research paper review process

The peer review process

The peer review process can be broadly summarized into 10 steps, although these steps can vary slightly between journals. Explore what’s involved, below.

Editor Feedback: “Reviewers should remember that they are representing the readers of the journal. Will the readers of this particular journal find this informative and useful?”

1. Submission of Paper

The corresponding or submitting author submits the paper to the journal. This is usually via an online system such as ScholarOne Manuscripts. Occasionally, journals may accept submissions by email.

2. Editorial Office Assessment

The Editorial Office checks that the paper adheres to the requirements described in the journal’s Author Guidelines. The quality of the paper is not assessed at this point.

3. Appraisal by the Editor-in-Chief (EIC)

The EIC checks assesses the paper, considering its scope, originality and merits. The EiC may reject the paper at this stage.

4. EIC Assigns an Associate Editor (AE)

Some journals have Associate Editors ( or equivalent ) who handle the peer review. If they do, they would be assigned at this stage.

5. Invitation to Reviewers

The handling editor sends invitations to individuals he or she believes would be appropriate reviewers. As responses are received, further invitations are issued, if necessary, until the required number of reviewers is secured– commonly this is 2, but there is some variation between journals.

6. Response to Invitations

Potential reviewers consider the invitation against their own expertise, conflicts of interest and availability. They then accept or decline the invitation to review. If possible, when declining, they might also suggest alternative reviewers.

7. Review is Conducted

The reviewer sets time aside to read the paper several times. The first read is used to form an initial impression of the work. If major problems are found at this stage, the reviewer may feel comfortable rejecting the paper without further work. Otherwise, they will read the paper several more times, taking notes to build a detailed point-by-point review. The review is then submitted to the journal, with the reviewer’s recommendation (e.g. to revise, accept or reject the paper).

8. Journal Evaluates the Reviews

The handling editor considers all the returned reviews before making a decision. If the reviews differ widely, the editor may invite an additional reviewer so as to get an extra opinion before making a decision.

9. The Decision is Communicated

The editor sends a decision email to the author including any relevant reviewer comments. Comments will be anonymous if the journal follows a single-anonymous or double-anonymous peer review model. Journals with following an open or transparent peer review model will share the identities of the reviewers with the author(s).

10. Next Steps

An editor's perspective.

Listen to a podcast from Roger Watson, Editor-in-Chief of Journal of Advanced Nursing, as he discusses 'The peer review process'.

If accepted , the paper is sent to production. If the article is rejected or sent back for either major or minor revision , the handling editor should include constructive comments from the reviewers to help the author improve the article. At this point, reviewers should also be sent an email or letter letting them know the outcome of their review. If the paper was sent back for revision , the reviewers should expect to receive a new version, unless they have opted out of further participation. However, where only minor changes were requested this follow-up review might be done by the handling editor.

• Insights blog

Understanding the peer review process

What is peer review a guide for authors.

The peer review process starts once you have submitted your paper to a journal.

After submission, your paper will be sent for assessment by independent experts in your field. The reviewers are asked to judge the validity, significance, and originality of your work.

Below we expand on what peer review is, and how it works.

What is peer review? And why is important?

Peer review is the independent assessment of your research paper by experts in your field. The purpose of peer review is to evaluate the paper’s quality and suitability for publication.

As well as peer review acting as a form of quality control for academic journals, it is a very useful source of feedback for you. The feedback can be used to improve your paper before it is published.

So at its best, peer review is a collaborative process, where authors engage in a dialogue with peers in their field, and receive constructive support to advance their work.

Use our free guide to discover how you can get the most out of the peer review process.

Why is peer review important?

Peer review is vitally important to uphold the high standards of scholarly communications, and maintain the quality of individual journals. It is also an important support for the researchers who author the papers.

Every journal depends on the hard work of reviewers who are the ones at the forefront of the peer review process. The reviewers are the ones who test and refine each article before publication. Even for very specialist journals, the editor can’t be an expert in the topic of every article submitted. So, the feedback and comments of carefully selected reviewers are an essential guide to inform the editor’s decision on a research paper.

There are also practical reasons why peer review is beneficial to you, the author. The peer review process can alert you to any errors in your work, or gaps in the literature you may have overlooked.

Researchers consistently tell us that their final published article is better than the version they submitted before peer review. 91% of respondents to a  Sense about Science peer review survey  said that their last paper was improved through peer review. A  Taylor & Francis study  supports this, finding that most researchers, across all subject areas, rated the contribution of peer review towards improving their article as 8 or above out of 10.

Enlarge the infographic

Choose the right journal for your research: Think. Check. Submit

We support Think. Check. Submit. , an initiative launched by a coalition of scholarly communications organizations. It provides the tools to help you choose the right journal for your work.

Think. Check. Submit. was established because there are some journals which do not provide the quality assurance and services that should be delivered by a reputable journal. In particular, many of these journals do not make sure there is thorough peer review or editor feedback process in place.

That means, if you submit to one of these journals, you will not benefit from helpful article feedback from your peers. It may also lead to others being skeptical about the validity of your published results.

You should therefore make sure that you submit your work to a journal you can trust. By using the checklist provided on the Think. Check. Submit. website , you can make an informed choice.

Peer review integrity at Taylor & Francis

Every full research article published in a Taylor & Francis journal has been through peer review, as outlined in the journal’s aims & scope information. This means that the article’s quality, validity, and relevance has been assessed by independent peers within the research field.

We believe in the integrity of peer review with every journal we publish, ascribing to the following statement:

All published research articles in this journal have undergone rigorous peer review, based on initial editor screening, anonymous refereeing by independent expert referees, and consequent revision by article authors when required.

Different types of peer review

Peer review takes different forms and each type has pros and cons. The type of peer review model used will often vary between journals, even of the same publisher. So, check your chosen journal’s peer-review policy before you submit , to make sure you know what to expect and are comfortable with your paper being reviewed in that way.

Every Taylor & Francis journal publishes a statement describing the type of peer review used by the journal within the aims & scope section on Taylor & Francis Online.

Below we go through the most common types of peer review.

Common types of peer review

Single-anonymous peer review.

This type of peer review is also called ‘single-blind review’. In this model, the reviewers know that you are the author of the article, but you don’t know the identities of the reviewers.

Single-anonymous review is most common for science and medicine journals.

Find out more about the pros and cons of  single-anonymous peer review .

Double-anonymous peer review

In this model, which is also known as ‘double-blind review’, the reviewers don’t know that you are the author of the article. And you don’t know who the reviewers are either. Double-anonymous review is particularly common in humanities and some social sciences’ journals.

Discover more about the pros and cons of  double-anonymous peer review .

If you are submitting your article for double-anonymous peer review, make sure you know  how to make your article anonymous .

Open peer review

There is no one agreed definition of open peer review. In fact,  a recent study  identified 122 different definitions of the term. Typically, it will mean that the reviewers know you are the author and also that their identity will be revealed to you at some point during the review or publication process.

Find out more about  open peer review .

Post-publication peer review

In post-publication peer review models, your paper may still go through one of the other types of peer review first. Alternatively, your paper may be published online almost immediately, after some basic checks. Either way, once it is published, there will then be an opportunity for invited reviewers (or even readers) to add their own comments or reviews.

You can learn about the pros and cons of  post-publication peer review here.

Registered Reports

The  Registered Reports  process splits peer review into two parts.

The first round of peer review takes place after you’ve designed your study, but before you’ve collected or analyzed any data. This allows you to get feedback on both the question you’re looking to answer, and the experiment you’ve designed to test it.

If your manuscript passes peer review, the journal will give you an in-principle acceptance (IPA). This indicates that your article will be published as long as you successfully complete your study according to the pre-registered methods and submit an evidence-based interpretation of the results.

Explore Registered Reports at Taylor & Francis .

F1000 Research: Open and post-publication peer review

F1000Research  is part of the Taylor & Francis Group. It operates an innovative peer review process which is fully transparent and takes place after an article has been published.

How it works

Before publication, authors are asked to  suggest at least five potential reviewers  who are experts in the field. The reviewers also need to be able to provide unbiased reports on the article.

Submitted articles are published rapidly, after passing a series of pre-publication checks that assess, originality, readability, author eligibility, and compliance with F1000Research’s policies and ethical guidelines.

Once the article is published, expert reviewers are formally invited to review.

The peer review process is entirely open and transparent. Each peer review report, plus the approval status selected by the reviewer, is published with the reviewer’s name and affiliation alongside the article.

Authors are encouraged to respond openly to the peer review reports and can publish revised versions of their article if they wish. New versions are clearly linked and easily navigable, so that readers and reviewers can quickly find the latest version of an article.

The article remains published regardless of the reviewers’ reports. Articles that pass peer review are indexed in Scopus, PubMed, Google Scholar and other bibliographic databases.

How our publishing process works for articles

1. Article submission

Submitting an article is easy with our single-page submission system.

The in-house editorial team carries out a basic check on each submission to ensure that all policies are adhered to.

2. Publication and data deposition

Once the authors have analysed the manuscript, the article (with its associated source data) is published within a week, enabling immediate viewing and caution.

3. Open peer review & user commenting

Expert reviewers are selected and invited. Their reports and names are published alongside the article, together with the authors’ responses and comments from registered users.

4. Article revision

Authors are encouraged to publish revised versions of their article. All versions of an article are linked and independently citable.

Articles that pass peer review are indexed in external databases such as PubMed, Scopus and Google Scholar.

Discover more about how the F1000Research model works .

Get to know the peer review process

Peer review follows a number of steps, beginning with submitting your article to a journal.

Step 1: Editor assessment

When your manuscript arrives at the journal’s editorial office it will receive an initial desk assessment by the journal’s editor or editorial office. They will check that it’s broadly suitable for the journal.

They will ask questions such as:

Is this the right journal for this article?

Does the paper cover a suitable topic according to the journal’s  aims & scope ?

Has the author followed the journal’s guidelines in the  instructions for authors ? They will check that your paper meets the basic requirements of the journal, such as word count, language clarity, and format.

Has the author included everything that’s needed for peer review? They will check that there is an abstract, author affiliation details, any figures, and research-funder information.

Does it make a significant contribution to the existing literature?

If your article doesn’t pass these initial checks the editor might reject the article immediately. This is known as a ‘desk reject’ and it is a decision made at the editor’s discretion, based on their substantial experience and subject expertise. By having this initial screening in place, it can enable a quick decision if your manuscript isn’t suitable for the journal. This means you can submit your article to another journal quickly.

If your article does pass the initial assessment, it will move to the next stage, and into peer review.

“As an editor, when you first get a submission, at one level you’re simply filtering. A fairly small proportion do not get sent out by me for review. Sometimes they simply fall outside the scope of the journal.”

– Michael Reiss, Founding Editor of Sex Education

Step 2: First round of peer review

Next, the editor will find and contact other researchers who are experts in your field, and will ask them to review the paper. A minimum of two independent reviewers is normally required for every research article. The aims and scope of each journal will outline their peer review policy in detail.

The reviewers will be asked to read and comment on your article. They may also be invited to advise the editor whether your article is suitable for publication in that journal.

So, what are the reviewers looking for?

This depends on the subject area, but they will be checking that:

Your work is original or new.

The study design and methodology are appropriate and described so that others could replicate what you have done.

You’ve engaged with all the relevant current scholarship.

The results are appropriately and clearly presented.

Your conclusions are reliable, significant, and supported by the research.

The paper fits the scope of the journal.

The work is of a high enough standard to be published in the journal.

If you have not already  shared your research data publicly , peer reviewers may request to see your datasets, to support validation of the results in your article.

Once the editor has received and considered the reviewer reports, as well as making their own assessment of your work, they will let you know their decision. The reviewer reports will be shared with you, along with any additional guidance from the editor.

If you get a straight acceptance, congratulations, your article is ready to move to publication. But, please note, that this isn’t common. Very often, you will need to revise your article and resubmit it. Or it may be that the editor decides your paper needs to be rejected by that journal.

Please note that the final editorial decision on a paper and the choice of who to invite to review is always the editor’s decision. For further details on this, please see  our peer review appeals and complaints policy.

Step 3: Revise and resubmit

It is very common for the editor and reviewers to have suggestions about how you can improve your paper before it is ready to be published. They might have only a few straightforward recommendations (‘minor amendments’) or require more substantial changes before your paper will be accepted for publication (‘major amendments’). Authors often tell us that the reviewers’ comments can be extremely helpful, to make sure that their article is of a high quality.

During this stage of the process you will have time to amend your article based on the reviewers’ comments, resubmitting it with any or all changes made. Make sure you know how to respond to reviewer comments, we cover this in the next section.

Once you resubmit your manuscript the editor will look through the revisions. They will often send it out for a second round of peer review, asking the reviewers to assess how you’ve responded to their comments.

After this, you may be asked to make further revisions, or the paper might be rejected if the editor thinks that the changes you’ve made are not adequate. However, if your revisions have now brought the paper up to the standard required by that journal, it then moves to the next stage.

If you do not intend to make the revisions suggested by the journal and resubmit your paper for consideration, please make sure you formally withdraw your paper from consideration by the journal before you submit elsewhere.

Make sure you resubmit

If you have not already shared your research data publicly , peer reviewers may request to see your datasets to support the validation of the results in your article.

Step 4: Accepted

And that’s it, you’ve made it through peer review. The next step is  production

How long does peer review take?

Editorial teams work very hard to progress papers through peer review as quickly as possible. But it is important to be aware that this part of the process can take time.

The first stage is for the editor to find suitably qualified expert reviewers who are available. Given the competing demands of research life, nobody can agree to every review request they receive. It’s therefore not uncommon for a paper to go through several cycles of requests before the editor finds reviewers who are both willing and able to accept.

Then, the reviewers who do accept the request, have to find time alongside their own research, teaching, and writing, to give your paper thorough consideration.

Please do keep this in mind if you don’t receive a decision on your paper as quickly as you would like. If you’ve submitted your paper via an online system, you can use it to track the progress of your paper through peer review. Otherwise, if you need an update on the status of your paper, please get in touch with the editor.

Many journals publish key dates alongside new articles, including when the paper was submitted, accepted, and published online. While you’re at the stage of choosing a journal to submit to, take a look at these dates for a range of recent articles published in the journals you’re considering. While each article will have a slightly different timeline, this may help you to get an idea of how long publication may take.

A 360⁰ view of peer review

Peer review is a process that involves various players – the author, the reviewer and the editor to name a few. And depending on which of these hats you have on, the process can look quite different.

To help you uncover the 360⁰ peer review view,  read these interviews  with an editor, author, and reviewer.

How to respond to reviewer comments

If the editor asks you to revise your article, you will be given time to make the required changes before resubmitting.

When you receive the reviewers’ comments, try not to take personal offence to any criticism of your article (even though that can be hard).

Some researchers find it helpful to put the reviewer report to one side for a few days after they’ve read it for the first time. Once you have had chance to digest the idea that your article requires further work, you can more easily address the reviewer comments objectively.

When you come back to the reviewer report, take time to read through the editor and reviewers’ advice carefully, deciding what changes you will make to your article in response. Taking their points on board will make sure your final article is as robust and impactful as possible.

Please make sure that you address all the reviewer and editor comments in your revisions.

It may be helpful to resubmit your article along with a two-column grid outlining how you’ve revised your manuscript. On one side of the grid list each of the reviewers’ comments and opposite them detail the alterations you’ve made in response. This method can help you to order your thoughts, and clearly demonstrate to the editor and reviewers that you’ve considered all of their feedback.

If there are any review comments which you don’t understand or don’t know how to respond to, please get in touch with the journal’s editor and ask for their advice.

What if you don’t agree with the reviewers’ comments?

If there’s a review comment that you don’t agree with, it is important that you don’t ignore it. Instead, include an explanation of why you haven’t made that change with your resubmission. The editor can then make an assessment and include your explanation when the amended article is sent back to the reviewers.

You are entitled to defend your position but, when you do, make sure that the tone of your explanation is assertive and persuasive, rather than defensive or aggressive.

“Where possible, a little constructive advice on how to make use of the views of the referees can make all the difference, and the editor has the responsibility of deciding when and how to do this.”

– Gary McCulloch, Editor, British Journal of Educational Studies

What if my paper is rejected?

Nobody enjoys having their paper rejected by a journal, but it is a fact of academic life. It happens to almost all researchers at some point in their career. So, it is important not to let the experience knock you back. Instead, try to use it as a valuable learning opportunity.

Take time to understand why your paper has been rejected

If a journal rejects your manuscript, it may be for one of many reasons. Make sure that you understand why your paper has been rejected so that you can learn from the experience. This is especially important if you are intending to submit the same article to a different journal.

Are there fundamental changes that need to be made before the paper is ready to be published, or was this simply a case of submitting to the wrong journal? If you are unsure why your article has been rejected, then please contact the journal’s editor for advice.

Some of the common reasons manuscripts are rejected

The author has submitted their paper to the wrong journal: it doesn’t fit the  aims & scope  or fails to engage with issues addressed by the journal.

The manuscript is not a true journal article, for instance it is too journalistic or clearly a thesis chapter.

The manuscript is too long or too short.

There is poor regard of the journal’s conventions, or for academic writing in general.

Poor style, grammar, punctuation or English throughout the manuscript. Get  English language editing  assistance.

The manuscript does not make any new contribution to the subject.

The research has not been properly contextualized.

There is a poor theoretical framework used. There are  actio nable recommendations to improve your manuscript .

The manuscript is poorly presented.

The manuscript is libelous or unethical.

Carefully consider where to submit next

When you made your original submission, you will probably have had a shortlist of journals you were considering. Return to that list but, before you move to your second choice, you may wish to assess whether any feedback you’ve received during peer review has changed your opinion. Your article may also be quite different if it has been through any rounds of revision. It can be helpful at this stage to re-read the  aims & scope  statements of your original shortlisted journals.

Once you have selected which journal to submit to next, make sure that you read through its information for authors and reformat your article to fit its requirements. Again, it is important to use the feedback from the peer review process to your advantage as you rewrite and reformat the manuscript.

Is ‘transferring’ an option?

A growing number of publishers offer a  transfer or cascade service  to authors when their paper is rejected. This process is designed for papers which aren’t suitable for the journal they were originally submitted to.

If your article falls into this category then one or more alternative journals from the same publisher will be suggested. You will have the option either to submit to one of those suggested journals for review or to withdraw your article.

If you choose to transfer your article this will usually save you time. You won’t need to enter all of the details into a new submission system. Once you’ve made any changes to your paper, bearing in mind previous editor or reviewer comments, the article will be submitted to the new journal on your behalf.

We have some more information about  article transfers, and also some FAQs about the Taylor & Francis transfer process.

Why you should become a peer reviewer

When you’re not in the middle of submitting or revising your own article, you should consider becoming a reviewer yourself.

There are many demands on a researcher’s time, so it is a legitimate question to ask why some of that precious time should be spent reviewing someone else’s work. How does being a reviewer help you in your career? Here are some of the benefits.

Keep up with the latest thinking As a reviewer you get an early view of the exciting new research being done in your field. Not only that, peer review gives you a role in helping to evaluate and improve this new work.

Improve your own writing Carefully reviewing articles written by other researchers can give you an insight into how you can make your own work better. Unlike when you are reading articles as part of your research, the process of reviewing encourages you to think critically about what makes an article good (or not so good). This could be related to writing style, presentation, or the clarity of explanations.

Boost your career While a lot of reviewing is anonymous, there are schemes to recognize the important contribution of reviewers. You can also include reviewing work on your resume. Your work as a reviewer will be of interest to appointment or promotion committees who are looking for evidence of service to the profession.

Become part of a journal’s community Many journals act as the center of a network of researchers who are in conversation about key themes and developments in the field. Becoming a reviewer is a great way to get involved with that group. This can give you the opportunity to build new connections for future collaborations. Being a regular reviewer may also be the first step to becoming a member of the journal’s editorial board.

Your research community needs you

Of course, being a reviewer is not just about the benefits it can bring you. The  Taylor & Francis peer review survey  found that these are the top 3 reasons why researchers choose to review:

Being an active member of the academic community Peer review is the bedrock of academic publishing. The work of reviewers is essential in helping every piece of research to become as good as it can be. By being a reviewer, you will play a vital part in advancing the research area that you care about.

Reciprocating the benefit Researchers regularly talk about the benefits to their own work from being reviewed by others. Gratitude to the reviewers who have improved your work is a great motivation to make one’s own contribution of service to the community.

Enjoying being able to help improve papers Reviewing is often anonymous, with only the editor knowing the important contribution you’ve made. However, many reviewers attest that it is work that makes them feel good, knowing that they have been able to support a fellow researcher.

How to be an effective peer reviewer

Our popular  guide to becoming a peer reviewer  covers everything you need to know to get started, including:

How to become a peer reviewer

Writing review reports: step-by-step

Ethical guidelines for peer reviewers

Reviewer recognition

Read the  Taylor & Francis reviewer guidelines .

“Reviewers are the lifeblood of any journal”

– Mike J. Smith, Editor-in-Chief of Journal of Maps

Further reading

We hope you’ve found this short introduction to peer review helpful. For further useful advice check out the following resources.

Further resources

Peer Review: the nuts and bolts A guide to peer review written by early career researchers, for early career researchers and published by Sense about Science.

A guide to becoming a peer reviewer An overview of what’s involved in becoming a reviewer for a Taylor & Francis journal.

Ethical guidelines for peer reviewer Produced by COPE, the Committee on Publication Ethics, setting out the standards all peer reviewers should follow.

Using peer review effectively: quick tips Advice available to staff and students at institutions with a Vitae membership.

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Insights topic: Peer review

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Behind the scenes of peer review: meet the Taylor & Francis Editorial office team

Peer review process

Introduction to peer review, what is peer review.

Peer review is the system used to assess the quality of a manuscript before it is published. Independent researchers in the relevant research area assess submitted manuscripts for originality, validity and significance to help editors determine whether a manuscript should be published in their journal.

How does it work?

When a manuscript is submitted to a journal, it is assessed to see if it meets the criteria for submission. If it does, the editorial team will select potential peer reviewers within the field of research to peer-review the manuscript and make recommendations.

There are four main types of peer review used by BMC:

Single-blind: the reviewers know the names of the authors, but the authors do not know who reviewed their manuscript unless the reviewer chooses to sign their report.

Double-blind: the reviewers do not know the names of the authors, and the authors do not know who reviewed their manuscript.

Open peer: authors know who the reviewers are, and the reviewers know who the authors are. If the manuscript is accepted, the named reviewer reports are published alongside the article and the authors’ response to the reviewer.

Transparent peer: the reviewers know the names of the authors, but the authors do not know who reviewed their manuscript unless the reviewer chooses to sign their report. If the manuscript is accepted, the anonymous reviewer reports are published alongside the article and the authors’ response to the reviewer.

Different journals use different types of peer review. You can find out which peer-review system is used by a particular journal in the journal’s ‘About’ page.

Why do peer review?

Peer review is an integral part of scientific publishing that confirms the validity of the manuscript. Peer reviewers are experts who volunteer their time to help improve the manuscripts they review. By undergoing peer review, manuscripts should become:

More robust - peer reviewers may point out gaps in a paper that require more explanation or additional experiments.

Easier to read - if parts of your paper are difficult to understand, reviewers can suggest changes.

More useful - peer reviewers also consider the importance of your paper to others in your field.

For more information and advice on how to get published, please see our blog series here .

How peer review works

The peer review process can be single-blind, double-blind, open or transparent.

You can find out which peer review system is used by a particular journal in the journal's 'About' page.

N. B. This diagram is a representation of the peer review process, and should not be taken as the definitive approach used by every journal.

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• Published: 12 March 2024

Peer review demystified: part 1

Nature Methods volume  21 ,  page 361 ( 2024 ) Cite this article

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• Biological techniques

Peer review is at the heart of publishing scientific papers. In this first installment of a two-part Editorial, we explain how we manage the process at Nature Methods .

The basic peer review process 1 has remained the same in the near two decades since Nature Methods was launched, although we have made many enhancements over the years.

Peer reviewers provide valuable technical advice on manuscripts and also often share their opinions on whether a paper will have a strong impact. However, the decision of whether to offer revision or rejection is made by the editors, all of whom have PhDs in subjects closely related to the fields in which we publish.

A key role of the editors is determining which points raised by the reviewers are crucial to address. Sometime reviewers ask for experiments that we think go beyond the scope of a Nature Methods paper, and so we overrule them. If a reviewer raises serious and reasonable technical concerns, however, we may reject the paper or require authors to provide a fix or a strong rebuttal. In cases of disagreements or opposing views, we may discuss the key concerns with the other experts. More generally, we carefully consider the reasons why a particular reviewer is positive or negative, and weigh that against the other reviewers’ opinions as well as our own original thoughts about the paper.

We appreciate that the quality of reviewer reports varies widely and despite our best efforts to ensure the process is fair, everyone has their own personal biases. If authors believe that a reviewer is strongly biased or has fundamentally misunderstood their work, we encourage you to reach out to your editor to discuss the situation.

How we choose peer reviewers

We try our best to invite three peer reviewers for every research paper. Sometimes it is difficult to find three or a reviewer may fail to deliver in a timely manner (this happens with unfortunate regularity!), and so we may make a decision on the basis of two reviews if they appear to be competent, fair and detailed.

Authors are welcome to suggest and exclude reviewers. We typically will not assign more than one suggested reviewer, but providing names can be a helpful starting point. Authors should suggest peers with expertise that matches the topic, and whom they believe will be unbiased — that is, not their former advisor, best friend, collaborator or fierce competitor. We always honor exclusions, within reason: as a rule of thumb, authors should exclude no more than five peers, and should not exclude whole institutions or whole fields of research. It is essential for the robustness of the scientific literature that reviewers with appropriate expertise be allowed to give feedback on manuscripts.

We do our best to find reviewers without any real or perceived conflict of interest with the authors. But we are not always aware of past relationships or new collaborations. We ask reviewers to declare these possible conflicts of interest to us before accepting to review a manuscript. If you are unsure of whether your connection to an author represents a conflict, ask the editor!

As a methods journal, we believe it is important for our papers to be assessed both by experts in the techniques used and by potential end users of the technology. For example, we will assign a computational expert in a particular field to review a paper that describes a new software tool for that field, and even ask for their help in evaluating its code 2 , 3 . But we will also try to invite a reviewer who is a potential user of the tool, who may raise important issues about impact or usability.

We try to minimize the number of rounds of peer review, out of respect for both the authors’ and reviewers’ time. The majority of papers that we publish have gone through two rounds of peer review. Some papers will need a third round of peer review if serious technical concerns still remain, but this is infrequent. If the editors are able to assess whether the reviewers’ concerns have been adequately addressed, we avoid going back to the reviewers.

How we review nonprimary papers

All primary research papers (Articles, Brief Communications, Resources, Analyses and Registered Reports — which have their own specialized review process 4 ) are subject to the rigorous process described above. Other types of content also go through peer review, although the process is a bit different.

Reviews and Perspectives are scholarly surveys of the literature and are always peer-reviewed. We ask reviewers to assess the accuracy, comprehensiveness and balance of Reviews. Perspectives may be more narrowly focused or advance the authors’ particular opinion, and thus balance is not always essential.

Comments and Correspondences are sometimes, but not always, peer reviewed. If reviewed, there will be a note in the paper; if the note is not present, the reader can assume it was not peer reviewed. Comments are opinion pieces, but experts in the subject often provide feedback on whether the authors’ position is well-supported by literature evidence. The Correspondence is a diverse format that we use to publish short reader comments or descriptions of computational tools or platforms. The more technical pieces are always peer reviewed, which often helps authors to optimize their tools or platforms before publication.

Other regular content — such as Points of Significance, Creature Columns, News & Views, and Research Briefings — is not peer reviewed, nor is regular content written by the editorial team (Research Highlights, Editorials, Lab & Life pieces and Technology Features).

For a general description of the editorial process, please revisit ‘How editors edit’ 5 . If you are a reviewer who would like more information about reviewing for Nature Methods , please read ‘The good referee’ 6 . Stay tuned for next month’s Editorial, where we will cover topics of reviewer diversity, credit and transparency.

Nat. Methods 3 , 329 (2006).

Nat. Methods 15 , 641 (2018).

Nat. Methods 12 , 1099 (2015).

Nat. Methods 19 , 131 (2022).

Nat. Methods 16 , 135 (2019).

Nat. Methods 15 , 91 (2018).

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Peer review demystified: part 1. Nat Methods 21 , 361 (2024). https://doi.org/10.1038/s41592-024-02225-6

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How to conduct a review

1. before you begin.

Before you accept or decline an invitation to review, consider the following questions:

Does the article match your  area of expertise ? Only accept if you feel you can provide a high-quality review.

Do you have a potential  conflict of interest   opens in new tab/window ? Disclose this to the editor when you respond.

Do you have  time ? Reviewing can be a lot of work — before you commit, make sure you can meet the deadline.

Do you need to find out more about reviewing and the peer review process ? If so, check out the free tutorials on the Elsevier Researcher Academy   opens in new tab/window , especially the Certified Peer Reviewer course   opens in new tab/window .

Respond to the invitation as soon as you can (even if it is to decline) — a delay in your decision slows down the review process and means more waiting for the author. If you do decline the invitation, it would be helpful if you could provide suggestions for alternative reviewers.

2. Managing your review

Confidential material.

If you accept, you must treat the materials you receive as confidential documents. This means you can’t share them with anyone without prior authorization from the editor. Since peer review is confidential, you also must not share information about the review with anyone without permission from the editors and authors.

Generative AI

Reviewing a scientific paper implies responsibilities that can only be attributed to humans. The critical thinking and assessment required for peer-review are outside the scope of generative AI and AI-assisted technologies, and there is a risk that the technology will generate incorrect, incomplete or biased conclusions. These considerations, together with the principle that submitted manuscripts are to be treated as confidential documents, underpins our Generative AI policies for reviewers and editors :

Reviewers or editors should not upload the manuscript or any part of it into a Generative AI tool, as there is no guarantee of where materials are being sent, saved, or viewed, or how they may be used in the future and this may violate the authors’ confidentiality, proprietary and/or data privacy rights. It may also violate the terms of use of the Generative AI tool.

This confidentiality requirement extends to the peer review report and any other communication about the manuscript, such as the notification or decision letters, as they may also contain confidential information about the manuscript and/or the authors. For this reason, they should not be uploaded into a Generative AI tool, even if it is just for the purpose of improving language and readability.

Generative AI should not be used to assist in the review, evaluation or decision-making process of a manuscript.

Elsevier embraces new AI-driven technologies that support reviewers and editors in the editorial process, such as those used during the screening process to conduct completeness and plagiarism checks and identify suitable reviewers. These identity-protected technologies conform to the  RELX Responsible AI Principles   opens in new tab/window . We continue to develop and adopt in-house or licensed technologies that can assist editors and reviewers while respecting confidentiality, proprietary and data privacy rights.

How to log in and access your review

Your review will be managed via an Elsevier submission system such as  Editorial Manager   opens in new tab/window . Elsevier journals use different submission systems so there is no one generic login link. To access the paper and deliver your review, click on the link in the invitation email you received which will bring you to the submission/reviewing system.

How to submit a review in Editorial Manager

Article- and journal-specific instructions.

When you sit down to write the review, make sure you familiarize yourself with any journal-specific guidelines (these will be noted in the journal’s guide for authors available on  each journal’s homepage ).

Some of our journals employ “structured peer review” whereby you will receive a series of questions to make it easier for you to convey recommendations for improvement in a structured manner. These questions address different aspects of the manuscript, such as the quality of the data analysis, reproducibility, and the overall clarity of the manuscript. You can find an overview of the core questions (note that journals may also employ additional journal-specific questions).

In any review, please keep the author (instead of the editor) in mind when preparing your replies as your comments will likely be included in the decision letter sent to the author. Also please make sure to reply to the questions in sufficient detail when you are submitting your report in Editorial Manager, to enable the author to most effectively improve the manuscript based on your comments.

If the journal in question does not require you to respond to a list of specific questions, you might find it useful to consider these questions and the below generic points when you are preparing your review report.

Full length research article

Examine the importance of the research question addressed in the manuscript (e.g., are objectives and justification clearly stated?)

Assess the originality (contribution, addition of knowledge to scientific literature or field) of the manuscript

Clearly identify the strengths and weaknesses of the method described in the manuscript

Make specific useful comments on the writing of the manuscript (e.g., writing, organization, figures, etc.)

Offer specific comments on the author’s interpretation of the results and conclusions drawn from the results

In case applicable, comment on the statistics (for example question if they are robust and fit-for-purpose and if the controls and sampling mechanisms are sufficient and well described)

Review article

Discuss the importance of the topic/scope of the review

Assess the originality of the review

Comment on the author's representation of the most relevant recent advances in the field; specifically, determine whether the references are relevant to the topic and cover both historical literature and more recent developments

Offer comments on the writing, organization, tables and figures of the manuscript

Comment on the author's interpretation of the results

In any case, your first task is to read the article. You might consider spot checking major issues by choosing which section to read first. Below we offer some tips about handling specific parts of the paper.

Methodology

If the manuscript you are reviewing is reporting an experiment, check the methods section first. The following cases are considered major flaws and should be flagged:

Unsound methodology

Discredited method

Missing processes known to be influential on the area of reported research

A conclusion drawn in contradiction to the statistical or qualitative evidence reported in the manuscript

For analytical papers examine the sampling report, which is mandated in time-dependent studies. For qualitative research make sure that a systematic data analysis is presented and sufficient descriptive elements with relevant quotes from interviews are listed in addition to the author’s narrative.

For more details and guidance you might find it helpful to view the  information on structured peer review .

Sex and gender reporting

Although it might sound late, given that the research has already been concluded and the data analyzed, the peer review process can still play an important role in ensuring effective, transparent and complete sex and gender reporting .

Assess manuscripts for inclusion of sex-disaggregated data and gender analysis. It could well be that the study was not designed to analyze sex and/or gender. Nevertheless, we recommend referees to consider if sex and gender are relevant to the topic of the study, and whether the study follows relevant guidelines, wherever applicable.

As a general principle,  the SAGER guidelines   opens in new tab/window  recommend careful use of the words “sex” and “gender” in order to avoid confusing both terms. The use of common definitions will improve the ability to conduct meta-analyses of published and archived data. The term “sex” should be used as a classification of male or female based on biological distinction to the extent that this is possible to confirm.  Per Heidari et al.   opens in new tab/window : "Gender refers to the socially constructed roles, behaviours and identities of female, male and gender-diverse people . It influences how people perceive themselves and each other, how they behave and interact and the distribution of power and resources in society. Gender is usually incorrectly conceptualized as a binary (female/ male) factor. In reality, there is a spectrum of gender identities and expressions defining how individuals identify themselves and express their gender."

Please check if authors have underlined in the methods section whether the sex of participants was defined based on self-report or assigned following external or internal examination of body characteristics, or through genetic testing or other means. In studies of animals, the term “sex” should be used. In cell biological, molecular biological or biochemical experiments, the origin and sex chromosome constitutions of cells or tissue cultures should be stated. If unknown, the reasons should be stated. In other disciplines, such as the testing of devices or technology, authors should explain whether it will be applied or used by all genders and if it has been tested with a user’s gender in mind. Please check whether the authors have done due diligence and reported any previous studies in the introductions that reveal or refute potential sex or gender differences, and the rationale why they have or have not examined these aspects in their study.

If the study included only one sex/gender, ensure this has been justified. If the study included more than one sex/gender, ensure data are reported for all the options that were recorded and that data disaggregated by sex/gender are reported in full, in the main text or in the appendix or supplementary materials. It is important to assess whether the methodology is appropriate to capture possible sex and gender aspects. As a peer reviewer you may request sub-group analysis if deemed necessary and check if all data are provided disaggregated by sex, as a minimum.

Finally, please also pay attention to whether authors have clearly justified in the discussion section any limitation of their study due to lack of any sex- and gender-based analysis and/or the implications on the generalizability and interpretation of their findings in light of that. It could be that the study was not designed to analyze sex and/or gender, nevertheless, it is important to consider if sex and gender are relevant to the topic of the study, and whether the study follows relevant guidelines, wherever applicable.

Research data and visualizations

Once you are satisfied that the methodology is sufficiently robust, examine any data in the form of figures, tables, or images. Authors may add  research data , including   data visualizations ,  to their submission to enable readers to interact and engage more closely with their research after publication. Please be aware that links to data might therefore be present in the submission files. These items should also receive your attention during the peer review process. Manuscripts may also contain database identifiers or accession numbers (e.g., genes) in relation to our  database linking program .

Critical issues in research data, which are considered to be major flaws can be related to insufficient data points, statistically non-significant variations and unclear data tables.

NB for certain types of visualization, preview tools are available, allowing you to inspect how files will display on ScienceDirect if the manuscript is accepted. For other data visualizations, there may be  other ways of inspecting the files .

Ethical considerations

Experiments including patient or animal data should properly be documented. Most journals require ethical approval by the author’s host organization. Please check journal-specific guidelines for such cases (available from the journal’s homepage,  accessible via the journal catalogue .

If you don’t spot any major flaws, take a break from the manuscript, giving you time to think. Consider the article from your own perspective. When you sit down to write the review, again make sure you familiarize yourself with any journal-specific guidelines (these will be noted in the journal’s guide for authors).

3. Structuring your review

Your review will help the editor decide whether or not to publish the article. It will also aid the author and allow them to improve their manuscript. Giving your overall opinion and general observations of the article is essential. Your comments should be courteous and constructive, and should not include any ad hominem remarks or personal details including your name (unless the journal you are invited to review for employs  open peer review ).

Providing insight into any deficiencies is important. You should explain and support your judgement so that both editors and authors are able to fully understand the reasoning behind your comments. You should indicate whether your comments are your own opinion or are reflected by the data and evidence.

The journal for which you are reviewing might have a specific format (e.g., questionnaire) or other instructions for how to structure your feedback. Below are some general tips on what to include/consider if no other guidelines apply. View the checklist .

Also, here is  an example of a published peer review report   opens in new tab/window .

Your recommendation

When you make a recommendation, it is worth considering the categories the editor will likely use for classifying the article:

Reject (explain your reasoning in your report)

Accept without revision

Revise — either major or minor (explain the revision that is required, and indicate to the editor whether you would be happy to review the revised article); if you are recommending a revision, you must furnish the author with a clear, sound explanation of why this is necessary

Bear in mind that there will be the opportunity to direct separate comments to both the editor and author. Once you are ready to submit your report, follow the instructions in the email  or visit our support center   opens in new tab/window  if you encounter any difficulties.

Access the support center   opens in new tab/window for further help.

The final decision

The editor ultimately decides whether to accept or reject the article. Elsevier plays no part in this decision. The editor will weigh all views and may call for another opinion or ask the author for a revised paper before making a decision. The submission system provides reviewers with a notification of the final decision, if the journal has opted in to this functionality.

4. After your review

Once you have delivered your review, you might want to make use of  Elsevier’s Reviewer Hub   opens in new tab/window  to ensure that you receive credit for your work. The platform provides a private profile page, certificates, editor recognition as well as discounts for Elsevier services.

Do not forget that, even after finalizing your review, you must treat the article and any linked files or data as confidential documents. This means you must not share them or information about the review with anyone without prior authorization from the editor.

Finally, we take the opportunity to thank you sincerely on behalf of the journal, editors and author(s) for the time you have taken to give your valuable input to the article.

Tools & resources

Free Certified Peer Reviewer course on Researcher Academy   opens in new tab/window

Reviewer checklist   opens in new tab/window

Articles from Reviewers' Update

Want to become a certified peer reviewer?

How to tackle your first review

Paving the way to increase diversity in journals – and research

Ten reasons to accept your (next) invitation to review

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Understanding Peer Review in Science

Peer review is an essential element of the scientific publishing process that helps ensure that research articles are evaluated, critiqued, and improved before release into the academic community. Take a look at the significance of peer review in scientific publications, the typical steps of the process, and and how to approach peer review if you are asked to assess a manuscript.

What Is Peer Review?

Peer review is the evaluation of work by peers, who are people with comparable experience and competency. Peers assess each others’ work in educational settings, in professional settings, and in the publishing world. The goal of peer review is improving quality, defining and maintaining standards, and helping people learn from one another.

In the context of scientific publication, peer review helps editors determine which submissions merit publication and improves the quality of manuscripts prior to their final release.

Types of Peer Review for Manuscripts

There are three main types of peer review:

• Single-blind review: The reviewers know the identities of the authors, but the authors do not know the identities of the reviewers.
• Double-blind review: Both the authors and reviewers remain anonymous to each other.
• Open peer review: The identities of both the authors and reviewers are disclosed, promoting transparency and collaboration.

There are advantages and disadvantages of each method. Anonymous reviews reduce bias but reduce collaboration, while open reviews are more transparent, but increase bias.

Key Elements of Peer Review

Proper selection of a peer group improves the outcome of the process:

• Expertise : Reviewers should possess adequate knowledge and experience in the relevant field to provide constructive feedback.
• Objectivity : Reviewers assess the manuscript impartially and without personal bias.
• Confidentiality : The peer review process maintains confidentiality to protect intellectual property and encourage honest feedback.
• Timeliness : Reviewers provide feedback within a reasonable timeframe to ensure timely publication.

Steps of the Peer Review Process

The typical peer review process for scientific publications involves the following steps:

• Submission : Authors submit their manuscript to a journal that aligns with their research topic.
• Editorial assessment : The journal editor examines the manuscript and determines whether or not it is suitable for publication. If it is not, the manuscript is rejected.
• Peer review : If it is suitable, the editor sends the article to peer reviewers who are experts in the relevant field.
• Reviewer feedback : Reviewers provide feedback, critique, and suggestions for improvement.
• Revision and resubmission : Authors address the feedback and make necessary revisions before resubmitting the manuscript.
• Final decision : The editor makes a final decision on whether to accept or reject the manuscript based on the revised version and reviewer comments.
• Publication : If accepted, the manuscript undergoes copyediting and formatting before being published in the journal.

Pros and Cons

While the goal of peer review is improving the quality of published research, the process isn’t without its drawbacks.

• Quality assurance : Peer review helps ensure the quality and reliability of published research.
• Error detection : The process identifies errors and flaws that the authors may have overlooked.
• Credibility : The scientific community generally considers peer-reviewed articles to be more credible.
• Professional development : Reviewers can learn from the work of others and enhance their own knowledge and understanding.
• Time-consuming : The peer review process can be lengthy, delaying the publication of potentially valuable research.
• Bias : Personal biases of reviews impact their evaluation of the manuscript.
• Inconsistency : Different reviewers may provide conflicting feedback, making it challenging for authors to address all concerns.
• Limited effectiveness : Peer review does not always detect significant errors or misconduct.
• Poaching : Some reviewers take an idea from a submission and gain publication before the authors of the original research.

Steps for Conducting Peer Review of an Article

Generally, an editor provides guidance when you are asked to provide peer review of a manuscript. Here are typical steps of the process.

• Accept the right assignment: Accept invitations to review articles that align with your area of expertise to ensure you can provide well-informed feedback.
• Manage your time: Allocate sufficient time to thoroughly read and evaluate the manuscript, while adhering to the journal’s deadline for providing feedback.
• Read the manuscript multiple times: First, read the manuscript for an overall understanding of the research. Then, read it more closely to assess the details, methodology, results, and conclusions.
• Evaluate the structure and organization: Check if the manuscript follows the journal’s guidelines and is structured logically, with clear headings, subheadings, and a coherent flow of information.
• Assess the quality of the research: Evaluate the research question, study design, methodology, data collection, analysis, and interpretation. Consider whether the methods are appropriate, the results are valid, and the conclusions are supported by the data.
• Examine the originality and relevance: Determine if the research offers new insights, builds on existing knowledge, and is relevant to the field.
• Check for clarity and consistency: Review the manuscript for clarity of writing, consistent terminology, and proper formatting of figures, tables, and references.
• Identify ethical issues: Look for potential ethical concerns, such as plagiarism, data fabrication, or conflicts of interest.
• Provide constructive feedback: Offer specific, actionable, and objective suggestions for improvement, highlighting both the strengths and weaknesses of the manuscript. Don’t be mean.
• Organize your review: Structure your review with an overview of your evaluation, followed by detailed comments and suggestions organized by section (e.g., introduction, methods, results, discussion, and conclusion).
• Be professional and respectful: Maintain a respectful tone in your feedback, avoiding personal criticism or derogatory language.
• Proofread your review: Before submitting your review, proofread it for typos, grammar, and clarity.
• Couzin-Frankel J (September 2013). “Biomedical publishing. Secretive and subjective, peer review proves resistant to study”. Science . 341 (6152): 1331. doi: 10.1126/science.341.6152.1331
• Lee, Carole J.; Sugimoto, Cassidy R.; Zhang, Guo; Cronin, Blaise (2013). “Bias in peer review”. Journal of the American Society for Information Science and Technology. 64 (1): 2–17. doi: 10.1002/asi.22784
• Slavov, Nikolai (2015). “Making the most of peer review”. eLife . 4: e12708. doi: 10.7554/eLife.12708
• Spier, Ray (2002). “The history of the peer-review process”. Trends in Biotechnology . 20 (8): 357–8. doi: 10.1016/S0167-7799(02)01985-6
• Squazzoni, Flaminio; Brezis, Elise; Marušić, Ana (2017). “Scientometrics of peer review”. Scientometrics . 113 (1): 501–502. doi: 10.1007/s11192-017-2518-4

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• Knowledge Base

Methodology

• How to Write a Literature Review | Guide, Examples, & Templates

How to Write a Literature Review | Guide, Examples, & Templates

Published on January 2, 2023 by Shona McCombes . Revised on September 11, 2023.

What is a literature review? A literature review is a survey of scholarly sources on a specific topic. It provides an overview of current knowledge, allowing you to identify relevant theories, methods, and gaps in the existing research that you can later apply to your paper, thesis, or dissertation topic .

There are five key steps to writing a literature review:

• Search for relevant literature
• Evaluate sources
• Identify themes, debates, and gaps
• Outline the structure
• Write your literature review

A good literature review doesn’t just summarize sources—it analyzes, synthesizes , and critically evaluates to give a clear picture of the state of knowledge on the subject.

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Table of contents

What is the purpose of a literature review, examples of literature reviews, step 1 – search for relevant literature, step 2 – evaluate and select sources, step 3 – identify themes, debates, and gaps, step 4 – outline your literature review’s structure, step 5 – write your literature review, free lecture slides, other interesting articles, frequently asked questions, introduction.

• Quick Run-through
• Step 1 & 2

When you write a thesis , dissertation , or research paper , you will likely have to conduct a literature review to situate your research within existing knowledge. The literature review gives you a chance to:

• Demonstrate your familiarity with the topic and its scholarly context
• Develop a theoretical framework and methodology for your research
• Position your work in relation to other researchers and theorists
• Show how your research addresses a gap or contributes to a debate
• Evaluate the current state of research and demonstrate your knowledge of the scholarly debates around your topic.

Writing literature reviews is a particularly important skill if you want to apply for graduate school or pursue a career in research. We’ve written a step-by-step guide that you can follow below.

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Writing literature reviews can be quite challenging! A good starting point could be to look at some examples, depending on what kind of literature review you’d like to write.

• Example literature review #1: “Why Do People Migrate? A Review of the Theoretical Literature” ( Theoretical literature review about the development of economic migration theory from the 1950s to today.)
• Example literature review #2: “Literature review as a research methodology: An overview and guidelines” ( Methodological literature review about interdisciplinary knowledge acquisition and production.)
• Example literature review #3: “The Use of Technology in English Language Learning: A Literature Review” ( Thematic literature review about the effects of technology on language acquisition.)
• Example literature review #4: “Learners’ Listening Comprehension Difficulties in English Language Learning: A Literature Review” ( Chronological literature review about how the concept of listening skills has changed over time.)

You can also check out our templates with literature review examples and sample outlines at the links below.

Download Word doc Download Google doc

Before you begin searching for literature, you need a clearly defined topic .

If you are writing the literature review section of a dissertation or research paper, you will search for literature related to your research problem and questions .

Make a list of keywords

Start by creating a list of keywords related to your research question. Include each of the key concepts or variables you’re interested in, and list any synonyms and related terms. You can add to this list as you discover new keywords in the process of your literature search.

• Social media, Facebook, Instagram, Twitter, Snapchat, TikTok
• Body image, self-perception, self-esteem, mental health
• Generation Z, teenagers, adolescents, youth

Search for relevant sources

Use your keywords to begin searching for sources. Some useful databases to search for journals and articles include:

• Your university’s library catalogue
• Google Scholar
• Project Muse (humanities and social sciences)
• Medline (life sciences and biomedicine)
• EconLit (economics)
• Inspec (physics, engineering and computer science)

You can also use boolean operators to help narrow down your search.

Make sure to read the abstract to find out whether an article is relevant to your question. When you find a useful book or article, you can check the bibliography to find other relevant sources.

You likely won’t be able to read absolutely everything that has been written on your topic, so it will be necessary to evaluate which sources are most relevant to your research question.

For each publication, ask yourself:

• What question or problem is the author addressing?
• What are the key concepts and how are they defined?
• What are the key theories, models, and methods?
• Does the research use established frameworks or take an innovative approach?
• What are the results and conclusions of the study?
• How does the publication relate to other literature in the field? Does it confirm, add to, or challenge established knowledge?
• What are the strengths and weaknesses of the research?

Make sure the sources you use are credible , and make sure you read any landmark studies and major theories in your field of research.

You can use our template to summarize and evaluate sources you’re thinking about using. Click on either button below to download.

Take notes and cite your sources

As you read, you should also begin the writing process. Take notes that you can later incorporate into the text of your literature review.

It is important to keep track of your sources with citations to avoid plagiarism . It can be helpful to make an annotated bibliography , where you compile full citation information and write a paragraph of summary and analysis for each source. This helps you remember what you read and saves time later in the process.

To begin organizing your literature review’s argument and structure, be sure you understand the connections and relationships between the sources you’ve read. Based on your reading and notes, you can look for:

• Trends and patterns (in theory, method or results): do certain approaches become more or less popular over time?
• Themes: what questions or concepts recur across the literature?
• Debates, conflicts and contradictions: where do sources disagree?
• Pivotal publications: are there any influential theories or studies that changed the direction of the field?
• Gaps: what is missing from the literature? Are there weaknesses that need to be addressed?

This step will help you work out the structure of your literature review and (if applicable) show how your own research will contribute to existing knowledge.

• Most research has focused on young women.
• There is an increasing interest in the visual aspects of social media.
• But there is still a lack of robust research on highly visual platforms like Instagram and Snapchat—this is a gap that you could address in your own research.

There are various approaches to organizing the body of a literature review. Depending on the length of your literature review, you can combine several of these strategies (for example, your overall structure might be thematic, but each theme is discussed chronologically).

Chronological

The simplest approach is to trace the development of the topic over time. However, if you choose this strategy, be careful to avoid simply listing and summarizing sources in order.

Try to analyze patterns, turning points and key debates that have shaped the direction of the field. Give your interpretation of how and why certain developments occurred.

If you have found some recurring central themes, you can organize your literature review into subsections that address different aspects of the topic.

For example, if you are reviewing literature about inequalities in migrant health outcomes, key themes might include healthcare policy, language barriers, cultural attitudes, legal status, and economic access.

Methodological

If you draw your sources from different disciplines or fields that use a variety of research methods , you might want to compare the results and conclusions that emerge from different approaches. For example:

• Look at what results have emerged in qualitative versus quantitative research
• Discuss how the topic has been approached by empirical versus theoretical scholarship
• Divide the literature into sociological, historical, and cultural sources

Theoretical

A literature review is often the foundation for a theoretical framework . You can use it to discuss various theories, models, and definitions of key concepts.

You might argue for the relevance of a specific theoretical approach, or combine various theoretical concepts to create a framework for your research.

Like any other academic text , your literature review should have an introduction , a main body, and a conclusion . What you include in each depends on the objective of your literature review.

The introduction should clearly establish the focus and purpose of the literature review.

Depending on the length of your literature review, you might want to divide the body into subsections. You can use a subheading for each theme, time period, or methodological approach.

As you write, you can follow these tips:

• Summarize and synthesize: give an overview of the main points of each source and combine them into a coherent whole
• Analyze and interpret: don’t just paraphrase other researchers — add your own interpretations where possible, discussing the significance of findings in relation to the literature as a whole
• Critically evaluate: mention the strengths and weaknesses of your sources
• Write in well-structured paragraphs: use transition words and topic sentences to draw connections, comparisons and contrasts

In the conclusion, you should summarize the key findings you have taken from the literature and emphasize their significance.

When you’ve finished writing and revising your literature review, don’t forget to proofread thoroughly before submitting. Not a language expert? Check out Scribbr’s professional proofreading services !

This article has been adapted into lecture slides that you can use to teach your students about writing a literature review.

Scribbr slides are free to use, customize, and distribute for educational purposes.

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If you want to know more about the research process , methodology , research bias , or statistics , make sure to check out some of our other articles with explanations and examples.

• Sampling methods
• Simple random sampling
• Stratified sampling
• Cluster sampling
• Likert scales
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A literature review is a survey of scholarly sources (such as books, journal articles, and theses) related to a specific topic or research question .

It is often written as part of a thesis, dissertation , or research paper , in order to situate your work in relation to existing knowledge.

There are several reasons to conduct a literature review at the beginning of a research project:

• To familiarize yourself with the current state of knowledge on your topic
• To ensure that you’re not just repeating what others have already done
• To identify gaps in knowledge and unresolved problems that your research can address
• To develop your theoretical framework and methodology
• To provide an overview of the key findings and debates on the topic

Writing the literature review shows your reader how your work relates to existing research and what new insights it will contribute.

The literature review usually comes near the beginning of your thesis or dissertation . After the introduction , it grounds your research in a scholarly field and leads directly to your theoretical framework or methodology .

A literature review is a survey of credible sources on a topic, often used in dissertations , theses, and research papers . Literature reviews give an overview of knowledge on a subject, helping you identify relevant theories and methods, as well as gaps in existing research. Literature reviews are set up similarly to other  academic texts , with an introduction , a main body, and a conclusion .

An  annotated bibliography is a list of  source references that has a short description (called an annotation ) for each of the sources. It is often assigned as part of the research process for a  paper .  

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How to... Understand the peer review process

The peer review process followed depends on the channel the author chooses for their research. We highlight the two models used by our journals, cases, and open research platform.

On this page

The peer review process, peer review models, our approach to article peer review.

Download and keep your step-by-step guide (PDF), or view the accessible version of the flowchart below.

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Accessible version of content

Peer review process .

Submission 

1. The editor rejects the submission if it doesn’t meet the journal’s editorial objectives. 

2. The editor selects up to three reviewers and asks them to evaluate the manuscript against a set of criteria. 

3. The editor receives the reviewers’ recommendations.

4. The editor makes the final decision, taking into account the reviewers’ recommendations. Go to 5 if accepted, 6 if rejected, or 7 if revisions are required.

Accept 

5. The editor notifies the author that their paper has been accepted > Publish

6. The editor notifies the author that their paper has been rejected and shares a copy of the reviewers’ comments > Reject

Revise 

7. The editor notifies the author that their paper requires revisions and shares a copy of the reviewers’ comments.

8. The author resubmits their revised paper.

9. The editor can either make a decision based on the revised paper or send the revised paper to the same reviewers. Go to 3.

Single-anonymous peer review

The names of the reviewers are hidden from the author. However, the name of the author is shared with the reviewers.

The fact that reviewers remain anonymous means they can speak honestly and impartially. Meanwhile, knowledge of an author’s identity can help reviewers place an article in the context of the author’s earlier work.

Double-anonymous peer review

The reviewers aren’t told the name of the author, and the author never learns the names of the reviewers.

Outside of the triple-anonymous model (see below), this is the surest way to ensure that the process is completely objective.

The focus remains on the content of the article, and the possibility of reviewer bias is eliminated. Reviewer bias may be favourable or unfavourable, conscious or unconscious.

Triple-anonymous peer review

The identities of the author, reviewers and editors remain hidden from each other. The author is usually identified only by a number and communication takes place through a website or submission system. This eliminates any potential bias.

Open peer review

This can vary in form. It may be as simple as making the author and reviewers known to one another, or the reviews – and the reviewers’ names – may be published alongside the article. The review process may take place pre- or post-publication, and reports may receive their own DOIs, making them discoverable and citable.

This offers complete transparency. Some believe that the knowledge that reports are going to be published encourages reviewers to produce higher-quality reports overall. The post-publication format publicly recognises the important work of the reviewers.

The majority of our journals have adopted a double-anonymous peer review model, with reviewers invited by the journal editor.

Quality peer review is constructive, non-confrontational and prompt. It means putting yourself in the position of the author and helping them to bring out the best in their paper.

Related topics

Reviewer hub.

Here you will find practical tips and guidance on all aspects of peer review. 

Become a reviewer

Whether this is your first time reviewing or you are a seasoned professional, we explain why you should say yes next time an editor asks you to review.

Reviewer guidelines

Our reviewers play a crucial role in the publication process with a wide range of responsibilities. We have developed some reviewer guidelines to support you at each stage of the process.

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A step-by-step guide to peer review: a template for patients and novice reviewers

1 General Medicine and Primary Care, Beth Israel Deaconess Medical Center, Boston, Massachusetts, USA

Charlotte Blease

2 Beth Israel Deaconess Medical Center, Boston, Massachusetts, USA

3 Harvard Medical School

While relatively novel, patient peer review has the potential to change the healthcare publishing paradigm. It can do this by helping researchers enlarge the pool of people who are welcome to read, understand and participate in healthcare research. Academic journals who are early adopters of patient peer review have already committed to placing a priority on using person-centred language in publicly available abstracts and focusing on translational and practical research.

A wide body of literature has shown that including people with lived experiences in a truly meaningful way can improve the quality and efficiency of health research. Traditionally considered only as ‘subjects’ of research, over the last 10–15 years, patients and care partners have increasingly been invited to contribute to the design and conduct of studies. Established institutions are increasingly recognising the distinctive expertise patients possess—many patients have acquired deep insights about their conditions, symptoms, medical treatments and quality of healthcare delivery. Among some funders, including the views of patients is now a requirement to ensure research proposals are meaningful to persons with the lived experience of illness. Further illustrating these developments, patients are now involved in reviewing and making recommendations as part of funding institutions, setting research agendas and priorities, being funded for and leading their own research and leading or coauthoring scholarly publications, and are now participating in the peer review process for academic journals. 1–5 Patients offer an outsider’s perspective within mainstream healthcare: they have fewer institutional, professional or social allegiances and conflicts of interest—factors recognised as compromising the quality of research. Patient involvement is essential to move away from rhetorical commitments to embrace a truly patient-centred healthcare ecosystem where everyone has a place at the table.

As people with lived health experiences climb a ladder of engagement in patient–researcher partnerships, they may be asked to act as peer reviewers of academic manuscripts. However, many of these individuals do not hold professional training in medicine, healthcare or science and have never encountered the peer review process. Little guidance exists for patients and care partners tasked with reviewing and providing input on manuscripts in search of publication.

In conversation, however, even experienced researchers confess that learning how to peer review is part of a hidden curriculum in academia—a skill outlined by no formal means but rather learnt by mimicry. 6 As such, as they learn the process, novices may pick up bad habits. In the case of peer review, learning is the result of reading large numbers of academic papers, occasional conversations with mentors or commonly “trial by fire” experienced via reviewer comments to their own submissions. Patient reviewers are rarely exposed to these experiences and can be at a loss for where to begin. As a result, some may forgo opportunities to provide valuable and highly insightful feedback on research publications. Although some journals are highly specific about how reviewers should structure their feedback, many publications—including top-tier medical journals—assume that all reviewers will know how to construct responses. Only a few forward-thinking journals actively seeking peer review from people with lived health experiences currently point to review tips designed for experienced professionals. 7

As people with lived health experiences are increasingly invited to participate in peer review, it is essential that they be supported in this process. The peer review template for patients and novice reviewers ( table 1 ) is a series of steps designed to create a workflow for the main components of peer review. A structured workflow can help a reviewer organise their thoughts and create space to engage in critical thinking. The template is a starting point for anyone new to peer review, and it should be modified, adapted and built on for individual preferences and unique journal requirements. Peer reviews are commonly submitted via website portals, which vary widely in design and functionality; as such, reviewers are encouraged to decide how to best use the template on a case-by-case basis. Journals may require reviewers to copy and paste responses from the template into a journal website or upload a clean copy of the template as an attachment. Note: If uploading the review as an attachment, remember to remove the template examples and writing prompts .

Peer review template for patients and other novice reviewers

It is important to point out that patient reviewers are not alone in facing challenges and a steep learning curve in performing peer review. Many health research agendas and, as a result, publications straddle disciplines, requiring peer reviewers with complementary expertise and training. Some experts may be highly equipped to critique particular aspects of research papers while unsuited to comment on other parts. Curiously, however, it is seldom a requirement that invited peer reviewers admit their own limitations to comment on different dimensions of papers. Relatedly, while we do not suggest that all patient peer reviewers will be equipped to critique every aspect of submitted manuscripts—though some may be fully competent to do so—we suggest that candour about limitations of expertise would also benefit the broader research community.

As novice reviewers gain experience, they may find themselves solicited for a growing number of reviews, much like their more experienced counterparts or mentors. 8 Serving as a patient or care partner reviewer can be a rewarding form of advocacy and will be crucial to harnessing the feedback and expertise of persons with lived health experiences. As we move into a future where online searches for information are a ubiquitous first step in searching for answers to health-related questions, patient and novice reviewers may become the much-needed link between academia and the lay public.

Acknowledgments

LS thanks the experienced and novice reviewers who encouraged her to publish this template.

Twitter: @TheLizArmy, @@crblease

Contributors: Both authors contributed substantially to the manuscript. LS conceived the idea and design and drafted the text. CB refined the idea and critically revised the text.

Funding: The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

Competing interests: The authors have read and understood the BMJ policy on declaration of interests and declare the following interests: LS is a member of the BMJ Patient Advisory Panel, serves as a BMJ patient reviewer and is an ad hoc patient reviewer for the Patient-Centered Outcomes Research Institute; CB is a Keane OpenNotes scholar; both LS and CB work on OpenNotes, a philanthropically funded research initiative focused on improving transparency in healthcare.

Provenance and peer review: Commissioned; externally peer reviewed.

Ethics statements

Patient consent for publication.

Not required.

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Selective laser sintering of polymers: process parameters, machine learning approaches, and future directions.

1. Introduction

2. principle of sls, 3. applications of sls, 4. materials fabricated by sls, 5. challenges associated with fabricating polymers using sls technology [ 26 , 33 , 38 , 39 ], 6. sls parameters, 6.1. laser power, 6.2. scanning speed, 6.3. layer thickness, 6.4. hatch spacing, 6.5. preheating temperature, 6.6. powder particle size distribution, 6.7. part bed temperature (t b ), 6.8. thermal distribution, 6.9. powder morphology, 7. porosity in sls parts, 8. machine learning ml for sls, 8.1. overview, 8.2. ml-based monitoring methods.

Click here to enlarge figure

8.3. Process Optimization

8.4. defect detections, 9. conclusions.

• The quality and properties of SLS-produced parts are heavily influenced by numerous process parameters. Key factors include laser power, scanning speed, layer thickness, hatch spacing, preheating temperatures, powder particle size distribution, and part bed temperatures. Careful optimization of these parameters is crucial for achieving desired part qualities such as density, strength, and surface finish.
• Laser power has a direct impact on density and hardness, with both properties increasing as laser power increases. Scan spacing is a crucial parameter that significantly impacts both the density and hardness of the printed parts. Experimental findings reveal that an increase in scan spacing leads to a decrease in both density and hardness. Consequently, to achieve optimal performance in these metrics, it is recommended to use a low scan spacing value, such as 0.1 mm.
• The relationship between bed temperature and hardness is linear, with hardness continuously increasing as bed temperatures increase. However, the effect of bed temperature on density is more complex, with density initially increasing and then decreasing as bed temperatures further increase.
• Scan count has a positive correlation with density, with higher scan counts resulting in increased density. However, the effect of scan count on hardness is dependent on bed temperature. At lower scan counts, increasing bed temperatures lead to increased hardness, while at higher scan counts (e.g., 2), the trend reverses, with hardness decreasing as bed temperatures increase.
• By leveraging the power of ML and optimizing the process parameters, SLS has the potential to revolutionize the manufacturing industry, enabling the production of high-quality, customized parts with improved efficiency and sustainability.
• ML techniques have demonstrated high accuracy in defect detection for SLS processes. For instance, the VGG16 convolutional neural network model achieved impressive metrics for powder bed defect classification, including accuracy (0.958), precision (0.939), recall (0.980), F1-score (0.959), and AUC value (0.982) [ 133 ].
• The challenges in applying ML to SLS are significant and quantifiable. Data availability is a major issue, with many ML studies on SLS relying on limited datasets, often ranging from a few hundred to a few thousand data points. This highlights the need for larger, more diverse datasets to improve model accuracy and generalizability.
• The development of high-performance polymers and composite materials.
• Focusing on increasing print speed and building larger parts or assemblies.
• The integration of SLS with other manufacturing technologies, such as robotic arms and automation systems, to enhance its capabilities and applications.

Author Contributions

Data availability statement, acknowledgments, conflicts of interest.

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Yehia, H.M.; Hamada, A.; Sebaey, T.A.; Abd-Elaziem, W. Selective Laser Sintering of Polymers: Process Parameters, Machine Learning Approaches, and Future Directions. J. Manuf. Mater. Process. 2024 , 8 , 197. https://doi.org/10.3390/jmmp8050197

Yehia HM, Hamada A, Sebaey TA, Abd-Elaziem W. Selective Laser Sintering of Polymers: Process Parameters, Machine Learning Approaches, and Future Directions. Journal of Manufacturing and Materials Processing . 2024; 8(5):197. https://doi.org/10.3390/jmmp8050197

Yehia, Hossam M., Atef Hamada, Tamer A. Sebaey, and Walaa Abd-Elaziem. 2024. "Selective Laser Sintering of Polymers: Process Parameters, Machine Learning Approaches, and Future Directions" Journal of Manufacturing and Materials Processing 8, no. 5: 197. https://doi.org/10.3390/jmmp8050197

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Ammonia inhibition in anaerobic digestion of organic waste: a review

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• Published: 09 September 2024

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• J. Yang 1 , 2 ,
• J. Zhang 1 ,
• Z. Cheng 1 ,
• W. Fu 1 &
• S. Wang 1  

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Anaerobic digestion (AD) has become the technology of choice for organic waste treatment as an environmentally beneficial and sustainable waste treatment technology. However, the nitrogen content of these organic waste streams is generally high. Ammonia is produced in the biodegradation of nitrogenous organic matter. Low concentrations of ammonia favour AD, but high concentrations can lead to digestive system failure. To address the issue of ammonia inhibition and ensure the stability of the digestive system, numerous physical, chemical, and biologicalmethods aimed at controlling ammonia levels and/or strengthening the biological processes have been proposedand developed. Literature evidence suggests that differences in AD reaction conditions and microbial sources result in different tolerances of the digestive system to ammonia and nitrogen. This paper summarises and compares the inhibitory effects of ammonia nitrogen under different conditions and the existing regulatory measures to alleviate ammonia nitrogen inhibition. In addition, since the core of the digestive system is microorganisms, this paper explains the mechanism of ammonia stress especially at the microbial level, and in this way, it explores the future direction of research using biofortification. This review provides a theoretical reference for solving the problem of ammonia nitrogen inhibition.

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Introduction

As the global population continues to grow and the economy develops at a rapid pace, municipal solid waste (MSW) generation has surged worldwide. Currently, the global generation of MSW is about 2.01 billion tonnes per year (ranging from 0.11 to 4.54 kg/capita/day), but it is projected to increase to 3.40 billion tonnes by 2050 (Rosas-Mendoza et al. 2024 ). Most of this MSW is openly wasted and very little of it is converted into useful resources. An estimated 93% of waste is either dumped or incinerated, predominantly in less developed countries (Khurram et al. 2024 ). Organic waste (OW) accounts for 42–69 percent of MSW. These include crop residues, animal manure, landscaping waste, municipal sludge, animal inclusions from slaughterhouses, food waste, etc. (Pongsopon et al. 2023 ; Khurram et al. 2024 ). Inadequate management of organic waste can lead to various environmental hazards and even threaten human health. It is estimated that about 5% of total greenhouse gas emissions are due to open dumping of OW (Sailer et al. 2021 ).

How to properly deal with organic waste has become an important issue for human beings to solve at present, the current conventional treatment technology includes measures such as crushing direct discharge, landfill, incineration and so on (Guo et al. 2023 ) but faces the problem of secondary pollution from siltation and blockage of urban sewage network, leachate leakage and dioxin and greenhouse gas emissions. Anaerobic digestion of organic waste has received widespread attention as an eco-friendly and economically viable method. the technology has been applied to treat a variety of wastes, including municipal sludge, poultry manure, food/fermentation industry wastes, and concentrated municipal wastewater. (Tiwari et al. 2023 ; Rivera et al. 2023 ; Paranjpe et al. 2023 ).As of 2017, there were at least 118 kitchen waste treatment projects in China with a scale of 50 t/d or more, of which 76.1% used anaerobic digestion technology. From 2009 to 2020, the number of anaerobic digestion plants in Europe increased from 6,227 to 19,000 (Zhang et al. 2023 ). Simply put, anaerobic digestion relies on the synergistic action of hydrolysing acidifying microorganisms and methanogenic microorganisms, a process that converts organic matter into biogas, biogas residue and a carbon-rich fermentation broth. As the technology continues to mature, it has become a key technology for reducing organic waste, recovering biomass energy, and producing biofuels and energy (O'Connor et al. 2021 ). Research indicates that AD is highly effective for treating and managing OW (Granzotto et al. 2021 ).

Ammonia inhibition is a challenging issue in the AD of nitrogen-rich substrates(e.g.food waste and animal waste) and hindersthe energy recovery from organic wastes. Ammonia is produced by the biodegradation of nitrogenous organic matter in organic waste. The low concentrations of ammonia favor AD, but high concentrations of ammonia can lead to digestive system failure (Li et al.. 2023 ; O'Connor et al. 2023 ). The two main forms of total ammonia nitrogen (TAN) are ammonium ions (NH 4 + ) and free ammonia (NH 3 , FAN), both of which can directly or indirectly cause inhibition in AD systems (Lendormi et al. 2022 ; Mlinar et al. 2022 ). The relationship between the two is expressed in Eq. ( 1 ) (Xiao et al. 2022a ). Typically, FAN is considered the primary cause of inhibition because hydrophobic FAN molecules may passively diffuse through cellular membranes, leading to proton imbalance and/or potassium deficiency (Shi et al. 2017 ). Additionally, FAN enters microbial cells by passive diffusion and is subsequently converted to NH4 + through binding with extracellular protons (H + ), resulting in alterations in intracellular pH. To maintain intracellular proton homeostasis, cells actively transport potassium ions out of the cell via energy-consuming potassium pumps in the cell membrane. This process increases the energy required for cellular maintenance and limits certain specific enzymatic reactions(Mlinar et al. 2022 ; Peng et al. 2023a ). Various inhibitory thresholds of total ammonia nitrogen (TAN) concentrations, ranging from 3.4 to 5.77 gL −1 , have been reported, resulting in severe methane yield losses in the AD process ranging from 39 to 100% (Li et al. 2023 ). These outcome variations can be attributed to differences in temperature, reactor configuration, and the microbial communities that develop in distinct systems [Li et al. 2023 ].

Recent studies have focused on regulatory strategies to mitigate ammonia inhibition during anaerobic digestion (AD) of nitrogenous organic wastes. Various physical, chemical, and biological methods have been developed to control ammonia levels and enhance biological processes. Examples include substrate dilution, adjustment of the carbon to nitrogen ratio, pH control and ammonia recovery through membrane distillation, in addition, other methods have been tested to enhance the biological process including bioaugmentation and domestication(Jo et al. 2022 ; Wang et al. 2023 ); addition of various support materials such as activated carbon and magnetite (Li et al. 2023 ); and provision of trace elements and use of blowdown processes (Meng et al. 2020 ; Pedizzi et al. 2017 ). However, few articles have discussed and summarised the mechanism of ammonia inhibition in the AD process of nitrogenous organic wastes at the microbial level. Fundamentally, the AD process is a multi-stage, multi-level biochemical process that is mainly influenced by microorganisms, which are the core of the AD system(Li et al. 2017 ). It has been found that certain microorganisms are resistant to ammonia nitrogen inhibition during AD. However, coherent and targeted regulatory mechanisms to alleviate ammonia nitrogen inhibition remain elusive due to the complexity and variability of substrate properties, microbial sources and reaction conditions. To address ammonia inhibition and ensure digestive system stability, it is imperative to understand the extent of ammonia inhibition under various conditions, investigate the mechanisms and patterns of ammonia inhibition, and develop potential future regulatory strategies (Qian et al. 2017 ).

Based on this, this paper firstly describes the ammonia inhibition under different temperature, pH and reactor conditions. And the study of process intensification to implement ammonia inhibition mitigation in AD systems is described in detail. In addition, since the digestion process is mainly dominated by microorganisms, this paper focuses on analysing the causes and patterns of ammonia inhibition in AD from a microbial perspective. Finally, this paper summarises the current strategies and measures for mitigating ammonia inhibition in AD of nitrogenous organic wastes and looks forward to future research directions. This review aims to provide theoretical guidance for mitigating ammonia inhibition in AD.Fig.  1 describes the structure of this review.

Technology Roadmap

Inhibitory effect of ammonia nitrogen on AD process under different conditions

Although ammonia nitrogen promotes the growth of microorganisms at certain concentrations, it can exceed a threshold during substrate degradation and become toxic to microorganisms. Specifically, Anaerobic microorganisms are favorable to AD at ammonia concentrations of 50 ~ 200 mg/L, experience no antagonistic effects at 200 ~ 1000 mg/L, and are inhibited at 1500 ~ 3000 mg/L, especially in high pH anaerobic systems. Moreover, When ammonia nitrogen exceeds 3000 mg/L, microorganisms are inhibited to varying degrees in the AD process under any pH condition (Sung et al. 2003; Procházka et al. 2012b ). However, differences in temperature, reactor configuration, and the ammonia nitrogen inhibition levels tolerated by microbial communities in different systems make it impossible to accurately define thresholds. Table 1 lists the extent of ammonia nitrogen inhibition by each type of substrate under different conditions (Alsouleman,K 2019 ).

Different reactor types

Conventional AD systems are mainly categorized into single-phase and two-phase systems. In single-phase AD systems, all four AD steps occur concurrently within one reactor, while in two-phase AD systems, the acidification and methanization phases are separated into two tandem reactors (Ren et al. 2018 ; Shen et al. 2013 ), thus providing suitable conditions for the survival of acid-producing bacteria and methanogenic archaea. However Single-phase reactors lead to an increase in system loading because each reaction occurs in the same reactor. The rapid degradation of nitrogen-containing organic matter in a short period results in a swift increase in ammonia nitrogen content, which inhibits the activity of methanogenic bacteria and affects subsequent reactions (Bouallagui et al. 2009 ). For the same reactor, different feeding methods result in varying resistance to ammonia inhibition. For instance, Tian et al. ( 2017 ) evaluated three reactors (batch, fed-batch, and continuous) operating at medium (37 °C) and high (55 °C) temperatures and found that the batch-fed reactor had twice the free ammonia concentration of the batch reactor and that the continuously stirred reactor was inhibited at lower ammonia levels. In contrast, The two-phase reactor has the advantage of buffering the load in the first stage, allowing a more stable feed rate into the second stage for methane production. Ding et al. ( 2021 ) investigated the feasibility of a two-stage system to digest high-solids food waste as the sole feedstock in long-term operation. Compared to a single-stage system, the two-stage system had a 33.3% increase in food waste load, an 18.2% increase in energy yield, and was more resistant to ammonia nitrogen inhibition. However, the increase in system load can also lead to instability in the two-phase system. Ganesh et al. ( 2014a ) conducted a comparative study between single-phase and two-phase digestion of fruit and vegetable wastes and found that the two-phase system showed instability with lower methane and energy yields when the system load reached 7.0 kg VS/m 3 d. Since high concentrations of ammonia nitrogen can inhibit the digestion system and limit its organic load, future research should focus on reactor design to increase the organic load and avoid ammonia nitrogen inhibition (Nasr et al. 2012 ; Shen et al. 2013 ; Christou et al. 2021 ; Adghim et al. 2022 ). Figure  2 depicts the suppression of ammonia nitrogen under different conditions.

Suppression of ammonia nitrogen under different conditions

Different pH

The pH of an anaerobic fermentation system affects both microbial activity and ammonia nitrogen concentration. Under normal reactor operation, if the digested substrate is mainly composed of proteins, the pH of the digestion reactor is generally high. However, When the system pH is elevated, the transition of ammonia nitrogen to free state NH 3 will further affect the activities of anaerobic microorganisms, thus causing the accumulation of volatile fatty acids (VFAs) (Zhang et al. 2018 ; Park et al. 2018 ). At pH 6.5–8.5, the free ammonia content in the system increases by 3–18 times with a pH increase of 0.6–1.3. Consequently, after the accumulation of VFAs exceeds the buffer threshold, the system's pH will decrease significantly, resulting in system destabilization (Tian et al. 2019 ). For example, Ho et al. ( 2012 ) increased the biogas production of the system from 200 mL/L at pH 8.3 to 680 mL/L at pH 6.5 by adjusting the pH of the AD reactor feed, a 2.4-fold increase. Therefore, controlling the system pH is key to alleviating ammonia and nitrogen inhibition and maintaining the smooth operation of AD.

Different temperatures

Temperature is a significant factor affecting the ammonia threshold of the digestion system, as it isdirectly related to the microbial growth rate and free ammonia concentration in the digestion system (Ye et al. 2022 ; Liu et al. 2024 ). Medium and high temperature digestion each have their own advantages and disadvantages. Specifically, medium-temperature AD is cost-effective, has slow ammonia nitrogen accumulation, and higher biogas purity, but the biogas production rate is slower, and the tolerance limit for ammonia nitrogen is lower than that of high temperature digestion. Additionally, it also cannot effectively kill pathogens in the digestive system. High-temperature digestion is faster and more productive due to more complete degradation of raw materials, and the high temperature can effectively kill pathogens in the system, which is conducive to the secondary utilization of digestate. However, the methane content of the gas produced is lower, and ammonia nitrogen accumulates faster, leading to inhibition. Kim et al. ( 2011 ) found that the degree of protein destruction was higher under thermophilic conditions, and ammonia nitrogen content was higher in thermophilic phases due to protein degradation at increased organic loading rates (OLR). Furthermore, when the temperature increases, it enhances microbial metabolism, and the free ammonia content in the system rises consequently, increasing the ammonia nitrogen content in the system (Massé. et al. 2014 ; Angelidaki and Ahring 1993 ). For high ammonia loading digesters, the low-temperature AD process tends to have less ammonia inhibition and is found to be more stable than high/moderate temperatures. High-temperature digestion (operating temperature: 50 °C ~ 55 °C) is more susceptible to ammonia inhibition than moderate temperature digestion (operating temperature: 35 °C ~ 40 °C), leading to an unstable digestion system (Montecchio et al. 2017 ). Li et al. ( 2022 ) investigated the effects of bio-reinforcement on ammonia nitrogen in the digestive system at moderate and high temperatures using sludge as the substrate to explore the mitigation of ammonia inhibition by bio-augmentation. The results showed that methane production decreased by about 21% and 28% under medium and high temperature conditions, respectively, and thermophilic microorganisms responded more strongly to ammonia inhibition.

Microbial domestication

Different inoculated microorganisms have varying levels of tolerance to ammonia nitrogen, and microorganisms domesticated with high concentrations of ammonia nitrogen can improve their resistance to it. Studies have shown (Yenigün and Demirel 2013 ; Christou et al. 2021 ) that domestication of the microbial community in AD systems by progressively increasing the ammonia levels can increase the community's tolerance to ammonia. Specifically, inoculation with undomesticated microorganisms completely inhibited the digestion process when the system TAN concentration reached1700 ~ 1800 mg/L, while inoculation with domesticated microorganisms increased the inhibition threshold of TAN concentration to 5000 mg/L. Corresponding to a FAN concentration of 256 mg/L, the acid production process remained stable, indicating that the digester could still operate stably under low methane production conditions. However, Complete inhibition was observed when the TAN concentration reached 6700 mg/L. The effectiveness of biofortification is significantly influenced by the microbial composition of consortia. Wang et al. ( 2023 ) obtained two microbial consortia (syntrophic microbial consortium, MC, and hydrogenotrophic methanogen consortium, SS) by pure culture domestication and applied them to a nitrogen-enriched AD system (TAN concentration > 8 g/L). The results indicated that AD performance was restored within 21 days for the MC treatment and 83 days for the SS treatment. Although domestication of microorganisms is an effective method for resisting ammonia inhibition, it should be noted that domestication at high ammonia concentrations may cause irreversible damage to the microbial community structure (Nie et al. 2015 ; Zhang et al. 2022 ; Poirier et al. 2017 ).

To summarize, in the practical application of AD, two-phase reactors stand out due to their strong buffering capacity. Nevertheless, when the system load continuously increases, two-phase reactors can be inhibited. In engineering applications, the system load can be equalized by choosing an intermittent feeding method to ensure stable reactor operation. The selection of temperature and pH should focus on the appropriate range for microorganisms in the digestive system; Specifically, medium and high temperatures enhance microbial activity, thereby increasing the free ammonia concentration in the system, which leads to higher ammonia–nitrogen concentrations. Furthermore, changes in pH indicate variations in ammonia nitrogen and volatile acid concentrations in the system. Therefore, to prevent ammonia nitrogen inhibition, pH adjustment should be carried out based on low-temperature digestion. To further enhance the system's tolerance to ammonia nitrogen, inoculation with microorganisms that have been domesticated with high ammonia nitrogen concentrations can be considered. Within a certain range, as the ammonia nitrogen concentration in the system gradually increases,microorganisms can adapt to higher ammonia nitrogen environments, thus resisting ammonia nitrogen inhibition.

Ammonia Stress Mechanism

High ammonia nitrogen concentrations are an important factor contributing to the instability of AD. The bacterial flora is closely related to the operational efficiency and methane production rate of AD. The system includes hydrolysis-acidification bacteria, hydrogen-producing acetogens, acetotrophic methanogenic archaea, and hydrogenotrophic methanogenic archaea (Yang et al. 2018 ; Tian et al. 2018b ). Specifically, high ammonia concentrations affect the structure of the bacterial population in the AD system, thereby reducing the efficiency of AD. For instance, it has been demonstrated that the microbial community within the AD system responds to high concentrations of ammonia nitrogen, with methanogens being more sensitive to ammonia stress than hydrolysis-acidification bacteria and hydrogen-producing acetogens. Under nitrogen stress, the pathway of methane production is altered, and the dominant community in the system shifts from acetotrophic methanogenic archaea to hydrogenotrophic methanogenic archaea (Wang et al. 2022 ). Figure  3 shows the mechanism of ammonia inhibition in the AD system.

Mechanism of ammonia inhibition in AD system

Effect of ammonia on VFAs accumulation in AD systems

When the anaerobic system is destabilized by ammonia inhibition, both inhibition of acid secretion and the accumulation of large amounts of short-chain fatty acids, such as propionic acid and butyric acid, are often observed. However, the inhibitory effect of high nitrogen stress on acetogenic bacteria is selective, affecting the overall abundance of acetogenic bacteria such as digestive Enterobacteriaceae of the genus Pelotomaculum, desulphurizing Enterobacteriaceae of the genus Desulfotomaculium, and acid-producing hydroxyacetic acid bacteria of the genus Pelotomaculum. High nitrogen stress also resulted in a reduction in the genera Syntrophomonas and Syntrophus, as well as the overall abundance of the phylum Ascomycetes. The relative abundance of Desulfovibrio, a phylum of Proteobacteria, has also decreased. According to the literature, propionate and butyrate are directly utilized by Desulfovibrio, which converts them into acetate (Chen et al. 2016 ). Therefore, high ammonia concentrations significantly reduce the abundance of propionate and butyrate oxidizing microorganisms and their accessory bacteria.

Changes in the efficiency of the AD process and microbial population dynamics in food waste were examined by Peng et al. ( 2018 ). The study found that higher ammonia nitrogen concentrations significantly reduced Methanosaeta species abundance and inhibited acetic acid metabolism in the AD system. The accumulation of acetic acid inhibits the degradation of other volatile fatty acids, such as propionic acid and valeric acid, leading to a complete disruption of the entire AD metabolic network and possibly the collapse of the AD reactor. Niu et al. ( 2015a ) found that high ammonia concentration increased the diversity of bacterial communities and enriched acid-producing bacteria. Furthermore, throughout the entire AD process, hydrogenotrophic methanogenic archaea predominated, and the lack of acetotrophic methanogenic archaea led to the accumulation of acetic acid and other volatile fatty acids. This accumulation resulted in a reduction in methane production and destabilization of the reactor. It is evident that hydrogenotrophic methanogenic archaea are more resistant to ammonia stress than acetotrophic methanogenic archaea, and blocking acetic acid metabolism is the main cause of AD system destabilization (Ziganshin et al. 2013 ). In addition, Among the microorganisms that metabolize acetic acid, Methanosaeta is the most sensitive to ammonia, with its activity inhibited when TAN exceeds 2000 mg/L. In contrast, Methanosarcina and syntrophic acetate-oxidizing bacteria (SAOB) are more tolerant to ammonia. Thus,the introduction of domesticated Methanosarcina and SAOB into the system can restore system stability.

Effects of ammonia nitrogen on the dominant bacterial species in the AD system

In recent years, many researchers have attempted to introduce ammonia–nitrogen interference into reactors to study the microbial community structure, dominant bacterial species, and metabolic functions of AD flora under nitrogen stress conditions. For example, Ruiz-Sánchez et al. ( 2018 ) studied bacteria and archaea in AD systems under ammonia stress. They assessed diversity and found that ammonia had different effects on microbial diversity in different groups and conflicting effects on microbial diversity for each metabolic function. By comparing differences in the number of dominant microorganisms in AD reaction systems operating at four ammonia concentrations, they found that the dominant acidifying hydrolysing bacteria were from the genera Sporocytophaga and Nitratalea in AD reaction systems with TAN < 3000 mg/L. Similarly, Buhlman et al. ( 2019 ) showed that the dominant methanogenic pathway shifted from acetotrophic methanogens to synthetic trophic acetate oxidative-hydrogenotrophic methanogens when ammonia nitrogen concentration increased to 6133–8366 mg/L (Wang et al. 2015 ; Lee et al. 2021 ). In addition, the results of Niu et al. ( 2015a ) showed that bacterial communities subjected to ammonia inhibition mainly exhibited resistant or redundant traits. During ammonia inhibition, the dominant bacterial species were replaced to ensure that the gas production efficiency of the anaerobic digestion (AD) system was restored.

Influence of ammonia nitrogen on the activity of intracellular enzymes of microorganisms

During the AD process, the biotransformation of organic substrates is facilitated by various enzymes within microbial cells, as illustrated in Fig.  3 . Recent studies have increasingly focused on the role of microbial enzymes during anaerobic fermentation under ammonia stress. Using integrated multi-omics analysis, Zhang et al. ( 2022 ) demonstrated that ammonia stress in the AD system significantly reduces the expression activity of methyl coenzyme M reductase in methanogenic filamentous bacteria, thereby inhibiting the conversion of acetate to methane (Yan et al. 2020 ). In addition, Ammonia also inhibits the methylmalonyl-CoA (MMC) pathway in Pelotomaculum by suppressing the expression of succinyl coenzyme A synthetase, leading to the inhibition of propionate oxidation. Acetic acid metabolism is particularly sensitive to ammonia stress in AD. High ammonia nitrogen concentrations inhibit the expression of methyl coenzyme M reductase in methanogenic filamentous bacteria, thereby inhibiting acetic acid metabolism. However, continuous ammonia stress shifts the dominant acetate-degrading microorganisms to methanotrophic octococci with higher ammonia tolerance. The microbial community can be continuously optimized to restore acetate metabolism through the acetate-methane (AM) pathway, facilitated by enzymes such as acetic acid kinase, phosphoacetyltransferase, and deaminase (Poirier et al. 2016 ).

Under ammonia stress, propionic acid accumulates in the system, significantly reducing the rate of methane production. The abundance of propionate-oxidizing bacteria (SPOB) decreases, along with the expression of methane-related enzymes that degrade propionate, indicating that propionate metabolism is highly inhibited and more sensitive to ammonia stress. Therefore, SPOBs are crucial in influencing the function of the AD system under ammonia stress. They inhibit propionate metabolism by suppressing the expression of succinyl coenzyme A synthetase and the conversion of methylmalonyl coenzyme A. Wang et al. ( 2023 ) found that the accumulation of short-chain fatty acids, such as propionic acid, affects the activity of methanogenic bacteria, indirectly leading to system destabilization. Excess free ammonia reduces the number and activity of filamentous methanogenic bacteria of the genus Methanosaeta, inhibiting acetate metabolism and leading to reactor destabilization. Excess long-chain fatty acids (LCFAs) in the system create a synergistic inhibitory effect with ammonia. The relative abundance of Petrimonas and Paraclostridium species decreases, suggesting that high ammonia concentrations inhibit the β-oxidation of LCFAs (Wu et al. 2019 ). The abundance of enzymes such as acetate kinase, phosphopyruvate acetyltransferase, pyruvate synthase, phosphopyruvate hydratase, phosphoglycerate dehydrogenase, and glycine hydroxymethyltransferase, which are associated with acetic acid dehydroxylation, is significantly reduced. This suggests that high concentrations of nitrogen and ammonium primarily inhibit the oxidation reaction step of methane under synergistic inhibition conditions (Capson-Tojo et al. 2020 ; Peng et al. 2023b ; Yu et al. 2021 ).

Effect of ammonia on microbial cell morphology

The effect of free ammonia on AD performance was investigated by Calli et al ( 2005 ).They found that the cluster structure of Methanosarcina Methanoctococcus spp. was significantly decomposed at free ammonia concentrations greater than 700 mg/L. Furthermore, scanning electron microscope observations revealed obvious wrinkles, deformations, and cracks on the surface of the cells, indicating the destruction of cellular integrity (Park et al. 2018 ). In contrast, cells in the blank control group were intact in shape, tightly packed, and had smooth surfaces. The breakdown of cellular integrity leads to the loss of protective functions, which in turn results in cell damage, lysis, and death (Yan et al. 2019 ). Therefore, this study suggests that the effect of high concentrations of ammonia nitrogen on microbial cell activity and morphological structure is a crucial factor in ammonia inhibition. Different species of methanogenic bacteria exhibit varying tolerance to ammonia nitrogen, likely due to differences in cell morphology. For instance, filamentous cells with a larger specific surface area, characteristic of acetotrophic methanogenic archaea of the genus Methanosaeta, allow free ammonia to diffuse more easily into the cell. On the other hand, Methanosarcina and some species of hydrogenotrophic methanogenic archaea can form cell clusters under high ammonia stress, creating ecological niches in the centers of these clusters that help resist ammonia toxicity.

Effect of ammonia nitrogen on lipid molecules of microbial cells

The cell membrane is the main route for the exchange of substances between the cell and its environment and is crucial for maintaining growth and proper function. During the AD process, organic substrates generally need to be transported into the cell by crossing the cell membrane to metabolize substances (Linda-M et al. 2001 ). In recent years, researchers have shown interest in the effects of high ammonia nitrogen exposure on the key properties of microbial cell membranes. For example, Liu et al. ( 2023 ) showed that the permeability of the acetotrophic methanogenic archaea cell membrane increased significantly under nitrogen stress, exhibiting significant depolarization of the membrane potential, as shown in Fig.  4 . Furthermore, fluorescence polarization detection was conducted, revealing that the fluorescence polarization of cells in the AD system under high concentrations of ammonium nitrogen was significantly higher than in the control group (Tian et al. 2018a ; Astals et al. 2021 ). These results indicate that high concentrations of ammonium nitrogen cause a significant decrease in cell membrane fluidity.

cell membrane transport under nitrogen stress

Controlling strategy

During the AD process, approximately 33–80% of nitrogenous organic matter, such as urea, proteins, amino acids, and nucleic acids, are hydrolyzed and fermented, releasing ammonia and nitrogen as end products. Ammonia inhibits the normal metabolic functions of anaerobic microorganisms by disrupting intracellular pH/proton and potassium (K + ) ion balances, exacerbating cellular energy depletion, and inhibiting the activities of specific enzymes associated with methanogenic metabolism. Furthermore, due to the heightened sensitivity of methanogenic bacteria to ammonia, it leads to the accumulation of volatile fatty acids, thereby exacerbating the deterioration of the AD process. Strategies to mitigate ammonia inhibition by adjusting operating parameters (e.g., temperature, pH, and carbon to nitrogen ratio) and employing physicochemical methods (e.g., dilution, precipitation, air stripping, membrane separation, and ion exchange) have been extensively studied and reviewed. (Xu et al. 2022 ) Table  2 lists several measures to mitigate ammonia inhibition in the AD process.

Adjustment of the C/N ratio

The adjustment of the carbon to nitrogen (C/N) ratio is considered a crucial measure in mitigating the inhibition caused by ammonia nitrogen during the AD of organic wastes. When the C/N ratio is too low, it can lead to the accumulation of ammonia nitrogen within the system, potentially inhibiting AD microorganisms (Capson-Tojo et al. 2020 ; Yang et al. 2022 ). Conversely, a fermentation feedstock with an excessively high C/N ratio may result in an inadequate nitrogen source within the system, leading to an underutilized carbon source. The combined AD of various organic wastes is recognized for its ability to enhance waste utilization efficiency and reduce the need for multiple treatment facilities, thereby resulting in cost savings (Peng et al. 2022 ). Moreover, it is acknowledged that this process aids in improving the stability of the digestion procedure by increasing the C/N ratio of the feedstock, effectively mitigating the effects of ammonia nitrogen inhibition. In a study conducted by Wang et al. ( 2022 ), it was observed that when kitchen waste and food waste were co-digested at a mass ratio of 2:1, there was a significant increase in the gas production rate and methane content compared to the AD of kitchen waste alone. Specifically, at an organic loading rate (OLR) of about 120 kg·d -1 , the average volumetric mean biogas production rates for the AD and co-digestion of the two substrates were measured at 2.02, 0.75, and 2.3 m 3 ·m -3 ·d -1 , respectively. Similarly, the average methane content in the produced biogas was recorded at 38.4%, 21.2%, and 63.8% for the AD of kitchen waste alone and the co-digestion of the two substrates, respectively (Mahdy et al. 2020 ). Furthermore, Beniche et al. ( 2021 ) observed promising outcomes from the co-digestion of food waste combined with leaves and stems of kale and cauliflower at a C/N ratio of 45. The resultant mixed substrate exhibited high biodegradability, reaching 98%. This co-digestion process yielded a methane production of 475 mL STP CH 4 /g VS, both of which displayed enhancements compared to the performance achieved through sole AD.

Selection of adapted microorganisms

Microbial domestication is a pivotal strategy in alleviating ammonia nitrogen inhibition within waste digestion systems. This approach involves the deliberate cultivation or introduction of microbial flora capable of tolerating elevated ammonia nitrogen concentrations, thereby significantly enhancing the system's ability to manage ammonia nitrogen effectively. An essential phase in the domestication process is the screening and cultivation of microorganisms resilient to high ammonia nitrogen concentrations (Chen et al. 2018 ). This is typically achieved by subjecting the microbial flora to gradually increasing levels of ammonia nitrogen. Jo et al. ( 2022 ) demonstrated the enhancement of methane production rates within the system, increasing from 154.6 ± 9.9 mL/g COD to 269.6 ± 3.6 mL/g COD through microbial domestication. This domestication led to observable alterations in both bacterial and archaeal populations. Notably, the transition of archaeal populations from Methanobacterium spp. to Methanosaeta spp. and Methanosarcina spp. occurred concurrently throughout the domestication process (Carballa et al. 2015 ; Peng et al. 2023c ). Wang et al.( 2023 ) applied two microbial communities (MC and SS) through domestication to a nitrogen-enriched AD system and found that the MC and SS treatments restored AD performance within 21 and 83 days, respectively. Analysis of the 13 C isotope indicated that both MC and SS enhanced the hydrogenotrophic pathway.

Ammonia removal

The accumulation of ammonia nitrogen can significantly impede the efficiency of AD and potentially cause process failure. Ameliorating ammonia nitrogen inhibition can be accomplished through several methods, including the addition of auxiliary materials such as clay, zeolite, and guano, as well as utilizing membrane reaction contactors, incorporating trace elements, and employing the blow-off method. For instance, the guano stone method involves leveraging magnesium and phosphorus within guano to create insoluble magnesium ammonium phosphate precipitation (Ye et al. 2024 ; Zhuo et al. 2018 ). This process effectively removes phosphorus and aids in denitrification, thereby mitigating ammonia nitrogen accumulation. Li et al. ( 2022 ) investigated the impact of guano stone precipitation in removing ammonia nitrogen from the anaerobic fermentation process of chicken manure. Their study revealed a significant reduction in the ammonia nitrogen concentration within the test group reactor, decreasing from 2,937 mg/L to 1,466 mg/L. Consequently, the average methane production improved by 18%, increasing to 0.39 L/g compared to the control group's 0.33 L/g. Furthermore, the addition of trace element Fe demonstrated an antagonistic effect on ammonia nitrogen, with this effect notably intensifying as the ammonia nitrogen concentration increased. Meng et al. ( 2020 ) explored the use of zero valent iron (ZVI, 150 µm) to enhance methanogenic capacity. The introduction of ZVI at 160 mM notably amplified cumulative methane production by 22.2% and further reduced the high-solid anaerobic digestion (HSAD) duration by 50.6%. Additionally, the blow-off method, which transfers ammonia nitrogen from the liquid phase to the gas phase, emerged as an efficient physical nitrogen removal process with low investment costs, relatively straightforward equipment requirements, and simple operational procedures. Pedizzi et al. ( 2017 ) implemented an air sidestream vapor stripping process to reduce ammonia nitrogen concentration. Their study demonstrated a successful reduction of ammonia nitrogen concentration from 2.4 ± 0.1 g N-TAN L -1 to 1.1 ± 0.1 g N-TAN L -1 , without compromising process stability. Furthermore, they achieved a reduction from 4.5 ± 2.0 g N-TAN L -1 to 2.0 ± 0.1 g N-TAN L -1 , highlighting the effectiveness of their approach. Similarly, Zhuang et al. ( 2018 ) demonstrated that the addition of magnetite nanoparticles resulted in a 36–58% increase in methane production compared to the control group. Additionally, it was observed that magnetite nanoparticles had a minimal impact on TAN concentration, suggesting that conductive materials have a relatively minor effect on ammonia levels, yet they diminish the inhibitory effects of ammonia nitrogen (Ngo et al. 2023 ; Provolo et al. 2017 ; Zhao et al. 2019 ).

limitations and future directions of research

Based on the above, the anaerobic digestion (AD) process is a complex biochemical system grounded in thermodynamic principles and driven by microorganisms that work synergistically through commensal linkages to maintain system stability. However, these microorganisms are highly sensitive to environmental fluctuations, which, given the complexity of the substrates and the stochastic nature of AD operating conditions, could adversely affect the performance of AD facilities. Consequently, there has been significant scientific interest in exploring whether the addition of microorganisms with specific biodegradative capabilities—a bioaugmentation strategy involving the introduction of specialized microbial functional groups into inhibited AD systems—can mitigate toxicity under high ammonia concentrations and enhance reactor performance. For instance, Methanoculleus and Methanosarcina have been identified as effective bioaugmentation agents for counteracting ammonia inhibition. Research has shown that adding Methanoculleus to an AD system utilizing municipal solid waste as a substrate can increase methane production by 21% (Wang et al. 2023 ). Additionally, the introduction of bacteria-rich bioadditives, such as propionic acid and butyric acid-degrading bacteria, has been found to accelerate the conversion of volatile fatty acids (VFAs) to methane. This acceleration is largely attributed to the interactions within the complex bacterial community, which help reduce hydrogen partial pressure (Li et al. 2022 ).

Although the bioaugmentation strategy of cultivating pure strains can be effective in enhancing the digestive performance of anaerobic reactors under ammonia stress, it is accompanied by some risks, i.e., it is difficult for a single archaea to colonise and rebuild the microbial community in systems suppressed by high ammonia and nitrogen concentrations (Wang et al. 2023 ). In addition, the cost and technical requirements (sterile environment, culture media) associated with the cultivation of pure strains are realities that have to be taken into account. Therefore, biofortification strategies using mixed microbial consortia (consisting of microorganisms that can tolerate inhibitory factors) are a more effective alternative in order to better fit the host microbial community as well as to improve the tolerance of the system to ammonia stress. For example, Yang et al. ( 2019 ) combined Methanobrevibacter and syntrophic acetate oxidizing bacteria (SAOB) (Syntrophaceticu schinkii) as a microbial consortium and methane yield was improved by 71%. However, whether these selected microbial consortia are stably able to function in high-ammonia inhibition systems needs to be confirmed by extensive experiments. For instance, Westerholm et al. ( 2012 ) combined some SAOBs (Clostridium ultunense sp,Tepidanaerobacter acetatoxydans, and Syntrophaceticus schinkii) with ahydrogenotrophic methanogen (Methanoculleus) to construct a microbial consortium as a biological additive but the digestion performance was not improved after adding the microbial consortium. Interestingly, microbial symbionts obtained through purposeful domestication are more closely related yet better adapted than artificial microbial symbionts. Consequently, they may yield superior results in enhancing digestive performance in AD systems with high TAN concentrations (Wang et al. 2023 ). However, there remains a knowledge gap regarding the effectiveness and potential mechanisms of microbial symbionts acquired through purposeful domestication as bioadditives for alleviating ammonia inhibition. Specifically, the potential mechanisms of microbial symbionts obtained through purposeful domestication in mitigating ammonia inhibition may be more complex than those of individual archaea or artificially assembled microbial symbionts, and the roles of individual members within a microbial symbiont in mitigating ammonia inhibition are still unknown. Much future research is needed. In addition, a reliable set of mathematical models to predict the efficiency of different biofortification would provide a more efficient solution for selecting a mixed microbial consortium that is tolerant to ammonia stress and thus improve the digestive performance of the AD system.

AD stands as a pivotal biological treatment method that effectively converts organic wastes into valuable biogas while simultaneously reducing waste volume. Nevertheless, the accumulation of ammonia nitrogen within this process can trigger ammonia nitrogen inhibition, thereby restricting the efficiency and stability of the digestion process. To mitigate this inhibition, various measures have been implemented. These include the adjustment of the C/N ratio, pH regulation, addition of VFAs, utilization of ammonia nitrogen adsorbents, adoption of ammonia nitrogen removal processes, alongside gas stripping and blow-off techniques. In essence, these collective measures collectively contribute to the reduction of ammonia–nitrogen concentrations and serve to alleviate the inhibition effects within the AD system. Adjusting the C/N ratio through co-elimination methods aids in diminishing the accumulation of ammonia nitrogen. pH regulation and the utilization of ammonia nitrogen adsorbents assist in preserving a neutral or alkaline environment, thereby reducing the concentration of free ammonia nitrogen. Additionally, vapor stripping and blow-off techniques work to facilitate the release of ammonia nitrogen by enhancing the rate of gas–liquid mass transfer. These approaches collectively enhance the efficiency and stability of the AD system.

In the future, there is potential for further exploration of novel methods and advanced technologies aimed at more effectively mitigating ammonia nitrogen inhibition. Particularly, the investigation of innovative microbial domestication strategies holds promise in enhancing microbial adaptation within high ammonia–nitrogen environments. Moreover, delving into the intricate interrelationships between ammonia nitrogen inhibition and various other waste treatment parameters can significantly contribute to the refinement and optimization of waste digestion systems. Ultimately, such endeavors are poised to significantly enhance the feasibility of mitigating ammonia nitrogen inhibition, thus fostering improvements in the efficiency and stability of the AD process.

Abbreviations

Municipal solid waste

Organic waste

• Anaerobic digestion

Total ammonia nitrogen

Free ammonia

Organic loading rates

Syntrophic microbial consortium

Hydrogenotrophic methanogen consortium

Syntrophic acetate-oxidizing bacteria

Wood-Ljungdahl pathway

Methylmalonyl-CoA

Acetate-methane

Propionate-oxidizing bacteria

Long-chain fatty acids

Volatile fatty acids

Carbon to nitrogen

Organic loading rate

Zero valent iron

High-solid anaerobic digestion

Syntrophic acetate oxidizing bacteria

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Acknowledgements

This work was supported by the Gansu Province University Industry Support Program Project (2020c-38); Gansu Province Key R&D Project Program (2021-0201-GXC-0145).

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Yang, J., Zhang, J., Du, X. et al. Ammonia inhibition in anaerobic digestion of organic waste: a review. Int. J. Environ. Sci. Technol. (2024). https://doi.org/10.1007/s13762-024-06029-1

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Received : 01 January 2024

Revised : 06 August 2024

Accepted : 27 August 2024

Published : 09 September 2024

DOI : https://doi.org/10.1007/s13762-024-06029-1

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