 2015 ; Esmaeeli et al. 2023 ). Kaur et al. ( 2020 ) reported that using a conventional alum coagulant and chitosan flocculant to treat wastewater from pulp and paper mills removed 81% TSS and 78% COD. Eskelinen et al. ( 2010 ) compared the removals of COD from simulated wastewater containing model compounds of wood extractives (abietic acid, linoleic acid, and β-sitosterol) using various chemical treatment methods, including ultrasonic (US) irradiation combined with Fenton-like oxidation (Fe 3+ /H 2 O 2 ), photo-Fenton degradation (Fe 3+ /H 2 O 2 /UV), chemical precipitation using CaO, and electrooxidation. Although the highest COD removal was obtained using chemical precipitation with CaO, combination with subsequent biological treatment is needed to meet the legislative COD limit of 200 mg/L. Ribeiro et al. ( 2020 ) reported maximum AOX removals of 85% and 95% by using the Fenton and photo-Fenton processes, respectively, to treat bleaching wastewater from a kraft pulp mill over a 10-min period. Studies were performed on increasing the COD removal efficiency by combining biological treatment with AOPs, such as O 3 , O 3 /UV, UV, UV/H 2 O 2 , heterogeneous photocatalysis (TiO 2 /UV and ZnO/UV), Fenton oxidation, and photo-Fenton oxidation. Specifically, O 3 treatment has been implemented at an industrial scale in pulp and paper mills (Hermosilla et al. 2014 ). Combining AOPs with biological treatment may improve the wastewater treatment efficiency and reduce costs (Hermosilla et al. 2014 ).

Textile industry

The textile industry provides essential goods to society, such as clothing and fabrics. Fabric is manufactured by successive processing of raw materials (cellulose, protein, and synthetic fibers) to yarn to greige fabric to the fabric product.

Fabric production involves dry and wet processes. The wet processes require a large quantity of water and generate highly polluted wastewater (Yaseen and Scholz 2019 ). Water consumption can range from 30 to 150 L/kg of cloth according to the type of fiber being processed, with the effluent containing 200–600 mg/L BOD, 1,000–1,600 mg/L total solids, and 30–50 mg/L TSS (Azanaw et al. 2022 ). Cotton is the main raw material used for fabric production, accounting for 60% of the earnings of the industry (Velusamy et al. 2021 ). Wet processing of cotton includes sizing, desizing, scouring, bleaching, mercerization, dyeing, printing, and finishing (Holkar et al. 2016 ). Wet processes generate wastewater with varying characteristics resulting from the differences in the raw materials and process used. Table 3 shows the characteristics of wastewaters from different wet processes (Correia et al. 1994 ; Kant 2012 ; Sarayu and Sandhya 2012 ; Holkar et al. 2016 ; Azanaw et al. 2022 ).

Wastewater from the dyeing and printing process is diverse because of the numerous dyes used and variations in the dyeing process. Dyes can be classified according to their chemical structure (such as azo, nitro, anthraquinone, cyanine, and carbonyl) or the type of fiber being dyed (cellulose, protein, and synthetic) and application (such as direct, acid, basic, cationic, direct, reactive, and mordant) (Correia et al. 1994 ; Mustroph 2014 ). Synthetic dyes have been widely used for their color range, brightness of color, and fastness (Kant 2012 ). Azo dyes are the most commonly used synthetic dyes because of a low production cost, color variety, and fastness (Piaskowski et al. 2018 ; Al-Tohamy et al. 2022 ), accounting for 60%–70% of the synthetic dye industry (Slama et al. 2021 ). Untreated dye wastewater is harmful to aquatic and terrestrial life as well as the human skin, liver, nervous system, kidney, and reproductive system. (Al-Tohamy et al. 2022 ). Untreated dye effluents block the transmission of sunlight in water bodies and inhibit photosynthesis, leading to oxygen depletion and low biodegradability by aerobic microorganisms and disruption of the aquatic ecosystem (Slama et al. 2021 ). Heavy metals, such as lead, chromium, cadmium, and copper, as well as metals may appear in wastewater because of the use of metal-complex dyes (Khan et al. 2022 ). These heavy metals are toxic and harmful to aquatic life and human health, causing health problems, such as cancer and cardiovascular disease (Khan et al. 2022 ). To prevent these problems and meet effluent standards, textile wastewater must be treated before being released into the environment or reused. Physical, biological, and chemical methods are used to treat textile wastewater.

Physical treatment methods, such as adsorption, membrane filtration, and ion exchange, can remove 85%–99% of dyes from effluent (Samsami et al. 2020 ). Adsorption using materials such as clay, zeolite, and activated carbon has been proposed as an efficient and low-cost method for the removal of heavy metals and dyes from wastewater (Velusamy et al. 2021 ). Membrane processes, such as microfiltration (MF), UF, nanofiltration (NF), and RO, for the removal of dyes, salts, and other auxiliary chemicals from textile wastewater have been investigated mostly in lab-scale studies and some pilot and full-scale studies, where over 95% removals of COD, turbidity, and color have been attained (Keskin et al. 2021 ). In particular, good performance has been reported using hybrid systems, such as AS followed by UF and RO (Keskin et al. 2021 ). Ion exchange using synthetic and natural resins for dye removal has been studied at the lab-scale but has a high cost and limited applicability to dyes (Khan et al. 2023 ; Singh et al. 2022 ).

Biological treatment of textile effluents includes aerobic, anaerobic, and combined processes using bacteria, fungi, and algae (Holkar et al. 2016 ; Bhatia et al. 2017 ). A wide variety of dyes can be degraded by using a combination of microorganisms that are compatible with and capable of degrading dyes and their intermediates. Biological treatment is low-cost, eco-friendly, and generates low quantities of sludge compared to other methods (Bhatia et al. 2017 ; Samsami et al. 2020 ). Generally, anaerobic treatment can be used to treat high-COD effluents and remove color, whereas aerobic treatment can be used to treat low-COD effluents. Rongrong et al. ( 2011 ) developed a lab-scale hybrid anaerobic baffled reactor for treating desizing effluents containing polyvinyl alcohol (PVA), achieving 42.0% COD removal while collecting methane-containing biogas, which can be used for other purposes. In the industrial treatment of textile wastewater, aerobic and anaerobic AS or MBRs are combined with technologies such as coagulation–flocculation, RO, and ozonation (Paździor et al. 2019 ). Compared to AS, MBRs can achieve higher biomass loadings by using supports on which biofilms can grow, and the presence of various microorganism species enables efficient removal of dyes, COD, BOD, TSS, phosphorus, and heavy metals (You et al. 2007 ).

Coagulation–flocculation has been used in the textile industry as a cost-effective method for color removal from textile wastewater despite excessive sludge generation (Verma et al. 2012 ). Coagulation–flocculation may be used after secondary biological treatment and before tertiary membrane filtration to prevent fouling by removing colloids, TSS, and color (Aragaw and Bogale 2023 ). Studies have been performed on using AOPs, such as ozone-based processes, H 2 O 2 , photocatalysis, and Fenton oxidation, to remove refractory pollutants from textile wastewater (Paździor et al. 2019 ). Bilińska et al. ( 2016 ) performed a comparative analysis on the removal of Reactive Black 5 using O 3 , UV/O 3 , O 3 /H 2 O 2 , O 3 /UV/H 2 O 2 , and H 2 O 2 /UV. The ozone-based processes—O 3 and O 3 /H 2 O 2 —were cost-effective and could be used as a pretreatment before biological treatment to remove color and improve biodegradability. Heterogeneous photocatalysis has been investigated for the treatment of dye wastewater using semiconductor photocatalysts (TiO 2 and ZnO, in particular) for their good photocatalytic activity and availability (Donkadokula et al. 2020 ).

Treatment of industrial wastewater

Industrial wastewater is treated using physical, biological, and chemical techniques. Each treatment method has advantages and disadvantages regarding factors such as the efficacy of removing specific pollutants, the treatment volume, ease of use, cost, energy usage, and chemical consumption. In many cases, these technologies are combined to achieve efficient removal of multiple types of pollutants, while reducing the total treatment cost.

• Physical methods

Physical methods involve the removal of contaminants by exploiting physical and mechanical properties for separation (Pirzadeh 2022 ). These methods include screening, comminution, grit removal, sedimentation, DAF, adsorption, ion exchange, membrane filtration, evaporation, distillation, and membrane distillation.

Screening, comminution, and grit removal

Screening, comminution, and grit removal are used as pretreatment methods before primary clarification. The main purpose of these methods is to protect downstream equipment and improve the effectiveness of subsequent treatment stages by removing or grinding large solids and debris.

During screening, large solids and debris are removed by passing wastewater through a series of screens or mesh filters to prevent damage to downstream pipes and equipment. Various types of screens, including coarse and fine, are used according to the size and characteristics of the solids and debris to be removed. Coarse screens, such as bar screens, usually have openings with sizes of 6-mm or larger (U.S. EPA 2003 ). Fine screens usually have openings of between 1.5 and 6 mm in size (U.S. EPA 2003 ). To remove finer solids, very fine screens with openings of between 0.2 and 5 mm can be used, and microscreens with openings of between 0.001 and 0.3 mm can be used to further treat the secondary effluent, which may still contain fine solids (Prabu et al. 2011 ). Typically, coarse screens are used near the inlet to capture large solids, followed by using fine screens to capture small particles. The debris captured on the screens is removed manually or mechanically and disposed of in landfills, incinerated, or ground and returned to the wastewater stream (Prabu et al. 2011 ). The collected solids typically contain various materials, such as plastic, paper, rags, food, and feces from human activity (Szostkova et al. 2012 ). Screening is used in the textile industry to remove large solids (such as yarn, lint, fibers, and rags) (Azanaw et al. 2022 ) and in the meat processing industry to remove bones and meat debris (Philipp et al. 2021 ).

Comminution can be used as an alternative to screening to reduce the size of solids in wastewater by grinding or shredding (Deluise et al. 2005 ). Comminutors consist of a screen and a rotating drum with slots and cutting teeth to shred solid materials that accumulate on the screen (McLeary 2004 ). Comminution itself does not remove solids from wastewater. The crushed solids are removed in a subsequent grit chamber and sedimentation tank (Ahmed et al. 2021b ). Considering that the ground particles are not removed and can damage the downstream equipment, comminutors are not commonly installed in newer WWTPs (McLeary 2004 ).

Grit chambers are used to remove heavy particles, such as sand and gravel, from wastewater to prevent damage to downstream equipment, such as pumps and pipes. There are several types of grit-chamber configurations, such as aerated, vortex, and horizontal flows as well as hydrocyclones (U.S. EPA 2003 ). To remove solids efficiently, the type of grit chamber used is determined by many factors, such as the particle characteristics, settling velocity, space availability, maintenance requirements, energy consumption, and cost. The variable particle density warrants direct measurement of the settling velocity (Plana et al. 2020 ).

Sedimentation

Sedimentation is a simple and common method for removing TSS from wastewater, which reduces BOD and COD (Jover-Smet et al. 2017 ). Primary sedimentation tanks, also known as primary settling tanks and primary clarifiers, are placed after screening and grit removal and before secondary biological treatment in conventional WWTPs. In sedimentation tanks, the wastewater velocity is reduced to cause TSS, organic matter, and other particles to settle to the bottom of the tank and form a layer of sludge. The clarified effluent is collected near the top of the tank around the wastewater surface.

Sedimentation tanks can be rectangular or circular. Rectangular tanks have a lower construction cost and can have a longer retention time but are less effective for treating wastewater with high TSS, whereas circular tanks have a lower maintenance cost and easier sludge collection but a shorter retention time. Short-circuiting is more likely to occur in circular tanks than in rectangular tanks (Hirom and Devi 2022 ). The efficiency of sedimentation tanks is determined by many parameters, such as the particle characteristics, settling velocity, tank dimensions, and wastewater and flow characteristics (Ferdowsi et al. 2022 ). Considering the complexity of the settling characteristics of suspended particles, experimental data have been used to develop empirical models for sedimentation tanks (Christoulas 1998 ; Martínez-González et al. 2009 ; Jover-Smet et al. 2017 ). As a result of advances in computational technology, computational fluid dynamics (CFD) has recently been used to model, design, and simulate sedimentation tanks, which has improved our understanding of tank hydrodynamics and thereby, tank design (Hirom and Devi 2022 ). The sedimented solids are mechanically removed by equipment, such as sludge scrapers and sludge pumps.

Secondary sedimentation tanks are also used in conventional treatment plants after secondary biological treatment, such as AS. The functions of secondary sedimentation tanks are settling sludge containing microorganisms to produce a clear effluent and thickening sludge for subsequent recirculation and storage (Patziger et al. 2012 ). Secondary sedimentation tanks have also been designed using empirical models (Gao and Stenstrom 2018 ). Computational fluid dynamics models have also been developed for secondary sedimentation tanks. These models have provided insights for tank design, such as the effect of the structure and position of the inlet on turbulence (de Almeida et al. 2020 ; Gao and Stenstrom 2018 ).

Sedimentation effectively clarifies wastewater by removing TSS and BOD 5 at ratios of 50%–70% and 25%–40%, respectively (Jover-Smet et al. 2017 ). Sedimentation tanks can have complex hydrodynamics but are relatively low-cost and simple to design and operate. However, the disadvantages of these tanks, such as long retention times for settling fine particles, may lead to large tank volumes. Very fine particles and dissolved content are difficult to remove using sedimentation tanks.

Dissolved air flotation

Dissolved air flotation is used to clarify wastewater, where small air bubbles are used to remove TSS, BOD, COD, oils, and grease. Bubbles are generated by dissolving and saturating air under pressure into water and releasing the air into a flotation tank. The generated air bubbles attach to particles, which consequently float to the surface. The floating particles are removed by a skimming device and disposed. Flocculants and coagulants, such as polymers, ferric chloride, and aluminum sulfate, are often added to the wastewater to aggregate suspended particles, oils, and grease (Musa and Idrus 2021 ). Dissolved air flotation is used to treat many industrial wastewaters, such as those produced by the pulp and paper (Miranda et al. 2009 ), petrochemical (Yu et al. 2017 ), mineral processing (Rajapakse et al. 2022 ), and food industries (Shrivastava et al. 2022 ). For example, DAF can remove 70%–80% BOD and 30%–90% COD from slaughterhouse wastewater (Musa and Idrus 2021 ). For papermill wastewaters, 80%–90% TSS removal can be achieved by removing particles, such as fines, fillers, and ink (Miranda et al. 2009 ). The effectiveness of DAF depends on factors such as the bubble size distribution, gas–liquid mass transfer, hydrodynamics, wastewater characteristics, and tank geometry (Rajapakse et al. 2022 ).

Dissolved air flotation offers advantages over sedimentation, such as a shorter retention time, smaller space requirements, and faster removal of small and low-density particles (Rodrigues and Rubio 2007 ; Crini and Lichtfouse 2019 ). However, there are challenges associated with DAF, such as high operation and maintenance costs resulting from the energy requirements and maintenance costs for equipment (Yu et al. 2017 ; Musa and Idrus 2021 ). The addition of coagulants and flocculants may incur additional costs for DAF.

Ion exchange

The ion-exchange process consists of using a resin to remove dissolved ions and pollutants from water through exchange with similarly charged ions. Ion-exchange resins are solid materials made of a polymer matrix with functional groups attached by covalent bonds (Carolin et al. 2017 ). Some common polymer matrices include polystyrene, polyacrylic, phenolic, and polyalkylamine resins (de Dardel and Arden 2008 ). The resins are porous and have a large specific surface area for effective ion exchange. Conventional ion-exchange resins are bead-shaped with typical diameters between 0.04 and more than 1 mm (Fink 2013 ).

Ion-exchange resins can be largely categorized into cationic and anionic types. Cation-exchange resins are commonly used in softening applications to replace magnesium and calcium ions with sodium ions (Samer 2015 ). At the industrial scale, wastewater is passed through a column packed with an ion-exchange resin. The saturated resin can be regenerated by flushing the column with a sodium solution (Samer 2015 ). Anion-exchange resins can be used to exchange negatively charged ions, such as nitrate, sulfate, chloride, and bicarbonate (Gomaa et al. 2021 ). Anion-exchange resins can be regenerated by treatment with a basic solution, such as a sodium hydroxide solution or an ammonium hydroxide solution. Cationic and anionic resins can be classified by their functional groups, including the strongly acidic cation (SAC) with a sulfonic group, weakly acidic cation (WAC) with a carboxylic group, strongly basic anion (SBA) with a quaternary ammonium group, and weakly basic anion (WBA) with primary, secondary, or tertiary amino groups, and chelating with functional groups, such as iminodiacetic and phosphinic groups (Chen et al. 2021 ). The resin is selected depending on criteria such as the target ions to be removed, the wastewater characteristics, economic feasibility, and the desired water quality.

Ion exchange has been used for numerous purposes, such as water softening, demineralization, deionization, deacidification, removal of impurities (such as nitrates and heavy metals), decoloring, separation, and dehydration (de Dardel and Arden 2008 ). Thus, ion exchange is used in many fields, such as agriculture, food processing, chemical synthesis, laboratory use, wastewater treatment, and hydrometallurgy (Ijanu et al. 2020 ). Studies have been performed on using ion exchange for industrial wastewater treatment, such as to treat textile-dyeing wastewater (Khan et al. 2023 ) and remove fluoride (Wan et al. 2021 ), phenol (Anku et al. 2017 ), and heavy metals (Barakat 2011 ). Ion exchange has been shown to be an effective wastewater treatment but is not widely used industrially because of drawbacks, such as high associated costs and the limited selectivity of conventional resins (Crini and Lichtfouse 2019 ; Khan et al. 2023 ).

Generally, adsorption refers to the change in concentration of a substance relative to those of neighboring phases at the interface between two phases, including liquid–gas, liquid–liquid, solid–liquid, and solid–gas interfaces (Dąbrowski 2001 ). Solid adsorbents are used to remove organic and inorganic pollutants from wastewater. There are two types of adsorption: physisorption and chemisorption. Physisorption is the adhesion of an adsorbate onto an adsorbent surface through van der Waals forces. Chemisorption involves the formation of covalent or ionic bonds between an adsorbate and an adsorbent. Physisorption is weak, nonspecific, and reversible, whereas chemisorption is strong, more specific, and often irreversible. Many factors affect the effectiveness of adsorption, such as the temperature, pH, use of stirring, contact time, adsorbent dosage, and initial concentration (Sukmana et al. 2021 ; Chai et al. 2021 ).

Adsorbents used in water treatment include natural and synthetic materials, such as activated carbon, clay, biosorbents, graphene oxide, and various nanomaterials. (Tran 2023 ). Among various available adsorbents, activated carbon is the most popular and is widely used for wastewater treatment because of advantages such as a high specific surface area, wide applicability to many pollutants, and regeneration ability (De Gisi et al. 2016 ). However, activated carbon is expensive and can be costly to regenerate (Chai et al. 2021 ). The nonselectivity of activated carbon must be traded off against its wide applicability. Studies have been performed on using low-cost adsorbents, such as agricultural wastes (e.g., orange peel, banana peel, and rice husks) and industrial wastes (e.g., flue ash, red mud and bagasse ash) (Rashid et al. 2021 ). Li et al. ( 2019 ) reviewed the key requirements and analyzed the feasibility of using bioadsorption for industrial-scale treatment of dye wastewater. Bioadsorption was determined to be a competitive technology if the adsorbent used has a good adsorption/desorption performance and is reused numerous times.

Adsorption can be implemented at various stages of industrial wastewater treatment depending on the target pollutant, concentration, and desired treated effluent quality. For example, adsorption can be combined with technologies such as ozone, DAF, and coagulation to treat papermill wastewater and remove toxic pollutants, color, and COD (Thompson et al. 2001 ). Adsorption may also be used for secondary or tertiary treatment of wastewater containing oil and grease, such as those from petroleum refineries and metalworks (Pintor et al. 2016 ).

Membrane filtration

Wastewater can be filtered through semipermeable membranes with various pore sizes to separate pollutants, such as colloids, microorganisms, organics, salts, and ions. Semipermeable membranes are selectively permeated by substances with particular sizes and charges (Breite et al. 2019 ). Wastewater can be treated using MF, UF, NF, and RO membranes (Keskin et al. 2021 ). Asymmetric membranes are used in a pressure-driven process for their high flux and mechanical stability (Strathmann 2005 ). The membranes can be composed of polymers (such as polyethylene, polytetrafluorethylene, and polypropylene) or inorganic materials (such as ceramics, zeolites, and silica) (Ezugbe and Rathilal 2020 ).

Microfiltration membranes have relatively large pore sizes of 0.1 μm or more and are often used to pretreat wastewater before UF, NF, and RO (Behroozi and Ataabadi 2021 ). These membranes can separate larger particles, such as TSS, colloids, and organic matter at relatively low operating pressures of 0.02 to 0.5 MPa (Zioui et al. 2023 ). Ultrafiltration membranes have pore sizes of 0.001 to 1 μm and are used to remove pollutants, such as TSS, organics, oils, and pigments (Ezugbe and Rathilal 2020 ). Nanofiltration membranes have pore sizes of 1 to 5 nm and can therefore separate pollutants with relatively low molecular weights and reject pollutants such as sugar, salt, minerals, heavy metals, oils, and dyes (Mulyanti and Susanto 2018 ). Reverse osmosis membranes have pore sizes of a few angstroms and can separate sodium and chloride ions, making RO a promising technology for seawater desalination (Jamaly et al. 2014 ). Reverse osmosis membranes have small pore sizes that enable removal of all pollutants but are susceptible to fouling, must be operated under high pressure, and are more expensive than other membranes (Nqombolo et al. 2018 ).

Membrane filtration is a versatile technology for industrial wastewater treatment and can be combined with other technologies to achieve sufficiently high water quality for reuse. For example, membranes have been used in combination with biological treatment in MBRs. Membrane filtration can be used to treat wastewater from many industries, including pulp and paper (Valderrama et al. 2021 ), textiles (Keskin et al. 2021 ), electroplating, and petroleum (Barakat 2011 ). Some advantages of membrane filtration are the production of high-quality treated water, a smaller footprint than that of conventional filtration, and the ability to be installed in existing WWTPs. Some challenges associated with membrane filtration are higher overall costs compared with those of conventional WWTPs and susceptibility to fouling (Othman et al. 2022 ).

Evaporation, distillation, and membrane distillation

Evaporation, distillation, and membrane distillation are thermal wastewater treatment processes that can separate and concentrate pollutants in wastewater. The evaporated wastewater may be collected by condensation.

Evaporation ponds are shallow, open-air basins that are lined with materials, such as clay and synthetic materials, to prevent wastewater seepage. Water is evaporated by solar irradiation, resulting in the concentration of contaminants, precipitation of crystalline salts, and sediment accumulation. The accumulated solids are removed regularly and disposed. Evaporation ponds have various applications, such as the treatment of oil-produced water and mine wastewater and the rejection of brine from desalination plants and other industrial wastewater (Izady et al. 2020 ). Advantages of evaporation ponds include straightforward application and low capital and operational costs. Some disadvantages of evaporation ponds are the environmental and health impact of the release of heavy metals, pesticides, VOCs, CO 2 , and CH 4 (Amoatey et al. 2021 ). Evaporation ponds require large land usage and solar irradiation and are therefore suitable for dry and warm locations with low-cost land (Abdeljalil et al. 2022 ).

Other evaporation techniques consist of using equipment such as flash evaporators, condensers, and distillation columns. Wastewater is converted to water vapor using heating devices, such as heat pumps and heating elements. The water vapor is then condensed and collected as distilled water. A portion of the wastewater may be left behind that contains unevaporated salts and other solids. Evaporation techniques can be used to treat a wide variety of wastewaters and are especially useful for desalination. However, steam generation has a large energy demand. Organics that evaporate at low temperatures may enter the treated water stream. To lower the energy demand for heating and removal of organics, Yang et al. ( 2018 ) proposed a desalination system with a low-temperature heat pump. A COD removal of 97% was achieved at 48 °C, producing 3 kg of treated water in 1 h for a power consumption of 250 W. Evaporation technologies have been developed to reduce energy consumption and improve the separation efficiency. Among evaporation technologies, multieffect evaporation is used most often because of maturity and high efficiency (Lu et al. 2017 ). Multieffect evaporation is implemented using a series of single-effect evaporators, where the vapor generated from one evaporator is used to heat the next evaporator to reduce energy consumption. Mechanical vapor recompression is an alternative emerging technology in which generated vapor is compressed and reused to heat the feed. Mechanical vapor recompression is mainly used for desalination and has advantages such as a higher energy efficiency than multieffect evaporation, compactness, and low-temperature operation (Liang et al. 2013 ). Some advantages of evaporation technologies are a high recovery rate, recoverability of both high-quality water and salts, wide applicability, and no use of supplementary materials. Some disadvantages of evaporation technologies are high capital costs, high energy consumption, and complexity (Mizuno et al. 2013 , 2015 ; Lu et al. 2017 ).

Membrane distillation is a thermal separation process in which a hydrophobic porous membrane is used to pass water vapor while rejecting pollutants. Membrane distillation can be divided into four types depending on the permeation side: direct contact in which both sides of the membrane surface contact vapor, air-gap in which the permeation side has an air gap, sweeping gas in which a cold inert gas is used to transfer vapor from the permeate side, and vacuum in which a vacuum is applied to the permeate side by a pump at a lower pressure than the saturation pressure of the volatile molecules (Yan et al. 2021 ). Membrane distillation has been considered one of the most promising technologies for the treatment of saline wastewater and can also be used to treat oily wastewater by being combined with other processes to reduce fouling (Kalla 2021 ). Some advantages of membrane distillation are low working pressures (to prevent fouling), high selectivity, and low sensitivity to the feed solute concentration. Some disadvantages of membrane distillation are a lower throughput than RO, pore-wetting risk, and a high energy demand (Shirazi and Dumée 2022 ).

• Chemical methods

Chemical treatment involves the use of chemicals, such as inorganics (iron and aluminum salts) and organics (cationic, anionic, and nonionic polymers). Some examples of chemical treatment methods are coagulation–flocculation and chemical precipitation (for increasing the settleability of pollutants), chemical oxidation, and AOPs (for degradation of organics, pH adjustment, and disinfection). Chemical treatment is often combined with other biological and physical treatment processes as a pretreatment or post-treatment strategy.

Chemical precipitation

Chemical precipitation removes dissolved pollutants from wastewater as solid particles. This technique is effective for the removal of heavy metals and is widely used in industry because of low costs and facile operation (Yadav et al. 2019 ). Counterions are added to reduce the solubility of dissolved ions, which are then removed through precipitate formation (Zueva 2018 ). These precipitates must be separated out using methods such as sedimentation and filtration. Flocculants may be used to improve the settleability of precipitants (Ojovan and Lee 2014 ). Chemical precipitation has the advantages of low capital costs and simple operation but also has disadvantages, such as the operating costs of using chemical precipitants and sludge disposal (Wang et al. 2005 ).

Dissolved metals can be precipitated as hydroxides, sulfides, and carbonates. The most widely used method for hydroxide precipitation involves the addition of alkaline agents, such as calcium hydroxide (lime) or sodium hydroxide (Dahman 2017 ). Lime is the most cost-effective alkaline agent for wastewater treatment (Zueva 2018 ). The addition of lime results in the formation of metal hydroxides and calcium ions:

The optimal pH for hydroxide precipitation depends on the dissolved metal. Considering the amphoteric nature of metal hydroxides, decreasing or increasing the pH may cause precipitates to resolubilize. Thus, it is challenging to treat wastewater containing different metals because multiple steps are required to remove the metal precipitates at their optimal pHs.

Sulfide precipitation involves the addition of sulfide ions usually generated from H 2 S, Na 2 S, CaS, (NH 4 ) 2 S, or NaHS (Estay et al. 2021 ). The sulfide ions react with metals to form metal sulfide precipitates:

Sulfide precipitation offers advantages over hydroxide precipitation, such as less soluble precipitates, faster reaction rates, and better settling, as well as disadvantages, such as the sensitivity of the reaction system to the sulfide dosage and problems associated with the usage of excess sulfide (Lewis 2010 ).

Carbonate precipitation is an alternate method involving the use of sodium carbonate or calcium carbonate. Sodium carbonate participates in the following reactions, producing a metal carbonate precipitate and CO 2 that can attach to and float the precipitate (Zueva 2018 ):

Quiton et al. ( 2022 ) compared the efficacies of carbonate and hydroxide precipitation for the removal of cobalt and copper from electroplating wastewater. Compared to hydroxide precipitation, carbonate precipitation achieved a higher removal efficiency of both metals at a lower pH of approximately 7–8, but generated a larger sludge volume.

Coagulation and flocculation

Unlike chemical precipitation, coagulation and flocculation remove TSS and colloids without a phase change. Coagulation and flocculation often occur simultaneously but are different treatment processes. Coagulation refers to the destabilization of a suspension or solution, whereas flocculation refers to the agglomeration of destabilized particles into large flocs (Bratby 2016 ). The generated flocs are separated by sedimentation, filtration, or air flotation.

Mixing is an important factor that affects the overall process performance. Coagulation and flocculation have different optimal mixing speeds and times. Rapid mixing is employed for coagulation, whereas slow mixing is employed for flocculation (Saritha et al. 2017 ). Yu et al. ( 2011 ) investigated the effect of rapid and slow mixing on coagulation and flocculation, using aluminum sulfate hydrate (alum) as a coagulant and kaolin clay as a model suspension. Increasing the time for rapid mixing decreased the final floc size, whereas increasing the speed of slow mixing decreased the floc size. Rapid mixing can cause floc breakage because of high shear and change in the floc surface properties, which affects the coagulation efficiency. Other important factors that affect the coagulation and flocculation efficiencies include the pH, coagulant and flocculant dosage, temperature, and the presence of anions (such as bicarbonate or sulfate) (Ersoy et al. 2009 ).

Conventional chemical coagulants include alum, ferric sulfate, ferric chloride, and polyaluminum chloride, to which natural coagulants and flocculants derived from animals, plants, and microorganism have been considered as alternatives (Badawi et al. 2023 ). Inorganic coagulants and flocculants have disadvantages, such as pH sensitivity, sludge generation, and leaching of metal ions from sludge to groundwater, which has resulted in increasing use of polymer flocculants that can form large flocs at low dosages (Maćczak et al. 2020 ). Commonly used flocculants include nonionic flocculants (such as polyacrylamide), cationic flocculants (such as polydiallyldimethylammonium chloride), and anionic flocculants (such as copolymers of acrylamide and ammonium) (Dao et al. 2016 ).

The advantages of coagulation and flocculation include simplicity, effective removal of colloids and suspended particles, and effective settling of sludge. The disadvantages of these processes include high operating costs incurred by continuous addition of coagulants and flocculants, large sludge generation, and sludge disposal costs (Iwuozor 2019 ).

Solvent extraction

Solvent extraction can be used to remove and recover valuable materials from wastewater. This technique has been used commercially for materials recovery, such as in the petroleum, wool, and pharmaceutical industries (Lo and Baird 2003 ). The first step in solvent extraction is contacting wastewater with an immiscible solvent. The solvent selectively extracts the target compound, known as the solute, from the wastewater. Sufficient contact results in a solute-rich solvent (the extract) and a solute-depleted effluent (the raffinate). The extract and raffinate are separated, the solute is recovered from the extract, and the solvent is recycled. The raffinate is the treated wastewater, which can be further treated, discharged, or recycled depending on the demanded water quality. Contact between the solvent and the wastewater and solute extraction are achieved using various extractors, such as mixer–settlers, agitated columns, and packed columns, whereas solvent regeneration and solute recovery can be accomplished by distillation and gas stripping (Chang 2020 ).

Solvent extraction is widely used for the treatment and recovery of wastewater with high concentrations of phenolic compounds, such as wastewater from coal gasification (Feng et al. 2017 ). Yang et al. ( 2006 ) developed a solvent extraction process using methyl isobutyl ketoneas the solvent to treat wastewater from coal gasification containing 5,000 mg/L phenol. In a trial plant with a wastewater flow of 2 t/h, 93% of phenols were recovered. The recovered phenols provided economic benefits that could compensate for the operational cost of the process. The advantages of using solvent extraction to treat wastewater include selectivity for specific pollutants, recovery of valuable materials, solvent regeneration, and no sludge generation (Chang 2020 ). The disadvantages of this process include the investment costs associated with the use of specialized equipment and solvents and the potential environmental and health impact of solvent use.

Electrochemical methods

Electrochemical technologies involve the application of electricity through electrodes. The most studied processes include electrocoagulation (EC), electroflotation (EF), electrochemical oxidation (EO), electroreduction (ER), and electrodialysis (ED) (Sillanpää and Shestakova 2017 ).

Electrochemical coagulation and electrochemical flotation

In EC and EF, an electric current is applied to an anode and cathode in a reactor to produce destabilization agents, such as Al and Fe, and gas bubbles (Emamjomeh and Sivakumar 2009 ). Under an applied electric current, metal cations are generated at the anode and hydroxide ions and hydrogen gas are produced at the cathode. In EC, metal hydroxides are generated by the combination of metal cations and hydroxide cations. The metal hydroxides neutralize charged contaminants, and the neutralized contaminants are adsorbed by sweep coagulation, resulting in the formation of flocs (Das et al. 2022 ). In EF, the flocs are separated by hydrogen gas bubbles that adhere to the flocs and float the flocs to the surface or by sedimentation (Emamjomeh and Sivakumar 2009 ).

Similar to conventional coagulation–flocculation, EC can be used to remove contaminants from wastewater, such as TSS, TOC, oils, heavy metals, COD, color, and turbidity (Das et al. 2022 ). The use of EC has been investigated to treat wastewater from the dairy, textile, petroleum, pulp and paper, and pharmaceutical industries (Boinpally et al. 2023 ). The advantages of EC include the effective removal of colloids, no addition of chemicals, low sludge generation, and simple operation. Electrocoagulation also has disadvantages, such as periodic replacement of the sacrificial anode, electrode fouling, the possibility of metal hydroxide dissolution, and power consumption (Sivaranjani et al. 2020 ; Boinpally et al. 2023 ).

Electrochemical oxidation

Electrochemical oxidation involves both direct and indirect oxidation. During direct oxidation, pollutants adsorb onto the anode surface and an electron is directly transferred between the anode and the pollutant. During indirect oxidation, reactive species, such as reactive oxidation species and chlorine active species, are generated at the electrode surface and react with pollutants (Garcia-Segura et al. 2018 ). Garcia-Segura et al. ( 2018 ) investigated using EO to treat wastewaters, such as those from the petroleum, pulp and paper, and pharmaceutical industries, by removing COD, TSS, and recalcitrant organics. Electrodes, especially the anode where oxidation occurs, are key EO components that affect the cost and efficiency of the process. Thus, effort has been expended in developing anode materials with high performance, low cost, and high stability (Qiao and Xiong 2021 ). The advantages of EO include the ability to degrade recalcitrant pollutants, a small footprint, no addition of chemicals, and reduced secondary pollution. The disadvantages of EO are high energy consumption, high cost of some electrodes (such as those based on noble metals and diamond), challenges with mass producing some electrodes, possible corrosion and fouling of electrodes, and electrode replacement.

Electrochemical reduction

Electrochemical reduction is an emerging technology that involves either direct or indirect reduction. During direct reduction, electron transfer occurs between the cathode and the pollutant adsorbed on the cathode. During indirect reduction, the cathode reduces a mediator, and the reduced mediator reduces the pollutant (Mousset and Doudrick 2020 ). Electrochemical reduction has been studied for the detoxification and conversion of toxic organics into value-added materials, denitrification, removal and recovery of metals (Xue et al. 2023 ), and decolorization (Sala and Gutiérrez-Bouzán 2012 ). The advantages of ER are no chemical addition, a low footprint, metal removal and recovery, and the conversion of pollutants into value-added materials. The disadvantages of ER include the high cost of noble-metal electrodes, high energy consumption, the need to control competition with the hydrogen evolution reaction, and the possibility of some corrosion. Compared to EO, ER has been the subject of fewer studies, cannot mineralize pollutants, and has slower kinetics (Xue et al. 2023 ).

Electrodialysis

Electrodialysis involves the application of an electric field and ion-exchange membranes t(IEMs) to separate ions, such as dissolved salts. Electrode compartments contacting the anode and cathode are placed on the outer sides of ED units. Between the electrodes, there are alternating layers of anion exchange membranes (AEMs) and cation exchange membranes (CEMs) separated by spacers. Under an applied electrical potential, cations migrate toward the cathode by passing through the CEMs and anions migrate toward the anode by passing through the AEMs. Cations are blocked by the AEMs, and anions are blocked by the CEMs, resulting in compartments with alternating concentrated and dilute solutions (Mohammadi et al. 2021 ). The number of cell pairs in an ED stack depends on the scale of the units, ranging from a few cell pairs for lab-scale units to several hundreds of pairs for pilot-scale units (Mohammadi et al. 2021 ). The IEMs are selective and separate molecules based on their charge.

Electrodialysis has been studied to treat industrial wastewater containing salts, such as those from oil and gas extraction, the petrochemical and coal mining industries, and power plants (Gurreri et al. 2020 ). The use of ED has been considered for the recovery of metals, such as those from the metal deposition and electroplating industries (Arana Juve et al. 2022 ). The advantages of ED include high salt removal, metal removal, low susceptibility to scaling, no addition of chemicals, and low operating pressures. The disadvantages of ED are high capital costs and energy demand, membrane fouling, and the inability to remove nonionic pollutants (Zhao et al. 2019 ; Mir and Bicer 2021 ; Arana Juve et al. 2022 ).

Chemical oxidation

Conventional chemical oxidation involves using various oxidizing agents to degrade organic pollutants in wastewater and disinfect biologically treated wastewater. Various oxidizing agents have been used, such as permanganate, O 3 , H 2 O 2 , chlorine, and persulfate (Devi et al. 2016 ). Chlorination has been commonly used to treat wastewater before discharge or reuse. Rodríguez‐Chueca et al. ( 2015 ) compared the efficacy of using chlorination to disinfect Escherichia coli in municipal wastewater with that of using NaClO and various AOPs, including UV, H 2 O 2 /solar irradiation, and photo-Fenton oxidation. Although the optimal disinfection/cost ratio was obtained using chlorination, chlorine can generate carcinogenic halogenated byproducts that pose health and environmental risks. H 2 O 2 is another oxidizing agent that has been used to reduce BOD, COD, and odors to further improve the quality of wastewater treated by physical or biological treatment (Ksibi 2006 ). Doltade et al. ( 2022 ) used O 3 and H 2 O 2 as oxidizing agents to treat wastewater from the polymer industry, achieving COD reductions of 85% and 91%, respectively, from an initial COD of approximately 1920 ppm and demonstrated the synergistic effect of combining the two oxidants.

The advantages of chemical oxidation are versatility and effectiveness in removing a wide range of organic contaminants, a relatively short contact time for treatment compared to biological methods, odor and color removal, and disinfection. The disadvantages of chemical oxidation are the environmental impact of the oxidizing agent used and intermediates produced, operation costs associated with the production, storage, transportation, and usage of oxidants, and the need to use pretreatment.

Advanced oxidation processes

Advanced oxidation processes are emerging water treatment methods that can degrade trace toxic organics in wastewater and upgrade treated wastewater for reuse. Reactive species, such as hydroxyl radicals (HO•), are generated and nonselectively oxidize organic pollutants in wastewater to nontoxic substances, such as CO 2 and H 2 O. These methods can effectively treat wastewater containing recalcitrant organics that are difficult to remove by conventional treatment, such as pharmaceuticals, pesticides, phenols, and dyes. As a result of growing interest in water reuse and the need to meet stricter water pollution regulations, AOPs are increasingly being used to upgrade effluents by removing persistent pollutants. Advanced oxidation processes include O 3 /H 2 O 2 /UV, Fenton oxidation, photocatalysis, and sonolysis.

O 3 , H 2 O 2 , and UV

The strong oxidants—O 3 and H 2 O 2 —can be used alone or in various combinations with each other and UV, such as O 3 /UV, H 2 O 2 /UV, O 3 /H 2 O 2 , and O 3 /H 2 O 2 /UV. During ozonation, organics can be degraded by direct oxidation (reaction with O 3 ) or by indirect oxidation (reaction with hydroxyl radicals) (Chiang et al. 2006 ). The combined use of O 3 , H 2 O 2 , and UV has been proposed to improve the organic removal efficiency (Matsumoto et al. 2021 ). These methods, especially O 3 /H 2 O 2 /UV, have been used to treat wastewater from the textile, pharmaceutical, petroleum, and aquaculture industries by efficiently removing emerging pollutants (Angeles Amaro-Soriano et al. 2021 ). The advantages of these technologies are nonselective oxidation of pollutants by radical species, complete mineralization, no generation of halogenated byproducts, disinfection, simple operation, and the ability to upgrade treated wastewater for reuse. The disadvantages are the capital and operating costs of ozone generation, UV irradiation, and H 2 O 2 usage as well as the risks posed by handling O 3 and H 2 O 2 .

Fenton oxidation

During Fenton oxidation, HO• radicals are generated by using H 2 O 2 and iron ions as a homogeneous catalyst under acidic and ambient conditions. The generally accepted mechanism is given below (Bautista et al. 2008 ):

Photo-Fenton oxidation, which combines Fenton oxidation with UV–vis irradiation, has been used to improve organic degradation. The generation of HO• is realized by the decomposition of H 2 O 2 under light irradiation and the regeneration of Fe 2+ through the following reactions:

Fenton and photo-Fenton oxidation have been investigated to treat wastewater from industries such as oil, textile, and pulp and paper (Machado et al. 2023 ). The advantages of Fenton oxidation include simplicity, effective pollutant degradation, availability of Fe 2+ and H 2 O 2 , and environmental safety. The disadvantages of this method include the need for sludge disposal, pH control, and high chemical inputs (Bello et al. 2019 ).

Photocatalysis

Heterogeneous photocatalysis involves the use of a semiconductor photocatalyst that is activated by light irradiation. The photocatalyst absorbs photons with sufficient energy to generate electron–hole pairs that participate in reactions. Organic pollutants may be degraded directly on a photocatalyst surface or indirectly by generated HO• (Oturan and Aaron 2014 ).

In photocatalytic applications, TiO 2 has been widely used for its chemical stability, durability, low cost, and nontoxicity (Nakata and Fujishima 2012 ). Alternative photocatalysts have been developed but many are impractical because of being based on expensive, rare, or toxic materials as well as fragility and chemical instability. Thus, TiO 2 remains a popular choice (Loeb et al. 2019 ).

Studies have been performed on increasing the removal of organic pollutants by using hybrid photocatalytic technologies, such as a photocatalytic circulating-bed biofilm reactor with photocatalytic-biological carriers (Marsolek et al. 2008 ), photocatalytic membrane reactor consisting of a photocatalyst deposited on ceramic membranes (Lim and Goei 2016 ), and sonophotocatalysis (the simultaneous application of photocatalysts and ultrasound (US)) (Kakavandi et al. 2019 ). In other studies, photocatalytic oxidation has been improved by using a US-generated mist (Itoh and Kojima 2019 ; Kato et al. 2023 ).

Photocatalysis offers advantages of nonselective degradation and mineralization of a wide range of organics, low chemical consumption because the photocatalyst can be reused, and the option to use solar irradiation. The disadvantages of photocatalysis are the need for efficient light irradiation of the photocatalyst, operational costs, energy input of artificial light sources, photocatalyst fouling, instability and safety concerns for some photocatalysts, photocatalyst recovery for slurry reactors, and equipment costs.

Sonolysis involves using US to degrade organic pollutants in wastewater. Irradiation by US results in cavitation; that is, the formation, growth, and collapse of bubbles generating hot spots of approximately 5,000 and 500 atm and shock waves (Suslick 1990 ). Cavitation causes thermal dissociation and the formation of radicals (e.g., HO•, O•, H•, and HO 2 •) that react with organic pollutants (Atalay and Ersöz 2021 ). The sonolysis frequency is an important parameter that determines whether physical or chemical effects are dominant. Low US frequencies of 20–80 kHz are considered to mainly cause physical effects, whereas higher US frequencies of 150–2000 kHz are considered to mainly cause chemical effects (Chatel et al. 2017 ). The application of multiple frequencies may simultaneously enhance the cavitational intensity and chemical and physical effects (Gogate and Patil 2016 ). Sonolysis has advantages of safety, eco-friendliness, no addition of chemicals, utility as a pretreatment to enhance biodegradability, and the ability to degrade recalcitrant organics. The disadvantages of sonolysis are equipment costs, high energy consumption, and conversion of cavitational energy producing chemical and physical effects, which have limited the full-scale application of this technique (Pang et al. 2011 ; Pirsaheb et al. 2023 ). Sonolysis is versatile and compatible with other treatment techniques, such as biological treatment, photocatalysis, and the use of UV, O 3 , and H 2 O 2 (Savun-Hekimoğlu 2020 ; Pirsaheb et al. 2023 ).

• Biological methods

Biological treatment uses microorganisms, such as bacteria, fungi, yeast, and algae, to remove pollutants from wastewater. Biological treatment of wastewater is mainly used to reduce organics but can also remove inorganic compounds, such as heavy metals (Singh et al. 2022 ). Biological treatment can effectively treat industrial wastewaters with high organic contents, such as those from the food, paper, and textile industries.

Biological treatment is typically categorized into aerobic and anaerobic types. Aerobic biological treatment consists of using microorganisms to convert organic pollutants into CO 2 , water, and biomass in the presence of oxygen, which is often supplied by mechanical aeration using air blowers and compressors. During anaerobic biological treatment, pollutants are metabolized by microorganisms in the absence of oxygen through anaerobic processes, including hydrolysis, acidogenesis, acetogenesis, and methanogenesis (Vijin Prabhu et al. 2021 ). As a result, biogas containing mainly CO 2 and CH 4 is produced (Vijin Prabhu et al. 2021 ). Aerobic treatment is usually used for low-strength effluents with CODs below 1000 mg/L, whereas anaerobic treatment is suitable for high-strength effluents with CODs above 4000 mg/L (Chan et al. 2009 ). Considering the greenhouse gas (GHG) emissions (CO 2 and CH 4 ), Cakir and Stenstrom ( 2005 ) reported that crossover points exist in the range of 300–700 mg/L ultimate BOD (BOD u ), depending on the aerobic treatment efficiency. They indicate that anaerobic treatment emits less GHG for wastewater with a BOD u value above the crossover point while aerobic treatment emits less GHG for a BOD u value below the crossover point.

Anaerobic treatment offers advantages over anaerobic treatment, including a lower energy demand, six-to-eightfold lower biomass production, a smaller reactor volume, and the production of biogas, which can be used as fuel (Ghangrekar and Behera 2013 ). Ranieri et al. ( 2021 ) investigated the electricity consumption of 202 WWTPs in Italy and found an electricity consumption of 1.02 kWh/m 3 for aerobic treatment and 0.43 kWh/m 3 for anaerobic treatment. Aerobic treatment offers advantages over anaerobic treatment, including a higher quality of treated wastewater, reduced odor (Martin et al. 2011 ), and nutrient removal (Aziz et al. 2019 ). Thus, anaerobic–aerobic systems can be used to efficiently remove organic content and increase effluent quality to meet discharge standards (Chan et al. 2009 ). Other combined systems include AOAO, AAO, and AAOO. Biological treatment is combined with physical and chemical treatment, typically as a secondary treatment, in WWTPs.

Aerobic digestion

Commonly used aerobic treatment methods include AS, aerated lagoons, the sequential batch reactor (SBR), trickling filter, MBR, rotating biological contactor (RBC), and aerobic MBBR.

The handling of excess sludge has been a key issue in aerobic treatment. Although sludge is most commonly disposed of in landfills, other handling methods have been proposed because of the environmental impact of sludge, to comply with environmental regulations, and the increasing costs of landfill disposal (Nguyen et al. 2022 ). Some alternative methods for sludge disposal include sludge thickening followed by anaerobic digestion and incineration as well as dewatering and drying followed by incineration (Hao et al. 2020 ). The main disposal methods used in the EU have been sludge reuse (such as in agriculture) and sludge incineration (Kelessidis and Stasinakis 2012 ).

Activated sludge

Activated sludge is one of the most used biological processes for treating solids and organic pollutants in wastewater (Zhang 2020 ; Islam and Mahdi 2022 ). Microorganisms suspended in aeration tanks are used to remove organic pollutants and nutrients from wastewater. Following the aeration tank, sedimentation tanks are used to separate sludge, where some of the sludge is returned to the aeration tank to maintain the concentration of microorganisms, and the rest of the sludge is removed. Depending on the water quality demand or standards, the supernatant from the sedimentation tank may either be discharged directly or undergo further treatment or disinfection before discharge. Biocarriers may be used to increase the efficiency of COD removal by AS. Jagaba et al. ( 2022 ) reported up to 88.4% removal of COD in wastewater from the pulp and paper industry using a hydraulic retention time (HRT) of 2 days and rice-straw activated carbon. The advantages of AS include low installation costs, high effluent quality, and a low footprint. The disadvantages of AS include high operating costs, sludge disposal, and sensitivity to effluent characteristics (Rezai and Allahkarami 2021 ).

Sequential batch reactor

The SBR is a variation on AS in which unit operations, such as aeration and sedimentation, are carried out in the same tank (Albahnasawi et al. 2023 ). There are five stages of SBR operation. In the filling phase, wastewater is added to the tank. In the reaction phase, pollutants are removed with or without mixing and aeration. In the settling phase, the tank acts as a clarifier without inflow or outflow. In the drawing phase, the supernatant is discharged and excess sludge is removed. The idle phase is used to switch between tanks for multiple tank systems (Singh and Srivastava 2011 ). Aeration can be flexibly controlled in SBRs to realize aerobic or anaerobic conditions. The advantages of the SBR include flexibility, control, a small footprint, and low costs because unit operations can be conducted in a single tank. The SBR also has disadvantages, such as complex operation, maintenance, and possible sludge discharge and blockages in aeration equipment (U.S. EPA 1999 ).

Trickling filter

In trickling filters, wastewater is distributed over a bed of solid media, such as rocks, gravel, or plastic. The solid media provide a surface on which microorganisms can grow and form a biofilm. Aerobic conditions are achieved by either upward or downward natural airflow, depending on the temperature and the difference in the humidity inside and outside the trickling filter. Alternately, mechanical ventilation using low-pressure fans can be used to provide consistent upward or downward airflow (Daigger and Boltz 2011 ). Sloughing occurs as the microbial layer thickens, and a portion of the biofilm falls off into the effluent (U.S. EPA 2000 ). The sloughed film is separated out in a secondary clarifier featuring the secondary sludge. Trickling filters have been employed to treat wastewater from the dairy industry, achieving 85% COD removal using a HRT of 10 days at 7–13 °C (Shahriari and Shokouhi 2015 ). The SBR has been used to remove various pollutants in wastewaters from mining, textile, and other industries (Dhokpande et al. 2014 ). Trickling filters offer advantages such as simplicity, reliability, and low power requirements as well as disadvantages of odor emission and the need for additional treatment and operator monitoring (Rezai and Allahkarami 2021 ).

Membrane bioreactor

Membrane bioreactors combine biological treatment and membrane filtration (MF and UF). Biological treatment removes pollutants from effluent, and the generated sludge is separated by membranes rather than by sedimentation. There are external and submerged MBRs. Tubular membranes are installed separately from the bioreactor in external MBRs, whereas hollow-fiber or flat-sheet membranes are immersed in the bioreactor in submerged MBRs (Martínez et al. 2020 ). Aeration is used to provide air and turbulence, which is crucial for preventing membrane fouling in submerged MBRs (Melin et al. 2006 ). The use of ceramic filters with enhanced resistance to fouling and anaerobic operation has been proposed to reduce the energy demands of MBRs (Judd 2008 ). Some advantages of MBRs include the high effluent quality produced by membranes, small sizes, and simplicity of automation (U.S. EPA 2007 ). In an MBR, the HRT and solids retention time (SRT) can be controlled independently, enabling higher sludge concentrations, longer sludge retention times, and the development of specialized microorganisms (Melin et al. 2006 ). Membrane bioreactors have disadvantages such as fouling, the need for membrane maintenance, foaming, electricity demand up to double that of CAS, and high capital and operational costs (Al-Asheh et al. 2021 ).

Rotating biological contactor

Rotating biological contactors use rotating cylindrical discs that are partially submerged in wastewater (Waqas et al. 2023 ). Microorganisms consume organic matter and form a biofilm on the disk surface. The rotating discs promote oxygen transfer to maintain aerobic conditions and provide turbulence to remove excess solids from the disc (Cortez et al. 2008 ). Thus, the cost of aeration is lower than that for AS. The effluent from an RBC is sent to a secondary sedimentation tank to remove TSS (Pathan et al. 2016 ). The advantages of RBCs include small land usage, high biomass concentrations, low energy consumption, short HRTs, and low operational and maintenance costs. The disadvantages of RBCs include low flexibility, the need for sludge removal, and sensitivity to wastewater characteristics (Mizyed 2021 ).

Aerobic moving bed biofilm reactor

In MBBRs, suspended plastic biofilm carriers are used to support microbial growth and the development of biofilms. The plastic carriers have a similar density to that of water and are maintained in suspension by aeration, liquid circulation, or mechanical mixing (Gzar et al. 2021 ). Biofilm sloughing occurs in MBBRs, as in trickling filters. Biomass from the MBBR effluent is removed using methods such as sedimentation, flotation, microscreening, and membrane filtration (Ødegaard et al. 2010 ). A major advantage of MBBRs is the ability to upgrade and increase the performance of existing WWTPs that use AS, eliminating the need to build new tanks (Falletti et al. 2014 ). Upgrading AS to MBBRs can decrease the HRT, increase the SRT, prevent clogging and channeling, and reduce capital costs (Ahmadi et al. 2011 ).

Aerated lagoons

Aerated lagoons consist of basins or ponds in which mechanical aeration is provided by devices, such as floating surface aerators and submerged diffusers. Aerated lagoons may be further classified into complete- and partial-mix lagoons. Complete-mix lagoons are aerated to maintain solids in suspension, which requires a high energy input. The aerators in partial-mix lagoons are designed to provide oxygen and do not provide sufficient turbulence to maintain solids in suspension (Alvarado et al. 2013 ). Consequently, sludge settling occurs and anaerobic zones form. Partial-mix lagoons have one-tenth the energy demand of complete-mix lagoons (U.S. EPA 2002 ). The advantages of aerated lagoons include lower operation and management costs than those of AS, lower sludge generation than other secondary treatment techniques, and lower land usage than those of stabilization ponds. The disadvantages of aerated lagoons are larger land usage compared to that of AS, lower nutrient removal compared to that of stabilization ponds, and energy input for aeration (U.S. EPA 2002 ).

Anaerobic digestion

As reviewed by van Lier et al. ( 2015 ), the development of continuous anaerobic reactors began with low-rate anaerobic reactors like the single-flow through tank designed by Karl Imhoff in 1905 and continuous stirred tank reactors (CSTR) that were prevalent until the 1960s. These low-rate reactors have similar HRTs and SRTs, and due to the low growth rate of bacteria, they require large volumes to provide sufficient biomass concentration (van Lier 2008 ).

To increase the biomass concentration and enhance treatment capacity, high-rate anaerobic reactors were developed, which decoupled SRT and HRT. The first high-rate anaerobic reactors to be developed were the anaerobic contact process (ACP), which has a secondary clarifier and recirculates sludge similar to AS, and the anaerobic filter (AF), which uses support materials for microbial growth (van Lier et al. 2015 ).

In the 1970s, Lettinga et al. ( 1976 , 1980 ) developed the UASB reactor. The expanded granular sludge bed (EGSB) reactor was developed to improve the UASB reactor so that higher loadings can be applied. High-rate anaerobic sludge bed reactors, including the UASB reactor, EGSB reactor, and their derivatives, have been the most popular anaerobic treatment for industrial wastewater, holding approximately 90% of the market share due to their compactness, simple operation, and ability to operate at high volumetric loading rates and short HRTs (van Lier 2008 ). Sludge retention in these reactors is increased by the effective separability and settleability of biomass.

The advantages of anaerobic reactors include efficient COD removal at high organic loadings, biogas production, low energy demand and operational costs, a small footprint, lower sludge production than AS, and generation of biologically stabilized sludge. The disadvantages of anaerobic reactors include the difficulty of controlling sludge granulation, the dependence of granulation on the wastewater quality, the need for pretreatment to remove TSS, sensitivity to shock loads (Hansen and Cheong 2007 ), temperature sensitivity, and long startup times.

The key conditions for anaerobic systems that can treat high COD loads include high sludge concentrations and retention, sufficient contact of biomass and wastewater, a high reaction rate, effective transfer of metabolic products from biofilms, adaptation of biomass to the wastewater characteristics, and favorable conditions for microorganisms (van Lier et al. 2020 ).

Upflow anaerobic sludge blanket

In UASB reactors, wastewater is fed from the bottom of the reactor and flows upward through a granular sludge bed and blanket. The biogas produced from the anaerobic digestion of organics provides mixing and facilitates the formation and maintenance of sludge granules (Daud et al. 2018 ). The formation of sludge granules is essential for UASB reactors because these granules support biofilms and provide buoyancy and settleability, facilitating the contact of biomass with wastewater and enabling biomass retention (Abbasi and Abbasi 2012 ). A gas–liquid–solid separator (GLSS), usually in the shape of a funnel or triangular prism, is installed in the upper section of the reactor to separate sludge granules, biogas, and water. The separated biogas and treated wastewater are discharged from the top of the reactor. The GLSS acts as a clarifier by separating and preventing mixing of the sludge granules and the effluent. The sludge concentration is maintained by the settling of the separated sludge granules. Efforts have been made to improve the GLSS design so that the sludge retention increases in the UASB reactor. Dos Santos et al. ( 2016 ) installed parallel plates at a 45° angle above the conventional separator, doubling the treatment capacity.

Expanded granular sludge bed

The EGSB reactor is a variant of the UASB reactor with a slight bed expansion resulting from a high superficial velocity of 4–10 m/h, which is achieved by using a tall reactor or effluent recirculation (Tauseef et al. 2013 ). An EGSB reactor has a specifically designed GLSS to separate biogas, effluent, and sludge. Compared to UASB reactors, EGSB reactors provide better contact between the biofilm and wastewater and may use higher sludge concentrations. Other advantages of EGSB reactors include a small footprint, the ability to function at high organic and hydraulic loadings, and the ability to treat wastewater containing lipids and toxic compounds (Mao et al. 2015 ). These advantages have resulted in an increasing number of installments of full-scale EGSB reactors compared to the declining use of UASB reactors (van Lier et al. 2020 ).

Conclusions

A comprehensive overview has been provided of the characteristics of wastewater from various industries and wastewater treatment technologies, highlighting their principles, applications, advantages, and disadvantages. A wide variety of physical, biological, and chemical treatment methods exist to treat the diverse pollutants found in industrial wastewater. Each technology offers unique advantages in terms of efficiency, cost-effectiveness, selectivity, environmental compatibility, and resource recovery. Differences in wastewater characteristics, local site-specific factors, and treatment objectives make it necessary to implement wastewater treatment facilities tailored to these needs. A single technique often cannot adequately achieve the treatment objectives, motivating the combination of various techniques. However, only a few methods are used to treat industrial wastewater for technological and economic reasons (Crini and Lichtfouse 2019 ). Ongoing research and innovation in wastewater treatment continue to drive sustainable water usage. It is necessary to develop more advanced treatment methods to remove a wide range of pollutants in industrial wastewater to protect the environment, increase the quality of treated wastewater for recycling, reduce the resource intensity of wastewater treatment, and recover valuable resources. The complexity of wastewater management warrants the realization of technological advances and interdisciplinary collaboration to develop holistic approaches considering technological, economic, and social factors.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article.

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This work was supported by the Japan Prize Foundation and JST SPRING (Grant number JPMJSP2108).

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Global Water, Sanitation and Hygiene Research Priorities and Learning Challenges under Sustainable Development Goal 6

Karen setty.

1 The Water Institute at University of North Carolina at Chapel Hill, Department of Environmental Sciences and Engineering, 166 Rosenau Hall, CB #7431, Chapel Hill, NC 27599-7431

Alejandro Jiménez

2 Stockholm International Water Institute (SIWI), Linnégatan 87A, Box 101 87, 100 55 Stockholm, Sweden

Juliet Willetts

3 University of Technology Sydney, Institute for Sustainable Futures, Bldg 10, 235 Jones St, Ultimo NSW 2007, Australia

Mats Leifels

4 Centre for Water and Environmental Research (ZWU), University Duisburg-Essen, Universitätsstr. 2, 45141, Essen, Germany and University of Alberta, School of Public Health, 3-300 Edmonton Clinic Health Academy, 11405 - 87 Ave, Edmonton, AB T6G 1C9, Canada

Jamie Bartram

5 The Water Institute at University of North Carolina at Chapel Hill, Department of Environmental Sciences and Engineering, 166 Rosenau Hall, CB #7431, Chapel Hill, NC 27599-7431

Associated Data

Motivation:.

Sanitation and Water for All (SWA) is a global partnership addressing challenges to universal water, sanitation, and hygiene (WaSH) access. Shortly following adoption of the United Nations’ Sustainable Development Goals, the Research and Learning (R&L) constituency of SWA undertook a systematic study to determine global research priorities and learning needs.

We aimed to identify priority topics where improved knowledge would aid achievement of Goal 6, by developing a global WaSH research agenda, and to describe evidence-use challenges among WaSH professionals.

Approach and Methods:

We delivered a tailored, semi-structured electronic questionnaire to representatives from countries, R&L institutions, and other SWA partners (external support agencies, civil society, and private sector). The survey gathered views from 76 respondents working in an estimated 36 countries across all world regions. Data were analyzed quantitatively and qualitatively to identify patterns and themes.

Most responses indicated lowered confidence on at least one Goal 6 target area, especially managing untreated wastewater and faecal sludge. Both brief and lengthy information formats were valued. WaSH information was perceived as conflicting or unreliable among non-R&L constituencies, suggesting differences in perceptions and information-seeking approaches. While the R&L constituency appeared saturated with learning and training opportunities, others perceived barriers to participating (e.g. not receiving notice or invitation). Research and other WaSH activities were frequently constrained by upward accountability to funders, while stakeholders were inconsistently included in research processes.

Policy implications:

This study offers insight into perceived research and decision challenges related to Goal 6 targets. It develops a unified research agenda focused on high priority topics, and recommends renewed attention to evidence synthesis, learning and implementation support, research engagement, and multisectoral coordination.

Introduction

The United Nations (UN) Sustainable Development Goals (SDGs) replaced the Millennium Development Goals (MDGs) at the end of 2015 ( UN General Assembly, 2015 ). Among the 17 SDGs, Goal 6 seeks to ‘ensure availability and sustainable management of water and sanitation for all’. Although some components of other SDGs also address or intermix with water, sanitation, and hygiene (WaSH), the Goal 6 targets in particular set out a clear agenda that will play an important role in framing WaSH development efforts from 2015 to 2030 ( UN, 2018 ). In comparison to the previous MDG target 7.C: ‘halve, by 2015, the proportion of the population without sustainable access to safe drinking water and basic sanitation’ ( UN, 2015 ), SDG 6 is more ambitious and may demand a shift in knowledge needs, potentially leaving WaSH professionals and institutions under-prepared.

This transition represented an opportune time for WaSH professionals to reflect on what activities might best support global achievement of Goal 6, as well as what factors might hinder its realization. While access to water and sanitation services has expanded over recent decades, progress has been hampered by population growth, among other factors, leaving many unserved or underserved ( World Health Organization & UNICEF, 2014 ). Inadequate water supplies and poor sanitation and hygiene continue to contribute to disease and deaths, especially in low-income countries and among children under five ( Troeger et al., 2017 ; Wolf et al., 2018 ). Efforts to improve these conditions are plagued by multiple challenges, including weak political support, insufficient national capacity, gaps in monitoring, and inadequate human resources ( WASH Impact Network, n.d. ; World Health Organization & UNICEF, 2014 ).

Sanitation and Water for All (SWA) was established in 2009, in part to address inefficiencies in WaSH sector coordination and to drive progress towards the MDGs. This global partnership of more than 170 entities works together to catalyse political leadership and action, improve accountability, and use scarce resources more effectively within the WaSH arena ( SWA, 2018 ). Partners agree to work towards a common vision of universal access to clean water and adequate sanitation. The growing SWA membership consists of diverse WaSH organizations, including civil society organizations (CSOs), national governments, multilaterals, development banks, foundations, private businesses, universities, and institutions. They typically join one of five SWA ‘constituencies’: countries, external support agencies, civil society, research and learning (R&L), or the private sector.

SWA’s R&L partners commit to strengthening the evidence base for WaSH ( SWA, 2018 ). Country partners represent low and middle-income country governments supporting domestic implementation of WaSH goals. Other SWA partners mobilize and allocate WaSH resources, influence political agendas, implement WaSH programmes or projects from regional to international levels, and/or conduct business that supports SWA objectives. SWA’s three ‘priority areas’ are political prioritization, government-led national planning processes, and development of a strong evidence base ( SWA, 2018 ). For the latter, SWA recognizes that decision-makers require high quality, up-to-date information to make appropriate and timely decisions.

Though linking scientific evidence to policy and practice outcomes is a common goal ( Hering, 2018 ; Nutley, Walter, & Davies, 2007 ), achieving this is challenging. Researchers often favour ‘supply-driven’ knowledge transfer, even when policy and practice needs are not adequately met, at the same time perceiving ‘demand-driven’ models as excessively constraining ( Hering, 2018 ). Urging researchers to satisfy only client needs may compromise the scientific process and objectivity ( Poch, Comas, Cortés, Sànchez-Marrè, & Rodríguez-Roda, 2017 ). Further, needs assessment may inadvertently truncate agendas if it focuses on the most achievable short-term needs at the expense of long-term and sometimes more critical issues ( Boyd, 2016 ). Timelines frequently differ, leading to a mismatch of research cycles with ‘policy windows’ ( Rose et al., 2017 ). More structured approaches for harvesting evidence requirements and integrating them into research agendas ( Huberman, 1994 ; Viergever, Olifson, Ghaffar, & Terry, 2010 ), along with professional capacity building for ‘boundary work’ ( Cash et al., 2003 ), could enhance progress toward WaSH goals.

Considering historic challenges and the diversity of WaSH actors worldwide, broad representation of stakeholders from multiple disciplines should feed into research prioritization ( Bryant, Sanson-Fisher, Walsh, & Stewart, 2014 ); however, some gaps stem from differences in norms of practice among actors. Policy-makers and their senior advisors have the closest interactive experience with how evidence informs decisions in their home countries, but may not be fully aware of their own future needs, or able to communicate them to researchers. Private businesses and CSOs often carry out WaSH projects, and gain familiarity with local context. In contrast, researchers often have more exposure to broad evidence and theory, but may be constrained by academic expectations, wherein funding availability and scientific advancement might take precedence over meeting the needs of end users ( DFID, n.d.-a , n.d.-b ; Kolsky, n.d. ; Smith, An, & Kawachi, 2013 ).

This study aimed to develop a global WaSH research agenda as collective guidance (e.g. for matching information needs to scientific bodies capable of fulfilling those needs). It also examined similarities and differences in perspectives among SWA constituencies to explore effective means for science communication and knowledge integration. Research questions asked:

• What evidence would accelerate progress on Goal 6?
• What type of evidence resources and delivery methods best support WaSH decision-making, research, or programme activities?
• Which barriers limit WaSH decision-making, research, or programme activities?
• How do respondent characteristics affect responses?

Overall, this study sought to backward-map research priorities critical to the pursuit of Goal 6, to draw out knowledge gaps and barriers, and to identify valued information resources and patterns of evidence use across SWA constituencies. Practically, this meant inferring needs based on recent experiences or activities rather than asking participants to anticipate future needs. An electronic survey was the primary mode of data collection, and questionnaires were developed using pragmatic, stakeholder-driven procedures ( Lewis, Weiner, Stanick, & Fischer, 2015 ). The first questionnaire targeted representatives of SWA partner countries. Tailored questionnaires then solicited feedback from other SWA partners, including (a) researchers, and (b) ‘all others’ (multilaterals, private businesses, CSOs, and funders). To facilitate follow-up, an in-depth interview guide (not shown) was developed, pilot-tested, and revised in parallel, lending some initial insight to the overall approach to survey question development.

Survey Development, Testing, and Translation

The survey consisted of an electronic questionnaire tailored to each of the three constituency groupings and developed using Qualtrics software. Each began with brief background and consent information, followed by introductory text citing some key differences between the MDGs and SDGs. Depending on constituency, the questionnaires consisted of either five or six sections moving from research priorities to decision challenges, learning challenges, funding and stakeholder interactions, and finally respondent characteristics. At the conclusion of the questionnaires, all respondents had the option to provide a first name and method of contact for follow-up, as well as to nominate a potential new R&L constituency member as part of an ongoing recruitment effort focused on institutions in the global South.

Survey questions and response categories (detailed in supplementary information ) mirrored the research questions. Table 1 gives the country constituency survey as an example, while the R&L and other questionnaires followed a similar format with minor modifications (described below). Questions sought to avoid unnecessary analytical leaps, focusing on recent personal experience. They sought both qualitative and quantitative information that could help describe important themes. Questionnaires comprised primarily closed-ended multiple-choice questions, typically with an ‘other’ write-in option, followed by an open-ended question to request elaboration and conclude each section. This combination of question types offered respondents suggestions for faster cognitive processing, while enabling free response to avoid limiting them to the suggested categories. The first question, for example, closely matched the Goal 6 targets ( Table 2 ), while the second question requested open description of any specific knowledge gaps or areas of concern.

Research questions mapped to country constituency questionnaire (full text offered as supplementary information )

Example of question development based on Goal 6 target areas

An informal survey work group within the R&L constituency and a survey expert from the Odum Institute for Social Science Research at UNC reviewed questions targeting country representatives for content, length, language, and clarity. They were then uploaded to the Qualtrics software platform and pilot-tested by the R&L work group (alpha testers) and external reviewers (beta testers), including international students and an information technology specialist. Reviewer feedback, especially to reduce length, was incorporated into the questionnaire design.

Following review of preliminary findings from the country constituency questionnaire, two modified versions were developed for the R&L constituency and ‘all other’ constituencies. This generally involved minor language adaptation (e.g. ‘decisions’ versus ‘research’ versus ‘activities’), to maintain comparability among versions. Although the country questionnaire was originally designed to avoid future projection, one question on future research priorities was added to the R&L and ‘all other’ versions, for the sake of comparison. In addition, funding and stakeholder interaction questions were added to explore potential drivers behind inter-constituency differences.

The final surveys consisted of 20-24 questions each. The country and ‘all other’ constituency questionnaires and recruitment messages were translated from English into French and Spanish to cover the three working languages of SWA, while the R&L survey and recruitment messages were offered in English only, at the work group’s request. Full-text English-version questionnaires are provided as supplementary information . Recruitment email text and questionnaires were reviewed by the UNC Office of Human Research Ethics (IRB #15-5808, exempt).

Survey Deployment

A printed pre-survey announcement was made available at the 15-17 March 2016 SWA Sector Ministers’ meeting in Addis Ababa, to engage respondents from the country constituency. The country questionnaire was deployed in May 2016, followed by preliminary data analysis, review of initial findings, and revision and translation of the remaining questionnaire and recruitment text. The R&L and ‘all other’ constituency questionnaires were then co-deployed from late September to early October 2016. Invitations shared a common anonymous link with all SWA representatives from each constituency, and invitees self-selected to participate. Following the two-week deployment, interpersonal recruitment led to one additional survey response, submitted three days after the deadline.

Data Cleaning and Analysis

Partial responses with answers to at least three questions (about 15% of the questionnaire) were included in the dataset, while those with fewer answers were excluded. Questions left blank were excluded from analysis, as was one duplicate response from the same respondent. All responses were de-identified, and respondents remained anonymous aside from automated IP address and geolocation collection by Qualtrics. Responses in French were translated to English; no responses were received in Spanish.

Data from related questions were matched across the three questionnaires, and analysed by question and constituency grouping. Data interpretation methods aimed to value quantitative and qualitative data as equivalent forms of insight. Quantitative (multiple-choice) responses were tallied and graphed using Microsoft Excel. Qualitative (open-ended) question responses were coded using line-by-line ‘in vivo’ codes (as close as possible to actual wording used by respondents), and tallied by constituency and frequency to identify trends. To achieve this, conventional qualitative content analysis and description ( Hsieh & Shannon, 2005 ; Kim, Sefcik, & Bradway, 2017 ) were combined with some elements of grounded theory ( Charmaz, 1996 ). A standard codebook was not developed; rather, each set of question responses was assessed inductively and not constrained to fit prior question responses. A single rater performed the qualitative data analysis with limited spot-checking by another member of the research team.

Although sections and questions differed slightly on each of the three questionnaires, responses to related questions were grouped into four primary categories: respondent characteristics, research priorities, learning and training, and funding and stakeholder interactions. Questions were assessed individually, and question-specific results were then summarized across these categories to interpret themes.

To develop a weighted sum for the research prioritization, values for each question 1 category (very confident = 3, somewhat confident = 2, not confident = 1, unsure = 0) were assigned and multiplied by the number of respondents selecting that category. To account for novel written-in categories from question 2 and achieve comparability of scales, target areas named as a ‘knowledge gap or area of concern’ were considered equivalent to ‘not confident’ and set to 1, while target areas that went unnamed were considered to correspond to moderate or greater confidence and set to 2.5. Although the calculation methods for questions 1 and 2 differed, the categories were integrated in one rank order based on their respective weighted sums. A scaled average score divided the weighted sums by the number of effective respondents.

Although overall confidence about knowledge and ability to work toward Goal 6 was high, with 86% of responses falling into the ‘very confident’ or ‘somewhat confident’ categories, only 7% of respondents were ‘very confident’ across all Goal 6 target areas included in question 1. Target 6.3, halving the proportion of untreated wastewater, was consistently named over multiple questions as an area of low confidence. Further, respondents extensively cited funding and financing as strong determinants of WaSH-related research and programming activities. Some perceptions (e.g. agreement across information sources) differed among constituencies, and cross-sector communications appeared somewhat challenged when it came to learning opportunities and research engagement.

Representativeness

Response rates were reasonable for an anonymous web-delivered survey, averaging 10% of those on the SWA mailing lists (about 759 individuals; Table 3 ). Actual response rates were probably higher because some email addresses may have been duplicates, no longer active, or for non-WaSH professionals (e.g. caterers). In total, 76 individuals responded (30 from the country constituency, six from the R&L constituency, and 40 from all other constituencies).

Response rates for three questionnaire deployments (country, R&L, and ‘all other’)

Survey respondents were working in an estimated 36 countries across all five UN world regions (excluding the sub-region of Latin America and the Caribbean). Approximately half (47%) of all responses and close to two-thirds of the country responses came from sub-Saharan Africa, mirroring the 2016 membership of SWA. Responses also came from Asia and the Middle East (21%), Europe (17%), North America (12%), and Oceania (3%). Some degree of misclassification via automated geolocation was likely, as contact information was not required of respondents, and two responses appeared to come from a work travel location. Respondents reported having diverse educational backgrounds and professional roles. Almost half had studied engineering or natural sciences, while other common fields included business, economics, medicine, public health, sociology, and political science.

This sample represented at most half of all SWA partner organizations ( Table 3 ). Respondents were not limited to one person per agency; therefore, the actual ratio probably was lower. Geolocations suggested that at most up to six people from the same country responded to any given questionnaire. Because the ‘all other’ survey went to multiple constituencies and only 75% marked their constituency affiliation, the breakdown for this survey was extrapolated from those who did respond ( Table 3 ). The smallest constituencies, R&L and private sector, had the lowest estimated rates of response per member organization (one in three). The ‘all other’ survey had the lowest rate of response per email address. Eighty per cent of the surveys included in the study were complete, while 20% were partial responses. The sample was deemed satisfactory for the study purposes; however, differences in constituency size and demographic question response rates primarily recommended stratification of the data into the three questionnaire groupings (countries, R&L, and others), rather than by educational background or current professional responsibilities.

Research Priorities

Overall confidence was high, but not equivalent across all target areas. Ninety-three per cent of respondents were less than ‘very confident’ about their knowledge/ability to work in at least one of the Goal 6 target areas (question 1), and 43% reported a specific area of concern or knowledge gap related to Goal 6 (question 2). We ranked the need for research under each target area using a weighted sum, where the lowest values corresponded to the least confidence ( Table 4 ). Uncertainty regarding managing untreated wastewater or faecal sludge was common. For example, one country respondent wrote, ‘I have specific knowledge gaps in the management of untreated wastewater… especially within the context of [country] where we don’t have treatment works. Wastewater is indiscriminately disposed of in settlements, open fields, and waterways’. Some top written-in concerns from question 2, such as managing wastewater and sludge and capacity building, reinforced the multiple-choice categories and were not double-counted. Others (namely financing and sustainability) represented novel categories and were added to the ranking ( Table 4 ). Written-in responses cited fewer than three times, such as interdependency with other SDGs, were not elevated as priorities ( Table 4 ).

Weighted ranking of confidence around Goal 6 targets from question 1 (n = 76), including two of the top written-in concerns from question 2 (n = 33, referenced as ‘added by respondents’). The lowest weighted sum corresponds to the least confidence.

Sub-categories were matched to each target area ( Figure 1 ), drawing from prominent information needs reported on questions 3, 4, and 5 (where more information would have aided the respondents’ work or decisions over the past six months). These included strategic planning and prioritization, sector coordination and collaboration, monitoring and evaluation, affordability (e.g. subsidies, tariffs), reaching poorest populations, and appropriate technologies. Additional sub-categories came from prominently reported recent or prospective work areas described under questions 6 (topics of recent research or decisions) and 7 (pressing future needs; R&L and ‘all other’ questionnaires only). These included equality and non-discrimination; WaSH in institutions and public spaces; WaSH finance or business models; resilience, security, and climate change; ecologically sustainable solutions; and universal access including remote areas.

Hierarchical outline of proposed WaSH research agenda under Goal 6 (full text research agenda offered as supplementary information )

The full resulting research agenda is provided as supplementary information . Research questions were drafted under each sub-category, using direct qualitative description from questions 6 and 7 if available. Rank ordering corresponds to the quantitative frequency of responses to questions 1–5. In the ranked outline structure of the research agenda ( Figure 1 ), we sought to balance inclusivity with brevity, based on separation in the frequency of responses. Thus, more sub-categories were included under the highest priority target area (managing untreated wastewater or faecal sludge). Sub-categories selected by a minority of respondents or unrelated to the main Goal 6 target areas were excluded.

Priorities were pooled equally among the 76 survey respondents to develop the research agenda, and responses generally agreed among constituencies. Statistical comparison was not feasible due to differences in sample size, but some priority knowledge areas clearly differed among respondents from different constituencies. When comparing the three sub-groupings, the R&L constituency had the least confidence about ending open defaecation (fully half were not confident), while the country constituency had the least confidence about managing untreated wastewater (only 14% were very confident) and the other constituencies had the least confidence about building national capacity (just 38% were very confident). Write-in recommendations for financing and ecosystem sustainability came mainly from the non-R&L constituencies. The country and ‘all other’ constituencies often mentioned WaSH finance or business models, as well as costing, budgeting, and donor management, while the R&L group did not broach this topic. One respondent described the intensity of financial limitations, writing ‘…with the growing economic crisis, it will be difficult to talk about universal access.… Only the ‘all other’ constituencies, which included CSOs, raised menstrual hygiene management and gender equality (frequency = 5).

When comparing questions about recent and future challenges (questions 6 and 7), the R&L and ‘all other’ constituencies viewed climate change and resilience, equity and inclusion, and WaSH in institutions and public spaces as more pressing under a future scenario. Sludge management and WaSH law or policy were instead perceived as more pressing in recent experience, including the country constituency (question 6), which may have been an effect of the recent adoption of Goal 6. Education and knowledge sharing, and utility management and service delivery, were more commonly reported as critical to future success, excluding the country constituency. For example, one respondent from a CSO wrote, ‘[half] of [the people in my country] do not know about this WaSH… we are [in the] process to educate our people’.

Learning and Training

Country respondents relied mainly on easy-to-access informational resources such as the internet or personal contacts to address WaSH questions, with secondary use of more distant resources such as contacts within a professional network. Country respondents viewed multilateral information sources (frequency = 43) and national information sources (frequency = 29) as the most ‘useful’ (defined as accessible, understandable, relevant, and/or sufficient) for addressing WaSH questions. Universities (frequency = 11) and news outlets (frequency = 2) were considered useful information sources less often. Respondents regarded partnership networks (frequency = 31), communications departments (frequency = 24), and stakeholders (frequency = 20) as the most important information disseminators. Stakeholders could include any party with an interest or concern in the work, whether or not they participate.

Synthesizing evidence and applying it remain important hurdles in practice. About 13% of respondents had no difficulty obtaining WaSH information, while the rest reported one or more barrier to seeking information and using it to inform decisions ( Figure 2 ). Information was often perceived as conflicting, unreliable, inaccessible, or outdated, especially among the ‘countries’ and ‘all other’ constituencies. This suggests differences in perceptions, approaches, and/or levels of practice at identifying and consolidating reliable information. Interestingly, lengthy or technical information was a less frequent cause of complaint than information that was too brief or general ( Figure 2 ). Still, reference to one’s particular country or situation, expert analysis or critique, and executive summaries or synopses were highly valued communication mechanisms, suggesting that both brief and technical information play a role in knowledge uptake.

Challenges reported in seeking WaSH information by constituency, permitting more than one response category (n = 62)

Constituencies differed regarding learning needs and access to training, wherein 20% reported no challenges ( Figure 3 ). In addition to more or broader funding opportunities, respondents from the country and ‘all other’ constituencies desired additional learning and training opportunities (e.g. discussion fora, training manuals, and courses; frequency = 9). These constituencies concurrently perceived barriers to participating, such as excessive cost or not receiving a notice or invitation ( Figure 3 ). Information synthesis was highly valued within the country constituency; in particular, in-person seminars or lectures were deemed useful by 87% of respondents. The R&L constituency members instead appeared more saturated with learning and training opportunities, where two-thirds were primarily limited by a lack of time ( Figure 3 ).

Barriers to seeking WaSH education or training opportunities by constituency, permitting more than one response category (n = 57)

Funding and Stakeholder Dynamics

Funding and stakeholder relationships revealed accountability imbalances among constituencies. WaSH requests for proposals often had narrow topic specificity ( Figure 4 ), while the R&L and other constituencies perceived lack of funding for undertaking desired WaSH work or activities ( Figure 5 ). When combined with country constituency results (n = 64), top-ranked non-informational barriers to undertaking WaSH activities included: (1) lack of financial resources or funding, (2) lack of technical or human resources, and/or (3) lack of political traction to pursue alternatives. Narrow requests for proposals likely increase research relevance to the funder, and may take into account (either informally or formally) the interests of the broader WaSH professional community.

Perceptions by constituency of the degree of topic specificity among WaSH requests for proposals (n = 26)

Obstacles to undertaking WaSH research or activities by constituency, permitting more than one response category (n = 37)

Partnerships and stakeholder involvement were important to receiving funding ( Figure 6 ). Still, stakeholders (a term broadly inclusive of end users, sponsors, and any affected parties) were not always included in research or other activities, especially at project start-up when the resources, scope, and methods are typically defined ( Table 5 ). These varied roles and inter-organizational dynamics may inadvertently exclude WaSH stakeholders, such as in-country end users, who do not fund or review proposals. One respondent from a CSO cited absent or weak downward accountability and ‘unhealthy competition for resources and visibility’ as challenges in the SDG era. Twelve respondents offered suggestions to help better match WaSH funding to the needs of end users, citing capacity building, opportunities for reflection, funding availability, and partnerships, while two did not perceive any dysfunction in these dynamics.

Reasons reported by respondents from funding organizations (‘all other’ constituencies) for declining to fund WaSH research or activities, permitting more than one response category (n = 20)

Reported stages at which stakeholders are involved in WaSH research or activities, permitting more than one response category (n = 36, R&L and other)

The research agenda ( Figure 1 ; supplementary information ) lists areas of priority, including categorization and relative ranking within the overarching ambition of clean water and sanitation. Importantly, some Goal 6 target areas correspond to weak confidence among surveyed WaSH professionals, suggesting a need for renewed attention to knowledge development and sharing. Managing untreated wastewater and sludge engendered the least confidence, and could be a focal area for R&L efforts. Underlying synergies, such as support for improved financing and equity, would support achievement of multiple targets. A literature review by Hutton and Chase (2016) matched many of the priorities found in this study, recommending increased focus on equity, financing strategies, social welfare consequences of poor WaSH services (especially regarding gender), synergies between WaSH and nutrition, sustainable behaviour change, slum environments, and poverty reduction. In contrast, it presented a more optimistic view toward potential options for untreated wastewater and faecal sludge management.

Economics and finance information needs figured prominently among research priorities, but were not reflected by respondents from, nor potentially membership in, the R&L constituency. A synthesis report backs the perception that current financial resources are inadequate to achieve Goal 6, and recommends increasing efficiency of existing financial resources while mobilizing additional and innovative forms of domestic and international finance ( UN Water, 2018 ). A critical analysis by Bartram, Brocklehurst, Bradley, Muller, and Evans (2018) similarly identified a lack of reference to financing needs in the Means of Implementation (MoI) for Goal 6. Our findings suggest development of a research agenda by researchers alone may not meet needs across all constituencies, and multidisciplinary, multi-stakeholder approaches are desirable to capture holistic requirements ( Bryant et al., 2014 ).

Notably, slightly different pictures were seen when inferring priorities from respondents’ actual recent needs versus broad future projection, which requires greater cognitive processing. Adding a question on future research priorities to the R&L and ‘all other’ versions offered a hypothetical comparison between different potential methods of research agenda construction. Relying solely on the researchers’ and other constituencies’ future priorities submitted via an open question (question 7) would have downplayed the need for evidence on faecal sludge management and open defaecation identified using a structured questionnaire; these topics were mentioned at frequencies of 4% and 2%, respectively. Retrospective question 6, in contrast, showed these were clear areas of concern for national WaSH professionals and others. This example illustrates the subtle differences in forward versus backward planning processes, as well as the benefits of considering diverse perspectives.

Learning and training findings suggested research translation is not a singular bottleneck, as information generation, information synthesis, and communication were all perceived as important needs for achieving Goal 6 targets. The review by Hutton and Chase (2016) called for additional evidence to support Goal 6 implementation, as well as evidence synthesis to support decision-making within specific contexts (e.g. at country or regional level within rural or urban areas). In addition to evidence about efficacy and effectiveness of proposed WaSH solutions (whether something could work at scale), local context (broadly inclusive of the enabling environment, people, and institutions) is an important influence on the eventual outcomes of public health interventions ( May, Johnson, & Finch, 2016 ; Pfadenhauer et al., 2017 ). For example, science–policy–practice gaps may stem from misalignment of institutions, incentives, and resources ( Ménard, Jimenez, & Tropp, 2018 ). In the spirit of quality improvement, improved coordination among actors often requires interactive and iterative problem solving at multiple time points, rather than a single push.

This finding generally agrees with existing literature. Simplistic linear models assuming research will be taken up and used by policy-makers and practitioners within a few years of its publication have been supplanted with a more complex and nuanced understanding ( Cairney, 2016 ; De Goede, van Bon-Martens, Mathijssen, Putters, & Van Oers, 2012 ; Georgalakis, 2016 ; Nutley et al., 2007 ). Such models include multi-way communication, knowledge translation, and mediation, which are best achieved via regular, structured, interpersonal interaction ( Cash et al., 2003 ; Gupta, 2014 ). Reflecting increased complexity in development goals, water governance models must embrace inclusive knowledge sharing, decision-making processes, coordination, and negotiated outcomes ( Tropp, 2007 ).

The mode and source of information also mattered to respondents. Both brief and lengthy formats were considered useful, as were perceived trustworthiness and accessibility of the information source. For example, the peer-reviewed scientific journal publications preferred by academics may effectively reach only fellow researchers, in part due the monetary barriers of paid subscription or fee-based open access models ( Tennant et al., 2016 ). Intermediary knowledge brokers can serve to translate, synthesize, and communicate findings across sectors ( Cash et al., 2003 ). These ‘boundary’ organizations or individuals at the science–policy–practice interface may include multilaterals and funding agencies, who vet the rigour of proposed research ( Figure 6 ) and help to disseminate it.

Distinct from information-related challenges, because availability of financial resources was a primary limiter across all constituencies when making decisions about whether to undertake WaSH activities, some respondents recommended detailing information on costs and potential financing avenues alongside WaSH recommendations. Similarly, a 2013 focus group discussion with SWA finance ministers about WaSH decision-making recommended ongoing, multi-ministry, multi-stakeholder dialogue, as well as modular briefing materials that make a strong case for WaSH as a sound investment and contributor to economic growth ( Brocklehurst, 2013 ). These findings led to development of a WaSH Policy Research Digest brief and webinar series coordinated by the UNC Water Institute.

This research priority-setting and learning challenges survey is one of several efforts to accelerate collective progress toward WaSH development goals. A broad consultative exercise cross-cutting all SDGs produced one research question directly relevant to WaSH: ‘What evidence is there that private sector finance has played a major role in the provision of basic services such as access to water, sanitation or energy, for the poorest quintile in lower-income countries?’ ( Oldekop et al., 2016 ). Specific to water and health, a research prioritization workshop involving students, academics, and practitioners was also held at the 10th International Water & Health Seminar in Cannes, France in June 2018.

The GLAAS process gathers and compares national-level WaSH data to help countries identify priorities and barriers to service provision, helping to promote a culture of accountability, partnership, and shared responsibility ( UN Water & World Health Organization, 2017 ). One key GLAAS finding in 2017 was that while national WaSH budgets are increasing, they are not on par with global aspirations. A secondary review of GLAAS survey data found accountability was more developed for water versus sanitation services, with little data provided on wastewater and faecal sludge management ( Jiménez, Livsey, Åhlén, Scharp, & Takane, 2018 ). To improve accountability, it recommended improved access to information, participatory policies, and increased regulatory capacity, as well as modification of the survey to better capture accountability mechanisms.

Other efforts addressing the use of evidence in decision making include the WASH PaLS programme of the United States Agency for International Development (USAID), funded in 2016 to enhance global learning and adoption of the evidence-based programmatic foundations needed to achieve the SDGs. The TRAction project (Translating Research into Action), also funded by USAID and launched in 2014, recently conducted a survey on Incentives for Engagement in Implementation Research and Delivery Science (IRDS). Australia’s Civil Society Water, Sanitation and Hygiene Fund placed substantial emphasis on global WaSH knowledge and learning from 2013-2018, funding research grants related to programming, webinars, and learning events.

Based on the observed differences among constituencies, we recommend evidence-based mechanisms for determining and vetting research priorities to enhance cost-effectiveness and speed progress toward global development goals. In agreement with this study, others recommend information collection and decision models as a starting point for research agenda construction ( Bryant et al., 2014 ; Doyle, 2005 ; Elder, Bengtsson, & Akenji, 2016 ). A common aspect of these designs involves transparently attracting and capturing the viewpoints of diverse stakeholders. For example, the European Commission has been promoting ‘responsible research and innovation’ since 2014 to ensure societal actors (e.g. researchers, citizens, policymakers, businesses) work together throughout the research process, helping to align processes and outcomes with the values, needs, and expectations of society ( European Commission, 2018 ).

Limitations

Respondents self-selected from SWA’s partnership network, and their views may not represent all SWA members or WaSH professionals worldwide. Low quality or intermittent internet access may have excluded some respondents. Differences among constituencies may have stemmed from the within-constituency sample (e.g. if co-workers from the same unit took the survey) or legitimate differences in topic representation based on limited constituency membership. Survey responses may have been affected by social desirability bias; for example, the rating for partnership networks as a communication outlet could have been affected by perceived expectations of SWA members. This was likely reduced by the anonymous nature of the survey; however, reporting bias was not explored via data triangulation, participant observation, or other means. Follow-up in-depth interviews or focus group discussions using the developed interview guide are recommended to improve the depth of responses and clarify some questions raised by this survey, and could help to target missing perspectives (e.g. from newer SWA partners in Latin America and the Caribbean).

Assessment of the psychometric properties of the questionnaire instrument, such as validity, would require test-retest replication under different scenarios ( Lewis et al., 2015 ). In addition, review of the full results by a second rater would have been beneficial, enabling assessment of inter-rater reliability. Sample size differences and the categorical nature of the data limited quantitative observations (e.g. tests of statistical significance). The basis for research prioritization likewise merged two different rating schemes, which limited its utility for relative comparison; however, pilot testers felt the survey design was too long when all possible options were included under the multiple-choice segment. When interpreting the results, greater emphasis should be placed on presence of the research agenda themes than their specific ordering.

A small degree of misclassification was possible, as some respondents (about 5%) reported current professional affiliations that did not match their expected constituency. This was most prevalent for the country constituency and may have reflected recent job changes. Few government ministers or advisors within the country constituency responded, representing perhaps 20% of country respondents, although high-level managerial and technical personnel were well represented, especially from ministries of water. Two of six respondents from the R&L constituency participated in survey development, but this posed a minimal concern based on the length of time between survey development and deployment.

Recommendations

In general, this study inductively explored research and learning needs rather than testing a pre-existing hypothesis, so the findings serve as a starting point for troubleshooting and future improvement. While SWA has defined ‘building blocks’ and ‘collaborative behaviours’ ( SWA, 2018 ), partners could further develop guidance and model good practices for promoting efficient exchange at the WaSH science–policy–practice interface. A need for mindful external accountability applies to all sectors. Some R&L organizations may be especially vulnerable to weak downward accountability, since publishing scientific literature and obtaining research funding are the primary drivers of academic career development. Commitments to support learning and progressive actions of others may have fewer or more indirect rewards, and publishers may not as readily accept applied research.

Findings point to some areas where partnership networks, including SWA, could assist in coalescing efforts among WaSH researchers, knowledge brokers, decision makers, practitioners, and others. These include:

• Fostering an enabling environment in which WaSH professionals have portals of access to a variety of established reference material and layers of interpersonal support;
• Opening up and promoting interactive seminars or webinars (e.g. offered by R&L institutions or networks such as SuSanA, the Rural Water Supply Network, and the UNC Water Institute) to offer up-to-date expert interpretation and information synthesis to peer scientists, practitioners, and stakeholders, potentially with messages tailored to different audiences;
• Helping WaSH professionals to connect more easily with others in their extended professional network, for example via personal referral or access to specific listservs of experts, especially when issues or questions cannot be immediately addressed by more proximate resources, or when preparing project proposals that require partnerships;
• Developing or recommending accountability mechanisms (e.g. grant criteria requesting evidence of past stakeholder satisfaction) that tie project follow-up, downward accountability, and applied (demand-driven) research to enhanced opportunities for future funding and publication;
• Promoting stakeholder involvement (or conscientious exclusion if warranted) in a consistent manner throughout all WaSH research or implementation projects and stages (e.g. via guidance on best practices);
• Facilitating communication and dissemination pathways for individual research or educational activities, especially from R&L institutions in the global South; and
• Assisting the country constituency, in particular, with increased opportunity for interpersonal interaction among their peers, especially to debrief and discuss how to implement new information or guidance.

R&L actions in progress include additional recruiting (especially of R&L institutions in the global South), establishing country-level focal points to improve within-country research engagement, and garnering additional external facilitation support as well as representation on the SWA steering committee. Such facilitation can stimulate active rather than passive networking. At a local level, researchers are likely able to individually discern whether answers to a given question are (1) currently missing but feasible to obtain, (2) available but lacking synthesis or communication, or (3) intractable. For example, R&L members could ground-truth the research agenda in their respective locations to construct dialogue about what evidence may or may not be needed in a given country or regional context ( Wickremasinghe et al., 2016 ). A secondary exercise at a global level to map evidence gaps within the WaSH literature (e.g. Rehfuess et al., 2016 ) and compare these to the suggested research agenda could help to identify which of these categories apply to each priority area on a larger scale or across different regions.

Conclusions

This study developed a high-priority WaSH research agenda based on broadly inclusive professional insight into learning needs during the period of SDG initiation, and characterized the status of research and learning dynamics among multi-sector WaSH professionals. Among targets of Goal 6, managing untreated wastewater and faecal sludge emerged as a top priority for knowledge generation and capacity building, forming a focal component of the research agenda. Several learning and training challenges became apparent, including difficulty interpreting conflicting sources of information and perceived exclusion of non-R&L professionals from educational or training opportunities. Based on learning preferences, packaging WaSH information in multiple formats (e.g. both brief and detailed information with in-person interpretation) is recommended, as well as providing follow-up opportunities for peer interaction, debriefing, and troubleshooting. Findings showed consistent evidence of upward accountability to organizations that offer WaSH research or project funding, alongside inconsistent evidence of downward accountability to all stakeholders. Funding and financing were widespread determinants of WaSH activities, recommending broad integration of these topics into research and development efforts. All WaSH professionals, institutions, and networks should reflect on how they could best contribute to a culture of learning that would help achieve progress towards Goal 6.

Supplementary Material

Acknowledgements.

This research was conducted in collaboration with Sanitation and Water for All (SWA), with special thanks to Clarissa Brocklehurst, Amanda Marlin, Alexandra Reis, and Sophie Thievenaz, as well as a volunteer working group from the SWA Research and Learning Constituency including: Sarah Dickin, Sara Marks, Patrick Moriarty, Eddy Perez, Erma Uytewaal, and Vidya Venkataramanan. We thank Osborn Kwena, Nur Aisyah Nasution, Jordan Dalton, and Teresa Edwards for their contributions to survey review. Funding for the study was generously provided by the University of North Carolina at Chapel Hill (UNC) Royster Society of Fellows and Center for European Studies, as well as the US National Institute of Environmental Health Sciences (grant T32ES007018).

A1. Full Questionnaire Transcript – SWA Country Partners

(Questions did not appear numbered or lettered. Numbering and lettering is shown for reference only.)

Please select a language and click below to proceed to the questionnaire.

What? This questionnaire identifies water, sanitation, and hygiene (WasH) research priorities for achieving Sustainable Development Goal 6, and related communication preferences.

How? It has six sections and should take about 20 minutes to complete. Please answer questions based on your expertise, and leave blank any questions you do not feel comfortable answering. Responses are confidential and no personal information will be included in summary reports.

Who? The survey is being conducted by the Water Institute at The University of North Carolina, Chapel Hill (UNC) and the Research & Learning constituency of Sanitation and Water for All (SWA). If you have any questions or concerns, please contact Karen Setty ( ude.cnu.evil@yttesk ).

SECTION 1: Introduction and Targets

Background (optional):.

Sustainable Development Goal 6 aims to “ensure availability and sustainable management of water and sanitation for all.” Unlike previous global goals, it:

• Seeks access for all people to improved water sources and sanitation, regardless of wealth, geography, gender, social class, age, and disability.
• Considers safety and security to be an important part of water and sanitation service provision.
• Encourages both international cooperation and local community participation to help build capacity for domestic water and sanitation management.

Q1 How confident are you in your knowledge/ability to work in each of the following target areas of Goal 6? (select one category for each row)

Q2 Do you have any specific knowledge gaps or areas of concern related to achieving Goal 6? (please describe)

SECTION 2: Information for Decision-Making

Q3 When you made WaSH-related decisions over the past six months, in which of these areas (related to governance and human resources) would more information have been helpful? (choose all that apply)

• ◻ Accountability
• ◻ Human resources
• ◻ Institutional change
• ◻ Participatory approaches
• ◻ Performance review
• ◻ Sector coordination/collaboration
• ◻ Strategic planning/prioritization
• ◻ Other (please describe) ____________________

Q4 When you made WaSH-related decisions over the past six months, in which of these areas (related to finance and information systems) would more information have been helpful? (choose all that apply)

• ◻ Affordability (e.g., subsidies, tariffs)
• ◻ Budgeting and costing
• ◻ Cost-benefit analysis
• ◻ Donor management
• ◻ Investment planning
• ◻ Market finance (e.g., capital markets)
• ◻ Monitoring and evaluation
• ◻ Public finance

Q5 When you made WaSH-related decisions over the past six months, in which of these technical areas would more information have been helpful? (choose all that apply)

• ◻ Appropriate technologies
• ◻ Behaviour change
• ◻ Children’s faeces
• ◻ Climate change
• ◻ Community-led total sanitation
• ◻ Disabled access
• ◻ Eliminating open defecation
• ◻ Emergencies and/or outbreaks
• ◻ Faecal sludge
• ◻ Food hygiene
• ◻ Handwashing
• ◻ Household water treatment
• ◻ Improved service levels/”service ladders”
• ◻ Cross-cultural approaches
• ◻ Marketing for sanitation
• ◻ Menstrual hygiene
• ◻ Reaching poorest populations
• ◻ Reliability of service
• ◻ Security for girls and women
• ◻ Temporary/emergency services
• ◻ Utilities in small towns
• ◻ WaSH impact on stunting/nutrition
• ◻ WaSH in health care facilities
• ◻ WaSH in rapidly growing cities
• ◻ WaSH in schools

Q6 What was the primary topic of the WaSH-related decision (or decisions) you made over the past six months? (please describe)

SECTION 3: Limitations to Decision-Making

Q7 Which challenges did you experience when seeking information to make WaSH-related decision/s over the past six months? (choose all that apply)

• ◻ Could not find/access information
• ◻ Different sources of information conflicted
• ◻ Information was not available for my region/situation
• ◻ Information was not trustworthy/reliable
• ◻ Information was outdated
• ◻ Information was too brief or general
• ◻ Information was too lengthy or technical
• ◻ None

Q8 Which other challenges did you experience when making WaSH-related decision/s over the past six months? (choose all that apply)

• ◻ Lacked adequate financial resources to consider alternative(s)
• ◻ Lacked cultural acceptance of alternative(s)
• ◻ Lacked political traction for alternative(s)
• ◻ Lacked technological alternative(s)
• ◻ Lacked time/capacity to evaluate alternative(s)

Q9 Based on these limitations, what knowledge or information might have helped with your decision/s? (please describe)

SECTION 4: Approaches to Gathering Information

Q10 Which organizations typically offer useful (e.g., accessible, understandable, relevant, and/or sufficient) information for addressing your WaSH-related questions? (choose all that apply)

• ◻ Global monitoring organizations (e.g. JMP, GLAAS)
• ◻ International civil society (non-governmental) organizations
• ◻ Local civil society or community organizations
• ◻ Multilateral organizations (e.g., World Bank, WHO, UNICEF)
• ◻ National monitoring agencies
• ◻ News outlets
• ◻ Other government ministries or departments
• ◻ Partnership networks (e.g., SWA)
• ◻ Private companies/consultants
• ◻ Universities (foreign)
• ◻ Universities (local)

Q11 Which of these actions are typically useful for addressing your WaSH-related questions? (choose all that apply)

• ◻ Ask a colleague/advisor in my office
• ◻ Call someone in my professional network
• ◻ Email a group of people (e.g., a listserv)
• ◻ Email someone in my professional network
• ◻ Initiate a new study/survey
• ◻ Organize a meeting or conference call
• ◻ Search the Internet

Q12 Which of these informational formats are typically useful for addressing your WaSH-related questions? (choose all that apply)

• ◻ Book/report
• ◻ Memorandum, bulletin, or flyer
• ◻ News (e.g., television, radio, newspaper)
• ◻ Online course/training module
• ◻ Online discussion forum
• ◻ Scientific or professional journal article
• ◻ Seminar/lecture
• ◻ Social media post (e.g., LinkedIn, Twitter)
• ◻ Tool or worksheet
• ◻ Webinar (virtual seminar/lecture)
• ◻ Website

Q13 What would make the WaSH-related information you accessed over the past six months more useful? (choose all that apply)

• ◻ An introduction (e.g., written, video)
• ◻ Discussion with my colleagues
• ◻ Email, mail, or social media alerts
• ◻ Executive summary/synopsis
• ◻ Expert analysis/critique
• ◻ Reference to my country/situation
• ◻ Translation for a non-specialist audience
• ◻ Translation into another language

Q14 What new resources, if any, would you like to have available for addressing your WaSH-related questions? (please describe)

SECTION 5: Learning and Training Needs

Q15 Which challenges did you experience when seeking WaSH-related training or educational opportunities over the past six months? (choose all that apply)

• ◻ Did not receive notice/invitation
• ◻ None offered
• ◻ Not relevant to my region/situation
• ◻ Too busy to participate
• ◻ Too expensive

Q16 How willing would you be to interact with WaSH researchers (e.g., to help plan studies and share new information)? (select one)

• ◻ Very willing
• ◻ Somewhat willing
• ◻ Somewhat unwilling
• ◻ Extremely unwilling

Q17 Other Comments

Do you have any other advice or comments? (if so, please describe)

SECTION 6: Your Professional Background

Q18 What is your educational specialization? (can select more than one)

• ◻ Business/Economics/Finance
• ◻ Engineering
• ◻ Humanities (e.g., Languages, Geography)
• ◻ Journalism
• ◻ Political Science
• ◻ Medicine/Public Health
• ◻ Natural Science/Mathematics
• ◻ Sociology/Anthropology

Q19 Which category best describes your current workplace? (select one)

• ○ Ministry of Finance
• ○ Ministry of Health
• ○ Ministry of Water
• ○ Other (please describe) ____________________

Q20 Which category best describes your current professional responsibilities? (select one)

• ○ Minister
• ○ Adviser in minister’s office
• ○ Director/manager
• ○ Technical staff

Thank you for completing this questionnaire!

Your feedback is important to helping us understand evidence needs to achieve Goal 6, ensuring availability and sustainable management of water and sanitation for all.

The following steps are optional. When ready, please click below to submit your responses.

Q21 May we contact you for a short follow-up interview (about 30 minutes)? If so, please enter your first name and preferred contact method. (Note: Information will be kept confidential.)

• E-mail address

Q22 Would you like to nominate a WaSH-related research and learning institution/s in your country to join the Sanitation and Water for All partnership? (Note: Information will only be used by SWA to reach out to potential new partners.)

• Institution
• Name of contact (if available)
• Contact information (if available)

A2. Full Questionnaire Transcript – SWA Research and Learning Partners

Please click below to proceed to the questionnaire.

How? It has five sections and should take about 15 minutes to complete. Please answer questions based on your expertise, and leave blank any questions you do not feel comfortable answering. Responses are confidential and no personal information will be included in summary reports.

Q2 Do you have any specific knowledge gaps or areas of concern related to Goal 6? (please describe)

SECTION 2: Recent and Future WaSH Research Needs

Q3 In which of these areas (related to governance and human resources) would more information have been helpful to your work over the past six months? (choose all that apply)

Q4 In which of these areas (related to finance and information systems) would more information have been helpful to your work over the past six months? (choose all that apply)

Q5 In which of these technical areas would more information have been helpful to your work over the past six months? (choose all that apply)

Q6 On what primary topics did you seek WaSH-related information over the past six months? (please describe)

Q7 From your perspective, what will be the most pressing WaSH-related research needs in coming years? (please describe)

SECTION 3: Communication and Interaction

Q8 Which organizations typically sponsor your research or educational activities? (choose all that apply)

• ◻ Companies or corporations
• ◻ Foundations or aid organizations
• ◻ Government agencies (domestic/federal)
• ◻ Government agencies (domestic/state or regional)
• ◻ Government agencies (foreign)
• ◻ Private donors/individuals

Q9 If you submit proposals for funding, are the requests for proposals typically open-ended or topic-specific? (choose one)

• ◻ Very open-ended
• ◻ Somewhat open-ended
• ◻ Somewhat topic-specific
• ◻ Very topic-specific
• ◻ Not applicable

Q10 Who typically disseminates the outcomes of your research or educational activities? (choose all that apply)

• ◻ Donors
• ◻ Independent media
• ◻ Internal communications department
• ◻ Partnership networks
• ◻ Scientific community
• ◻ Stakeholders

Q11 How often are stakeholders involved in your research or educational activities? (choose one)

• ○ >90% of the time
• ○ 70-90% of the time
• ○ 30-70% of the time
• ○ 10-30% of the time
• ○ <10% of the time
• ○ Unsure

Q12 At which stages are stakeholders typically involved in your research or educational activities? (choose all that apply)

• ◻ Scoping
• ◻ Design
• ◻ Implementation
• ◻ Analysis/Interpretation
• ◻ Dissemination

Q13 What would help better match your research and educational activities to the needs of end users? (please describe)

SECTION 4: Research and Learning Challenges

Q14 Which challenges did you experience when seeking WaSH-related information over the past six months? (choose all that apply)

• ◻ Other (please describe)) ____________________

Q16 What obstacles might affect your ability to undertake research or educational activities? (choose all that apply)

• ◻ Broader political climate
• ◻ Lack of funding
• ◻ Lack of interest among higher-ups
• ◻ Lack of partnership opportunities
• ◻ Lack of technical or human resources
• ◻ Lack of stakeholder buy-in

Q17 What new resources, if any, would help to specifically address these needs? (please describe)

Q18 Other Comments

SECTION 5: Your Professional Background

Q19 What is your educational specialization? (can select more than one)

Q20 What is the scale or scope of your current workplace? (select one)

• ○ Local/regional
• ○ National
• ○ International

Q21 Which category best describes your current professional responsibilities? (select one)

• ○ Director/administrator
• ○ Project manager
• ○ Research/technical staff

Q22 Did you participate in the Research and Learning survey working group? (select one)

• ○ Yes
• ○ No

Q23 May we contact you for a short follow-up interview (about 30 minutes)? If so, please enter your first name and preferred contact method. (Note: Information will be kept confidential.)

• Skype™

Q24 Would you like to nominate a WaSH-related research and learning institution/s to join the Sanitation and Water for All partnership? (Note: Information will only be used by SWA to reach out to potential new partners.)

A3. Full Questionnaire Transcript – All Other SWA Partners

Section 2: wash information needs.

Q7 From your perspective, what will be the most pressing WaSH-related evidence needs over the next several years? (please describe)

SECTION 3: Information Gathering Challenges

Q8 What obstacles might affect your ability to undertake WaSH-related activities? (choose all that apply)

Q9 Which challenges did you experience when seeking WaSH-related information over the past six months? (choose all that apply)

Q10 What would make the WaSH-related information you accessed over the past six months more useful? (choose all that apply)

Q11 Which challenges did you experience when seeking WaSH-related training or educational opportunities over the past six months? (choose all that apply)

Q12 What new informational or training resources, if any, would you like to have available for addressing WaSH-related questions? (please describe)

SECTION 4: Funding Activities

Q13 If your organization funds WaSH research or activities, which types of organizations do you typically sponsor? (choose all that apply)

• ◻ Civil society organizations
• ◻ Community-based organizations
• ◻ Government agencies (federal)
• ◻ Government agencies (state or regional)
• ◻ Private companies or corporations
• ◻ Universities

Q14 If your organization funds WaSH research or activities, are the requests for proposals typically open-ended or topic-specific? (choose one)

• ○ Very open-ended
• ○ Somewhat open-ended
• ○ Somewhat topic-specific
• ○ Very topic-specific
• ○ Not applicable

Q15 If your organization funds WaSH research or activities, why might you decline to fund a certain activity? (choose all that apply)

• ◻ Doesn’t match interests of donors/constituents
• ◻ Doesn’t match organizational directions
• ◻ Lack of plans for partnership/stakeholder involvement
• ◻ Lack of knowledge/trust in applicant’s organization
• ◻ Lack of scientific rigor or pre-proposal planning

Q16 What would help better match your organization’s WaSH-related work to existing needs? (please describe)

SECTION 5: Communication and Interactions

Q17 How often are stakeholders involved in your WaSH-related work? (choose one)

Q18 At which stages are stakeholders typically involved in your WaSH-related work? (choose all that apply)

Q19 Who typically disseminates the outcomes of your WaSH-related work? (choose all that apply)

Q20 How often do you interact with WaSH researchers (e.g., to help plan studies and share new information)? (select one)

• ○ Once a week
• ○ Once a month
• ○ Once every few months
• ○ Once a year

Q21 Other Comments

Q22 What is your educational specialization? (can select more than one)

Q23 Which category best describes your current workplace? (select one)

• ○ Civil society organization (or network)
• ○ Community-based organization (or network)
• ○ External support or funding agency
• ○ Private sector organization (or network)

Q24 Which category best describes your current professional responsibilities? (select one)

Q25 May we contact you for a short follow-up interview (about 30 minutes)? If so, please enter your first name and preferred contact method. (Note: Information will be kept confidential.)

Q26 Would you like to nominate a WaSH-related research and learning institution/s to join the Sanitation and Water for All partnership? (Note: Information will only be used by SWA to reach out to potential new partners.)

A4. Research Agenda

• Which approaches to safe wastewater faecal sludge disposal or geographical/population priorities will have the greatest impact on reducing faecal pollution and disease transmission by 2030?
• Can changes in global raw sewage or faecal discharge be quantified over time? What are the key drivers of this change?
• What cost-recovery mechanisms and demonstrated business models (including cost scenarios) are available to cities or communities interested in tackling untreated wastewater or faecal sludge discharges? (Q7)
• What payment mechanisms or payment options engender the greatest buy-in for new sanitation services? How can payment for sanitation services best be stabilized over time?
• Are adequate decision support tools in place to determine the best wastewater treatment scheme for a given location, whether traditional or unconventional? What are the key decision criteria? Have such solutions been reliably costed?
• How can nutrients in wastewater/faecal sludge safely be redistributed and reused for crop production? (Q6)
• Can risk management approaches such as sanitation safety planning help control environmental impacts from accidental sewage/sewerage release?
• Which marketing or behaviour change approaches have been the most successful and why might they succeed or fail in a different context? (Q7)
• What is the nature and magnitude of the links between WaSH improvements and community health status? (Q6, Q7)
• What are the public health and economic benefits of menstrual hygiene management? (Q7) How can stigmas about menstrual hygiene management be tackled?
• Which individuals/populations experience a disproportional burden from negative outcomes linked to WaSH-related stunting? (Q7) Can better prevention and treatment mechanisms be developed to reduce these impacts?
• Can disaggregated data be generated to ensure equality, non-discrimination, and targeting of services? (Q7)
• Are government accountability measures for WaSH achievements working? Are any countries falling through the cracks?
• What bottlenecks prevent actors from putting collaborative behaviours into practice? (Q7)
• What options are available for increasing access to improved public sanitation facilities (including menstrual hygiene management) in heavily populated areas versus more remote, rural areas? (Q7)
• What conventional or unconventional WaSH options are available for serving remote populations? (Q7) Have any been proven more successful than others? What decision criteria are recommended?
• How can WaSH targets be integrated with programming on other targets, such as food and energy security? (Q7)
• Are emergency management or response plans in place to address recent or future WaSH-related disease epidemics? (Q6)
• How should WaSH services be financed in the case of extremely poor, marginalized, or transient peoples?
• Given economic, political, and climate uncertainty, what financing options or portfolio of options are most resilient? How can dips or lags in financing best be weathered?
• What benefits can be gained from proactive risk management approaches such as water safety planning? (Q6)
• Where can we find successful case studies of scaling up high quality WaSH services in an equitable manner? How were these services delivered? (Q7)
• How might demographic trends hinder the ability to maintain and improve WaSH service levels? (Q7)
• What is the long-term cost-effectiveness and health impact of communal water points versus in-home piped access? (Q7)
• Do all new WaSH services consider ecological sustainability, including different scenarios of climate change? If not, why not? (Q7)
• How can existing WaSH facilities and water/wastewater treatment processes be retrofit to enhance efficiency and reduce environmental impacts?
• How can communities include diverse citizens in WaSH decisions, especially young people and women? (Q7) What are the benefits of diversity and inclusion?
• What behaviour change mechanisms work across diverse slum environments? (Q7)
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Ranchers reported abandoned oil wells spewing wastewater. A new study blames fracking.

An SMU study is the first scientific proof of a phenomenon local landowners have long warned was occurring.

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Fracking wastewater, injected underground for permanent disposal, traveled 12 miles through geological faults before bursting to the surface through a previously plugged West Texas oil well in 2022, according to a new study from Southern Methodist University.

It’s the first study to draw specific links between wastewater injection and recent blowouts in the Permian Basin, the nation’s top producing oil field, where old oil wells have lately begun to spray salty water.

It raises concerns about the possibility of widespread groundwater contamination in West Texas and increases the urgency for oil producers to find alternative outlets for the millions of gallons of toxic wastewater that come from Permian Basin oil wells every day.

“We established a significant link between wastewater injection and oil well blowouts in the Permian Basin,” wrote the authors of the study , funded in part by NASA and published last month in the journal Geophysical Research Letters. The finding suggests "a potential for more blowouts in the near future,” it said.

For years, the Texas agency that regulates the oil and gas extraction industry has refrained from putting forth an explanation for the blowout phenomenon, even as a chorus of local landowners alleged that wastewater injections were driving the flows of gassy brine onto the surface of their properties since about 2022.

Injection disposal is currently the primary outlet for the tremendous amount of oilfield wastewater, also known as produced water , that flows from fracked oil wells in West Texas. Thousands of injection wells dot the Permian Basin, each reviewed and permitted by Texas’ oilfield regulator, the Texas Railroad Commission.

Oil producers are exploring alternatives — a small portion of produced water is reused in fracking , and Texas is in the process of permitting facilities that will treat produced water and release it into rivers and streams. Still, underground injection remains the cheapest and most popular method by far.

A scientific connection has solidified between the practice of injection disposal and the increasing strength and frequency of earthquakes nearby. In the Permian Basin, a steady crescendo of tremors peaked last November with magnitude 5.4 earthquake, the state’s strongest in 30 years, triggering heightened restrictions on injections in the area.

The link between injections and surface blowouts, however, has remained unconfirmed, despite widespread suspicions . The latest study marks a big step forward in scientific documentation.

“It just validates what we’ve been saying,” Sarah Stogner, an oil and gas attorney who ran an unsuccessful campaign for a seat on the Railroad Commission in 2022, said about the latest study.

For the last three years, Stogner has represented the Antina Cattle Ranch, where dozens of abandoned oil wells have been spraying back to life. Stogner persistently alleged that nearby wastewater injection was responsible. But she couldn’t prove it.

Now a scientific consensus is beginning to fall in behind her.

“Our work independently comes to this same conclusion in different areas [of the Permian Basin],” said Katie Smye, a geologist with the Center for Injection and Seismicity Research at the University of Texas at Austin, citing several upcoming papers she and her colleagues will release at major geoscience conferences in the coming year. “There is a link between injection and surface flows in some cases.”

In a study published December 2023, Smye and others reported “linear surface deformation features” in parts of the Permian Basin — the ground was swelling along channels that suggested pressure moving through underground faults. Some of those were ancient geological faults, Smye said; others appeared to be created by recent human activity. Many of them were growing, heaving and bulging, the research showed.

When that channel of underground pressure hits an old oil well that is broken or improperly plugged, it can shoot to the surface.

“This is reaching a critical point in the Permian Basin,” Smye said. “The scale of injection needs is increasing.”

About 15 million barrels, or 630 million gallons, of produced water are injected for disposal in the Permian Basin every day, Smye said.

A Railroad Commission spokesperson, Patty Ramon, said in a statement the agency is “talking to operators in the Crane County area regarding geology and other data they maintain, reviewing satellite imagery, and analyzing RRC records such as well plugging information.

“We will be continuing this type of analysis in our commitment to ensuring environmental protection,” Ramon said.

Blowout in 2022 sparks study

The SMU study examined a January 2022 blowout in Crane County that gushed almost 15 million gallons of brine before it was capped, according to the paper. That would fill about 23 Olympic-sized swimming pools.

The study traced the cause of the blowout to a cluster of nine injection wells about 12 miles to the northeast. Researchers pulled publicly available data on injection volumes at those wells and found they lined up closely to surface swelling that preceded the blowout. Seven of the wells belong to Goodnight Midstream and two belong to Blackbeard Operating, according to Railroad Commission records.

A spokesperson for Blackbeard said the company “is committed to ensuring prudent operations” and “will continue to operate its assets in accordance with all applicable laws and in coordination with all applicable regulatory agencies.”

A spokesperson for Goodnight said the company is dedicated to operating its facilities responsibly and in full compliance with legal requirements.

"We work proactively with regulatory agencies and industry workgroups to ensure our operations are at the forefront of geologically sustainable solutions," the spokesperson said. "Our commitment to safety, environmental stewardship, and community engagement is the foundation for everything we do."

According to the paper, injection at those nine wells began in 2018 at a rate of about 362,000 gallons per day and doubled to 720,000 gallons per day in late 2019. In late 2020 it doubled again to 1.5 million gallons — two Olympic-sized swimming pools crammed underground everyday — which is when the ground near the blowout site began to inflate.

The study found that the volume injected matched the volume of the surface bulge 12 miles away.

“These observations suggest that this group of injection wells to the NW of the study area, injecting into the San Andres and Glorieta formations, is responsible for the surface deformation in the region,” the study said.

Those wells reached a depth between 4,300 and 3,300 feet. But the SMU study found that the source of the bulge in the earth was much shallower, between 2,300 and 1,600 feet underground.

“This suggests the leakage of wastewater from the San Andres or Glorieta formations to the shallow formations,” the study said.

The bottom of the Rustler Aquifer, the lowest usable source of groundwater in the Permian, sits between 800 and 1,000 feet underground. The SMU study did not examine the possibility of groundwater contamination.

“Our findings highlight the need for stricter regulations on wastewater injection practices and proper management of abandoned wells,” the study said.

Todd Staples, president of the Texas Oil and Gas Association, said the Railroad Commission “is taking appropriate action by thoroughly gathering and reviewing data to address the issues experienced in Crane County.”

He said the industry cooperates with the Railroad Commission by providing data to help analyze geological formations. “In addition, the industry and academia continue to explore alternatives to wastewater injection through market-based water reuse and recycling as well as innovative pilot programs,” Staples said.

Ranchers report damaged land

West Texas ranchers who own land where contaminated water is seeping from underground are beginning to worry it will soon become uninhabitable .

Last February, saltwater flooded parts of Bill Wight’s ranch, about 50 miles southwest of Odessa. The lifelong rancher purchased the land in 2012, hoping to pass it on to his kids. He told The Texas Tribune he wasn’t sure how much of the ranch would survive the leaking wells.

When it was clear the flow of water threatened the property last December, he asked the Railroad Commission to seal the well the water had leaked from. It took the commission months and millions of dollars to plug the well.

His brother, Schuyler Wight, faces a similar predicament at his ranch roughly 60 miles to the west in Pecos County. He has asked the Railroad Commission for years to investigate the multiple abandoned leaking wells on his property. The liquid has eroded the equipment on the surface and killed the plants. After the water dried up, the ground was crusted white from salt.

“It’s what we’ve known all along,” Schuyler Wight said. “What we’re doing is not sustainable.”

Ashley Watt, owner of a ranch 50 miles east of Schuyler Wight’s ranch in Crane County, told the Texas Railroad Commission during a 2022 meeting that she believed excessive injection by nearby oil producers was causing the fluids to spray from abandoned oil wells on her property.

A Railroad Commission staff member said the agency asked operators to check for a source of the leak. The operators told the commission they did not find any. The Railroad Commission during the meeting also said they did not find a well in the agency’s database, and that the nearest injection wells were less than two miles away.

The agency instructed staff to prevent truckers from accessing those injection sites, telling operators to find others “until further notice.”

The wells continue to leak.

Laura Briggs, who also owns a ranch in Pecos County less than half a mile east of Schuyler Wight’s place, said she has seen five old wells start leaking water since 2015. The Railroad Commission plugged two of them, she said, but one began to leak through the seal again.

Briggs has repeatedly given testimony and submitted documentation to the Railroad Commission asking for help. Based on her experience, she believes the subterranean problems in West Texas are much more than what the Railroad Commission can handle.

“If I could do one thing differently, we would have gotten a mobile home so it was easier to get the hell out of here,” Briggs said. “If this [ranch] goes leaking, we just have to leave and nobody will buy the property, no insurance will cover it, you’re just done.”

Despite those problems, the Railroad Commission approved 400 new disposal wells in the Permian Basin alone in 2021 , according to agency documents, and 480 in 2022 .

Threats to groundwater

The use of injection wells for disposal has expanded immensely with the practice of fracking, according to Dominic DiGiulio, a geoscientist who worked for 30 years at the U.S. Environmental Protection Agency. But DiGiulio said these wells are still regulated under rules from the 1970s and ’80s. Increasingly, he said, those rules appear insufficient.

“West Texas isn’t the only place where this is happening,” DiGiulio said. “Overpressurization of aquifers due to disposal of produced water is a problem.”

In 2022, DiGiulio conducted a review of Ohio’s wastewater injection program for the group Physicians, Scientists, and Engineers for Healthy Energy and found the same two problems there: Injected fluids were leaking from some formations meant to contain them, and excessive injections were causing other formations to become overpressurized.

There was one big difference with Texas. In November 2021, DiGiulio’s study said, Ohio had just 228 injection wells for wastewater disposal. Texas, meanwhile, had 13,585 in 2022, according to Railroad Commission documents .

The primary threat posed by produced water migrating from injection wells is groundwater contamination. If deep formations fail to contain the toxic waste injected into them, that waste could end up in shallow freshwater aquifers.

It could happen two ways, DiGiulio said. If the wastewater enters the inside of an old oil well through corroded holes in the casing, it can travel up the steel pipe to the surface, spilling and seeping into the ground. If the wastewater moves up the outside of an old oil well, through the cement that surrounds the steel pipe, it could already be flowing into the aquifer.

That would be bad news for West Texas, which depends almost entirely on groundwater for drinking and crop irrigation.

“Once groundwater contamination happens, it’s too expensive to remediate,” DiGiulio said. “So when it occurs, that’s basically it. You’ve ruined that resource.”

Disclosure: Southern Methodist University and the University of Texas at Austin have been financial supporters of The Texas Tribune, a nonprofit, nonpartisan news organization that is funded in part by donations from members, foundations and corporate sponsors. Financial supporters play no role in the Tribune's journalism. Find a complete list of them here .

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Using Wastewater as a Fertiliser to Increase Crop Production

Food security is a global concern. Experts estimate that farmers will need to produce more food in the next 35 years than ever before in human history. This is because the global population has increased significantly. According to the United Nations (UN), it is three times larger than it was in the mid-twentieth century. Medical advancements have led to a decline in mortality and an increase in longevity, which has resulted in the increase. Climate change is also affecting food production in some countries. These changes affect crop growth differently depending on the location. For example, some regions experience ideal growing conditions that propagate crop growth. However, some regions experience extreme weather conditions such as floods, droughts, and extreme temperatures, which can cause crop loss.

Extreme weather can destroy huge crop yields, resulting in shortages of certain foods in some locations. In 2010 and 2012 , warm evening temperatures affected corn yields across the U.S. Corn Belt. Additionally, premature budding due to a mild winter caused $220 million in losses of Michigan cherries in 2012.

The increased demand for food can also contribute to climate change. As crop losses increase, the need for food imports also increases. This elevates the food miles on some produce as well as the global carbon footprint. Furthermore, it may also lead to the elevated use of nitrogen-based fertilisers to propagate crop growth, polluting the atmosphere.

Additionally, crop losses or the destruction of farmland caused by extreme weather can result in the expansion of farmland by cutting down forests and destroying habitats and ecosystems.

Recently, researchers from the U.S. published an article in the Open Access journal Agriculture exploring urban agriculture methods, more specifically the use of aquaponics waste water as a growth medium for lettuce in hydroponic systems.

What is urban agriculture?

The United Nations (UN) defines urban agriculture as:

“Practices that yield food and other outputs through agricultural production and related processes (transformation, distribution, marketing, recycling), taking place on land and other spaces within cities and surrounding regions.”

Urban agriculture has great potential as it allows localised crops to be produced in densely populated areas year-round. However, it does have a variety of challenges to overcome, including high capital requirements, especially for vertical farming and controlled-environment agriculture, and being energy intensive due to the requirement of artificial lighting and fossil fuel-based synthetic fertilisers.

Urban agriculture can take many forms, from creating gardens in built-up areas via community, rooftop, or back gardens to vertical farms and hydroponics. Vertical farms are ideal for built-up areas as they are designed so that crops can grow on top of each other. Moreover, they can grow inside tray systems or towers, which utilise the best of a small space.

Hydroponics for sustainable crop growth

The U.S. Department of Agriculture (USDA) defines hydroponics as:

“Hydroponics is the technique of growing plants using a water-based nutrient solution rather than soil and can include an aggregate substrate or growing media, such as vermiculite, coconut coir, or perlite. Hydroponic production systems are used by small farmers, hobbyists, and commercial enterprises.”

Plant researchers have used this technique for centuries to study plant physiology. Research suggests that the technique could date back to the Hanging Gardens of Babylon, built in 600 BC. It’s a sustainable solution to propagating crop growth as it uses less water and requires less land. In addition, it reduces the need for pesticides and herbicides, reducing pollution into the atmosphere. Furthermore, it reduces water pollution and waste by decreasing runoff and nutrient leaching.

Aquaponics for sustainable crop growth

Aquaponics is a process that can be traced back to indigenous communities across the globe.

It is a sustainable, highly engineered water-based agriculture system that combines rearing fish in tanks and hydroponics. The water used to rear the fish is rich with nutrients, so it is used as a natural fertiliser for the plants. In return, the plants purify the water for the fish.

Using wastewater to propagate crop growth in a hydroponic system

The study mentioned about investigated the prospects of developing a hydroponic system that uses wastewater to propagate the growth of buttercrunch lettuce ( Lactuca sativa L.). Three different sources generated the wastewater: Chicago High School for Agricultural Sciences (CHSAS), Bevier Café, and the University of Illinois (UIUC) hydrothermal liquefaction (HTL) plant.

The researchers used aquaponic effluents from CHSAS and Bevier Café. The UIUC HTL plant processed the third wastewater source.

HTL processes wet biomass into sustainable fuel sources in the form of biocrude oil. The wet biomass is converted thermochemically via a hot pressurised water environment, breaking the solid biopolymeric structure into liquid components and producing biocrude oil. This process also results in by-products, including the aqueous phase, or Hydrothermal Liquefaction Aqueous Phase (HTL-AP), which has potential for use in crop production systems.

Researchers suggest that HTL-AP has potential for crop production because it has been thermally treated to eliminate pathogens and bacteria via HTL while retaining essential plant nutrients

“We’ve previously shown that it’s possible to grow lettuce hydroponically using treated wastewater; however, it doesn’t grow as quickly and effectively as it could. There are likely to be some toxic compounds inhibiting plant growth, and there are also not enough nutrients in a plant-available form,” said Professor Paul Davidson

The researchers sealed buttercrunch lettuce seeds in Ziplock bags along with paper towels saturated with the aquaponic effluent samples from each source as well as the HTL-AP and two controls. The study aimed to investigate if wastewater could propagate the germination of the lettuce seeds.

“We wanted to see if the naturally occurring microbes from fish waste in aquaponic systems could help convert the nutrients in HTL-AP into forms that plants can absorb. We focused on using wastewater for lettuce seed germination. Eventually, we’ll observe different stages of crop growth, including full-grown lettuce and other crops,” said Liam Reynolds lead author on the paper.

The study identified that the CHSAS aquaponic effluent performed better than Bevier Café. CHSAS has a bigger aquaponic system generating effluents with higher concentrations of nitrate and ammonia. The researchers also identified that the seed germination was not inhibited by solutions containing 2‒8% HTL-AP. They noted that these solutions performed similarly to one of the controls, standard hydroponic fertiliser. Furthermore, they also suggest that solutions of higher concentrations may result in inhibitory effects in plants, and lower concentrations may not have enough nutrients to sustain plant growth. In addition, they state that it’s clear that further research is needed regarding the combination of wastewaters to provide a reliable and sustainable fertiliser.

“We found that solutions containing up to 8% HTL-AP are still viable for plant growth, at least in the germination phase. This is a higher percentage of HTL-AP than anybody has demonstrated before,” “This makes it possible to recycle a waste stream that would otherwise go to a wastewater treatment plant, which takes up resources, or it would be discharged into the environment causing pollution.” – Professor Davidson

Alternative nutrient sources are needed to increase the circularity of global food production systems as well as decrease the reliance on chemical fertilisers derived from fossil fuels or mined from the earth.

If you would like to read more on hydroponics and sustainable agriculture or would like to submit research in this area, please see the Special Issue in Agriculture : Innovative Hydroponic Systems for Sustainable Agriculture .

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How did life begin on earth a lightning strike of an idea..

Yahya Chaudhry

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Researchers mimic early conditions on barren planet to test hypothesis of ancient electrochemistry

About four billion years ago, Earth resembled the set of a summer sci-fi blockbuster. The planet’s surface was a harsh and barren landscape, recovering from hellish asteroid strikes, teeming with volcanic eruptions, and lacking enough nutrients to sustain even the simplest forms of life.

The atmosphere was composed predominantly of inert gases like nitrogen and carbon dioxide, meaning they did not easily engage in chemical reactions necessary to form the complex organic molecules that are the building blocks of life. Scientists have long sought to discover the key factors that enabled the planet’s chemistry to change enough to form and sustain life.

Now, new research zeroes in on how lightning strikes may have served as a vital spark, transforming the atmosphere of early Earth into a hotbed of chemical activity. In the study, published in Proceedings of the National Academy of Sciences , a team of Harvard scientists identified lightning-induced plasma electrochemistry as a potential source of reactive carbon and nitrogen compounds necessary for the emergence and survival of early life.

“The origin of life is one of the great unanswered questions facing chemistry,” said George M. Whitesides, senior author and the Woodford L. and Ann A. Flowers University Research Professor in the Department of Chemistry and Chemical Biology. How the fundamental building blocks of “nucleic acids, proteins, and metabolites emerged spontaneously remains unanswered.”

One of the most popular answers to this question is summarized in the so-called RNA World hypothesis, Whitesides said. That is the idea that available forms of the elements, such as water, soluble electrolytes, and common gases, formed the first biomolecules. In their study, the researchers found that lightning could provide accessible forms of nitrogen and carbon that led to the emergence and survival of biomolecules.

A plasma vessel used to mimic cloud-to-ground lightning and its resulting electrochemical reactions. The setup uses two electrodes, with one in the gas phase and the other submerged in water enriched with inorganic salts.

Credit: Haihui Joy Jiang

Researchers designed a plasma electrochemical setup that allowed them to mimic conditions of the early Earth and study the role lightning strikes might have had on its chemistry. They were able to generate high-energy sparks between gas and liquid phases — akin to the cloud-to-ground lightning strikes that would have been common billions of years ago.

The scientists discovered that their simulated lightning strikes could transform stable gases like carbon dioxide and nitrogen into highly reactive compounds. They found that carbon dioxide could be reduced to carbon monoxide and formic acid, while nitrogen could be converted into nitrate, nitrite, and ammonium ions.

These reactions occurred most efficiently at the interfaces between gas, liquid, and solid phases — regions where lightning strikes would naturally concentrate these products. This suggests that lightning strikes could have locally generated high concentrations of these vital molecules, providing diverse raw materials for the earliest forms of life to develop and thrive.

“Given what we’ve shown about interfacial lightning strikes, we are introducing different subsets of molecules, different concentrations, and different plausible pathways to life in the origin of life community,” said Thomas C. Underwood, co-lead author and Whitesides Lab postdoctoral fellow. “As opposed to saying that there’s one mechanism to create chemically reactive molecules and one key intermediate, we suggest that there is likely more than one reactive molecule that might have contributed to the pathway to life.”

The findings align with previous research suggesting that other energy sources, such as ultraviolet radiation, deep-sea vents, volcanoes, and asteroid impacts, could have also contributed to the formation of biologically relevant molecules. However, the unique advantage of cloud-to-ground lightning is its ability to drive high-voltage electrochemistry across different interfaces, connecting the atmosphere, oceans, and land.

The research adds a significant piece to the puzzle of life’s origins. By demonstrating how lightning could have contributed to the availability of essential nutrients, the study opens new avenues for understanding the chemical pathways that led to the emergence of life on Earth. As the research team continues to explore these reactions, they hope to uncover more about the early conditions that made life possible and to improve modern applications.

“Building on our work, we are now experimentally looking at how plasma electrochemical reactions may influence nitrogen isotopes in products, which has a potential geological relevance,” said co-lead author Haihui Joy Jiang, a former Whitesides lab postdoctoral fellow. “We are also interested in this research from an energy-efficiency and environmentally friendly perspective on chemical production. We are studying plasma as a tool to develop new methods of making chemicals and to drive green chemical processes, such as producing fertilizer used today.”

Harvard co-authors included Professor Dimitar D. Sasselov in the Department of Astronomy and Professor James G. Anderson in the Department of Chemistry and Chemical Biology, Department of Earth and Planetary Sciences, and the Harvard John A. Paulson School of Engineering and Applied Sciences.

The study not only sheds light on the past but also has implications for the search for life on other planets. Processes the researchers described could potentially contribute to the emergence of life beyond Earth.

“Lightning has been observed on Jupiter and Saturn; plasmas and plasma-induced chemistry can exist beyond our solar system,” Jiang said. “Moving forward, our setup is useful for mimicking environmental conditions of different planets, as well as exploring reaction pathways triggered by lightning and its analogs.”

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Office: Vehicle Technologies Office FOA number: DE-FOA-0003383 Link to apply:  Apply on EERE Exchange FOA Amount: $42,950,000

The U.S. Department of Energy (DOE) announced $43 million in funding for projects that will advance research, development, demonstration, and deployment (RDD&D) in several areas critical to the future of advanced batteries. The funding will drive innovations in low-cost electric vehicle (EV) battery electrode, cell, or pack manufacturing; improve battery safety and reduce cascading failures; and strengthen the domestic supply chain of inexpensive and abundant battery materials. This funding is aligned with strategies detailed in the U.S. National Blueprint for Transportation Decarbonization , which is a landmark interagency framework of strategies and actions to remove all emissions from the transportation sector by 2050, by advancing battery technologies that can power safe and efficient zero-emission EVs. 

DOE’s Vehicle Technologies Office (VTO) will supply the funding and build on the Office of Energy Efficiency and Renewable Energy’s leadership in RDD&D of new technologies leading to efficient, clean, and affordable mobility options. Topic areas in VTO’s Fiscal Year 2024 Batteries funding opportunity include:

• Improving 12V lead-acid battery service life and performance requirements to meet critical safety features while reducing cost. 
• Developing cell, module, pack, vehicle, or structural strategies that reduce cascading effects of thermal issues that could lead to EV fires. 
• Conducting research and development that would reduce the manufacturing cost at the electrode, cell, or pack level by at least 30% compared to the state of the art, and increasing throughput by at least 50% compared to the state of the art.
• Researching, fabricating, and testing silicon-based lithium battery cells that meet EV battery performance requirements.
• Developing high energy density cathodes containing metal chalcogenide, oxide, or halide materials that surpass the energy density of state-of-the art nickel cathodes. 

As part of this approach, VTO encourages the participation of underserved communities and underrepresented groups. Applicants are highly encouraged to include individuals from groups historically underrepresented in STEM on their project teams.

Learn more about this funding opportunity and other funding opportunities within DOE’s Office of Energy Efficiency and Renewable Energy .

Topic Areas

This FOA has five topic areas:

Topic 1 Improved 12 Volt Lead Acid Batteries for Safety-Critical Electric Vehicle Applications,  focused on improving the service life and performance requirements to meet critical safety features while reducing cost ($10 million).

Topic 2 Develop Vehicle or Structural Level Strategies to Reduce the Likelihood of the Cascading Effects of Electric Vehicle Fires, focused on university-led teams conducting research at the cell, pack, and vehicle level ($3.9 million).

Topic 3 Battery Electrode, Cell, and Pack Manufacturing Cost Reduction, focused on developing improved manufacturing technologies for EV battery electrodes, cells, and packs ($12.5 million). 

Topic 4 Silicon-Based Anodes for Lithium-Ion Batteries, focused on researching, fabricating, and testing lithium battery cells implementing silicon electrodes with a commercially available cathode technology to achieve cell and cost performance targets (more than 350 Wh/kg of usable energy with a cell cost target of less than $70/kWh) ($12.5 million). 

Topic 5 High Energy Density Conversion Cathodes,  focused on developing high energy density battery cells containing metal chalcogenide, oxide, or halide cathodes by solving key challenges for the cathode, electrolyte, electrode integrity, or safety ($4.05 million). 

Additional Information

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• Sign up for the Office of Energy Efficiency and Renewable Energy (EERE) email list  to get notified of new EERE funding opportunities. Also sign up for the VTO GO! newsletter to stay current with the latest VTO news.