Review Reports - Assessing Biogas Production Potential from Organic Waste and Livestock Byproducts in a Serbian Municipality: Implications for Sustainable Food Systems

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The title of the paper meets the scoop of the journal but is not consistent with the objective of the study (title: Optimizing Biogas Production from Organic Waste and Live-2 stock Byproducts, objective: to investigate the biogas production potential of various substrates)
From waste to energy: the significance of substrate selection. In this section, the author talks about potential rather than conversion into energy; the title of this section should be changed to be more consistent with the content.
Too many gratuitous assertions without being supported by references
Manure from livestock farms

These characteristics (Manure from livestock farms) would be better understood if they were placed in a single table (combining tables 1, 2 and 3 into a single table) for a better understanding of the potential of each substrate.

Similarly for Energy plants, tables 4, 5, 6, and 7, 8, 9 can be combined into a single table.

Optimizing Biogas Production: Case from Practice. In this part, the author should present all the relevant information on the specific case (is it a project, if so when did it start, who implemented it, why, for whom, how? ....)
In addition, parts 2 and 3 should be balanced in terms of form
According to the review, what would be the best strategy for optimizing biogas production: substrate selection, production conditions, etc. The authors should conclude in this respect.

Author Response

Dear Editor and reviewer,

We appreciate the reviewers' comments and suggestions. Considering that two reviewers suggested the manuscript could be improved, while one reviewer indicated that the manuscript must be improved, we made corrections accordingly. In total, we addressed 40 questions, responding to each from our perspective adequately. We believe that the manuscript, in its current form, is acceptable for publication.

We look forward to hearing your response.

Reviewer 1.

Q.1.The title of the paper meets the scoop of the journal but is not consistent with the objective of the study (title: Optimizing Biogas Production from Organic Waste and Livestock Byproducts, objective: to investigate the biogas production potential of various substrates).

Response to Reviewer Comment 1:

We appreciate the reviewer’s insightful suggestion regarding the inconsistency between the paper’s title and objective. The current title, “Optimizing Biogas Production from Organic Waste and Livestock Byproducts,” suggests a focus on process optimization, while the study primarily investigates the biogas production potential of different substrates. To ensure alignment, we propose revising the title to:

“Assessing Biogas Production Potential from Organic Waste and Livestock Byproducts in a Serbian Municipality: Implications for Sustainable Food Systems”

This revised title more accurately conveys the study’s focus on assessing the biogas production potential of various substrates while maintaining relevance to the journal’s scope. We hope this adjustment resolves the concern.

Q.2. From waste to energy: the significance of substrate selection. In this section, the author talks about potential rather than conversion into energy; the title of this section should be changed to be more consistent with the content.

Response to Reviewer Comment 2:

Since the section primarily discusses the biogas production potential of various substrates rather than the conversion process itself, a more precise title could be:

“Evaluating Substrate Potential for Biogas Production"

Q.3. Too many gratuitous assertions without being supported by references

Response to Reviewer Comment 3:

The manuscript currently contains 71 references, which we believe is sufficient. We have ensured that the most important statements are properly cited. However, we kindly ask the reviewer to indicate specific sentences that require additional citations.

Q.4. Manure from livestock farms

Q.5. Similarly for Energy plants, tables 4, 5, 6, and 7, 8, 9 can be combined into a single table.

Response to Reviewer Comment 4

Thank you for your valuable suggestion. We have synthesized the data from Tables 1, 2, and 3 into a single table to enhance clarity and provide a comprehensive overview of the characteristics of manure from livestock farms. Similarly, data from Tables 4, 5, 6, and 7, 8, have been consolidated into a single table for energy plants, ensuring a more structured and accessible presentation of the potential of each substrate. The data from Table 9 cannot be synthesized into Table 3 due to its large volume and complexity, and must therefore be presented separately.

Q.6. Optimizing Biogas Production: Case from Practice. In this part, the author should present all the relevant information on the specific case (is it a project, if so when did it start, who implemented it, why, for whom, how? ....)

Response to Reviewer Comment 5

We appreciate the reviewer’s feedback. To enhance clarity and provide a more detailed context, we have expanded this section to include all relevant information about the specific case. We now specify whether it is a project, its start date, the implementing entities, its objectives, target beneficiaries, and the methodology used. These additions ensure a comprehensive understanding of the practical application of biogas production optimization."

A practical example of optimizing biogas production can be observed in the implementation of a biomass utilization project in Čačak, Serbia. This initiative, titled "Use of Biomass for Cogeneration in a Wastewater Treatment Facility," was developed as part of a broader effort to enhance renewable energy applications in wastewater management. The project was implemented under the coordination of the United Nations Development Programme (UNDP), with a specific focus on improving energy efficiency and sustainability through the integration of biomass-based cogeneration.

Project Overview

Project Type: Biomass utilization for cogeneration
Implementation Period: August 15, 2023 – October 26, 2023
Implementing Agency: United Nations Development Programme (UNDP)
Lead Consultant: Srećko Ćurčić, engaged as the National Consultant for Biomass Data Collection
Location: Čačak, Serbia
Funding and Contractual Agreement: The project was executed under an individual contract signed between UNDP and the lead consultant on August 8, 2023, with a total budget allocation of USD 9,990.00.

Objectives and Scope

The primary objective of the project was to evaluate the potential for biomass-based cogeneration within the wastewater treatment facility in Čačak, enabling a more sustainable and energy-efficient process. The project aimed to:

Assess the availability and quality of biomass resources in the region.
Develop a feasibility study on integrating biomass into the existing wastewater treatment infrastructure.
Provide technical recommendations for optimizing biogas production and energy recovery.
Support policy and decision-making for future large-scale implementation of biomass utilization.

Implementation and Methodology

The project followed a structured approach to data collection and analysis:

Biomass Resource Assessment: A comprehensive survey was conducted to identify the types and quantities of biomass available in the region, focusing on agricultural residues, wood waste, and organic sludge from wastewater treatment.
Technology Evaluation: Different cogeneration technologies were reviewed, with an emphasis on their applicability to wastewater treatment facilities.
Energy Yield Estimation: Calculations were performed to estimate the potential biogas yield from the identified biomass sources, considering parameters such as methane production efficiency and system integration.
Operational Recommendations: Based on the findings, strategies were proposed to optimize the anaerobic digestion process, improve biogas capture efficiency, and enhance overall energy output.

Outcomes and Impact

The study provided a comprehensive assessment of biomass potential in Čačak and its feasibility for cogeneration. Key takeaways included:

The identification of sustainable biomass sources that can supplement anaerobic digestion in wastewater treatment.
Recommendations for integrating biomass-based energy production to reduce reliance on fossil fuels and enhance environmental sustainability.
Insights into cost-effectiveness and regulatory considerations for scaling up biomass cogeneration.

Conclusion

This case study highlights the practical steps taken to optimize biogas production in a real-world setting. By leveraging biomass resources for cogeneration, the project demonstrated a viable pathway toward energy-efficient wastewater treatment. The findings and methodologies developed in this initiative can serve as a model for similar projects seeking to enhance biogas production through strategic biomass utilization.

Q.7. In addition, parts 2 and 3 should be balanced in terms of form

Response to Reviewer Comment 6

Thank you for your valuable suggestion. To improve balance between Parts 2 and 3, we have expanded Part 3 by including comprehensive details on the case study, such as project background (Who initiated it? When? Where?, What problems were addressed?), methodology, key optimization strategies, and outcomes (highlight practical implications of optimization strategies, and/or lessons learned). Additionally, we have ensured consistency in formatting, subheadings, and data presentation across both sections. These adjustments create a more structured and coherent flow between the two parts.

Q.8. According to the review, what would be the best strategy for optimizing biogas production: substrate selection, production conditions, etc. The authors should conclude in this respect.

Response to Reviewer Comment 7

Thank you for your insightful comment. We have revised the conclusion to explicitly outline the best strategies for optimizing biogas production. Based on our findings, the most effective approach combines:

Substrate selection – Choosing high-energy organic waste and livestock byproducts with optimal C/N ratios to enhance methane yield.

Production conditions – Maintaining optimal temperature, pH, and retention time to maximize microbial efficiency.

Co-digestion strategies – Mixing substrates with complementary characteristics to improve biogas yield and process stability.

Process monitoring and control – Implementing real-time monitoring of key parameters (e.g., gas composition, pH) to adjust conditions proactively.

These strategies collectively enhance biogas production efficiency while ensuring sustainability. We have incorporated these points in the revised conclusion to provide a clearer perspective on the key takeaways of our study.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript provides a relevant discussion on biogas production and a case study. However, the introduction does not position the study within the current research landscape. It should explicitly reference prior studies (2022–2024, Scopus/WoS) to highlight the research gap being addressed.

The abstract lacks a clear statement of the research problem and does not specify the methodology used. Indicating whether this is a systematic, meta-analysis, or narrative review would enhance clarity. Additionally, the study’s contribution should be briefly summarized in relation to the existing literature.

Consider adding a concise paragraph outlining how your study builds upon or diverges from previous research. Also, avoid excessive keywords—adjust them to meet journal requirements and prevent repetition with the title.

For the Introduction
Sentences are too long; abbreviations can improve readability.
References should be in parentheses (), not brackets [].
Numerical values (e.g., line 58) should be contextualized with updated data.
Line 71 lacks a smooth transition from the previous paragraph.
Paragraph (82–91) contains claims that need supporting references.
Avoid using "us, we, our" (e.g., line 95).
The study does not clearly present its innovative contribution.

For the Research Design and Methodology
The methodology lacks details on dataset selection. Specify criteria or mention databases like Scopus/WoS.
The title in line 105 must be revised.
Lines 144-145 should be accompanied by a flow diagram to clarify the experimental steps.
The biogas yield assessment lacks transparency. Include details on:
Control conditions and comparative basis for evaluating production.
Statistical methods used and their justification.
A sensitivity analysis to assess potential variability sources.

Coherence and Strength of Arguments
Section 2.1 is disorganized. Short and isolated paragraphs (174-175, 176-178) should be reformatted for better readability, particularly regarding the number of animals per farm and their implications.
Use "Adapted from" instead of "Adopted by" (line 183), and ensure all exponents are in superscript format.
Table 2 should be compared with other sources, and values in the last column should be presented as a range rather than fixed.
The discussion lacks a critical evaluation of limitations. Adding a "Study Limitations" subsection would strengthen credibility.
Some claims (e.g., on methane yield potential) need numerical comparisons to similar studies to emphasize novelty.

Presentation of Empirical Results
Tables 4-6 should be unified and analyzed together rather than presented in isolation. The same applies to Tables 7-9.
Table 10 is unnecessary as it does not contribute significant information.
Lines 295-321 cite the census but do not provide meaningful discussion. This section should summarize findings, integrate relevant discussion, and ensure all values are expressed in SI units.
The discussion of results remains too superficial. Rather than summarizing findings, provide a deeper analysis of trends, patterns, and comparisons with existing literature.
Improve table and figure legends by specifying data sources and ensuring they provide enough detail for independent interpretation.
Table 11: Define "Accepted value" to clarify its meaning.
Figure 1 requires a wider map for better understanding by international readers. Include essential elements such as scale, color meanings, north direction, and positioning system.
Figures 2 and 3 should be created by the authors. Figure 4 is unnecessary as it presents basic information.
Ensure unit and terminology consistency throughout the manuscript (e.g., format “m³/day” uniformly).
The results section should compare findings to current industry challenges to emphasize their impact. Consider adding a comparative analysis against recent studies to contextualize relevance and limitations.
Define abbreviations early (e.g., CHP in line 481 should be introduced earlier).

The literature review includes relevant sources but lacks engagement with recent studies. To better align with the latest advancements, incorporate post-2023 publications.
Ensure all in-text citations match the reference list and verify missing or redundant references.

Comments on the Quality of English Language

Finally, the manuscript is well-written but contains minor grammatical inconsistencies. Consider professional language editing to improve clarity and fluency.
Avoid redundant phrases and overly complex sentence structures to enhance readability.

Author Response

Dear Editor and reviewer,

We look forward to hearing your response.

Q.9. The manuscript provides a relevant discussion on biogas production and a case study. However, the introduction does not position the study within the current research landscape. It should explicitly reference prior studies (2022–2024, Scopus/WoS) to highlight the research gap being addressed.

Response to Reviewer Comment 1.

Thank you for your valuable feedback. We have revised the introduction to better position our study within the current research landscape. Specifically, we have incorporated references from recent studies (2020–2025) sourced from Scopus and Web of Science. These references highlight recent advancements in biogas production, key challenges related to substrate selection and optimization, and the necessity for improved strategies in practical applications. In particular, our study addresses the research gap related to the optimization of biogas production from diverse organic waste substrates and livestock byproducts, which has not been extensively explored in the context of Serbia. By focusing on the specific context of the municipality of Čačak, Serbia, we aim to provide localized insights that can inform broader applications of biogas production strategies. The integration of updated literature strengthens the justification for our study, emphasizing its contribution to enhancing biogas production efficiency and sustainability in local municipalities.

Q.10. The abstract lacks a clear statement of the research problem and does not specify the methodology used. Indicating whether this is a systematic, meta-analysis, or narrative review would enhance clarity. Additionally, the study’s contribution should be briefly summarized in relation to the existing literature.

Response to Reviewer Comment 2.

Thank you for your insightful feedback regarding the abstract. In response, we have revised it to include a clear statement of the research problem, specify the methodology used, and summarize the study's contribution in relation to existing literature. The revised abstract now clearly articulates that the study investigates the optimization of biogas production from various organic waste substrates and livestock byproducts, addressing gaps in current knowledge regarding substrate selection and its impact on biogas yield. We have specified that a narrative review methodology was employed, analyzing recent literature from 2020 to 2025. Furthermore, we have highlighted how the practical case study conducted in the municipality of Čačak provides valuable insights into effective biogas production strategies that can be broadly applied in similar contexts. The findings reveal significant variations in biogas production potential among different substrates, emphasizing the importance of strategic selection and management practices. By integrating these elements, the revised abstract presents a clearer understanding of the substrate optimization process and its implications for enhancing biogas production strategies in local municipalities.

Q.11. Consider adding a concise paragraph outlining how your study builds upon or diverges from previous research. Also, avoid excessive keywords—adjust them to meet journal requirements and prevent repetition with the title.

Response to Reviewer Comment 3.

Thank you for your valuable suggestions. In response, we have added a concise paragraph that outlines how our study builds upon previous research in the field of biogas production. Our study extends the existing literature by providing a comprehensive analysis of the biogas production potential from a diverse range of substrates, specifically focusing on organic waste and livestock byproducts in the context of a Serbian municipality. While previous studies have explored various substrates, our research uniquely integrates a practical case study in Čačak, offering new insights into the optimization of biogas production strategies that can be broadly applied in similar regional contexts. This approach not only addresses the gaps identified in prior research but also highlights the importance of localized solutions in enhancing biogas production efficiency. Additionally, we have revised the keywords to align more closely with journal requirements and to avoid repetition with the title. The updated keywords are now: Biogas production, optimization, substrates, livestock byproducts, organic waste, and Serbia.

Q.12. For the Introduction

Sentences are too long; abbreviations can improve readability.

Response to Reviewer Comment

Thank you for your feedback regarding sentence length and the use of abbreviations. After carefully reading and considering your suggestion, we respectfully disagree with the assessment that our sentences are excessively long. We believe that the current structure effectively conveys complex ideas necessary for our study. Additionally, we have employed abbreviations where appropriate to enhance readability without compromising clarity. We appreciate your understanding and value your input in refining our manuscript.

Q.13. References should be in parentheses (), not brackets [].

Response to Reviewer Comment

We appreciate your suggestion regarding the formatting of references. We agree with your recommendation; however, we would like to clarify that after the peer review process and upon acceptance of the manuscript for publication, the references will be numbered in the order of their appearance in the text. This approach aligns with the journal's formatting guidelines and ensures consistency throughout the manuscript. Thank you for your understanding, and we look forward to any further feedback you may have.

Q.14. Numerical values (e.g., line 58) should be contextualized with updated data.

Response to Reviewer Comment

We appreciate your feedback regarding the need for contextualization of numerical values, such as those presented in line 58. This statement has been rewritten to include updated data and is properly cited to ensure accuracy and relevance.

Q.15. Line 71 lacks a smooth transition from the previous paragraph.

Response to Reviewer Comment

Thank you for your valuable feedback regarding the transition in line 71. We acknowledge that a smoother transition from the previous paragraph is necessary to enhance the coherence of the text. We will revise this section to ensure that the introduction of biogas as a sustainable and versatile alternative for energy production and waste management flows more naturally from the preceding content. This will involve incorporating connecting phrases and contextual information that effectively bridge the two paragraphs. We appreciate your insights, and we are committed to improving the clarity and readability of the manuscript.

Q.16. Paragraph (82–91) contains claims that need supporting references.

Response to Reviewer Comment

Thank you for your feedback regarding the need for supporting references in paragraphs 82–91. We have ensured that adequate references are provided to substantiate the claims made in these sections. Each assertion is now backed by relevant literature to enhance the credibility and rigor of the discussion. If you have any further suggestions or require additional adjustments, please let us know.

Q.17. Avoid using "us, we, our" (e.g., line 95).

Response to Reviewer Comment

Thank you for the feedback regarding the use of personal pronouns. This section has been completely rewritten in accordance with the reviewer’s suggestions to enhance clarity and objectivity. If there are any additional concerns or further adjustments needed, please indicate them.

Q.18. The study does not clearly present its innovative contribution.

Response to Reviewer Comment

Thank you for your feedback regarding the clarity of the study's innovative contributions. To address this concern, the manuscript has been revised to explicitly highlight the unique aspects of the research, including specific methodologies, findings, and their implications for the field. This clearer presentation aims to effectively convey the innovative contributions of the study.

Q.19. For the Research Design and Methodology

The methodology lacks details on dataset selection. Specify criteria or mention databases like Scopus/WoS.
The title in line 105 must be revised.

Response to Reviewer Comment

Q.20. Lines 144-145 should be accompanied by a flow diagram to clarify the experimental steps.

Response to Reviewer Comment

Thank you for the suggestion regarding the inclusion of a flow diagram to clarify the experimental steps. However, it is not feasible to provide a flow diagram for this particular statement due to the complexity and variability of the experimental methodologies involved. Instead, a detailed description of the experimental procedures and the rationale behind the chosen methodologies will be included in the manuscript to ensure clarity and understanding of how different substrate characteristics influence biogas yields and contribute to best practices applicable globally.

Q.21. The biogas yield assessment lacks transparency. Include details on:

Control conditions and comparative basis for evaluating production.
Statistical methods used and their justification.
A sensitivity analysis to assess potential variability sources.

Response to Reviewer Comment

We appreciate the reviewer’s feedback regarding the need for greater transparency in the biogas yield assessment. Below, we provide the requested clarifications:

Control Conditions and Comparative Basis:
- The biogas yield values presented in the manuscript are derived from literature sources and previous experimental findings, as referenced throughout the text (e.g., Martinov, 2020; Seadi et al., 2008; Kafle & Kim, 2013).
- The comparative basis for evaluating biogas production relies on standard methodologies used in biogas research, where methane yield is expressed per unit of fresh matter, dry matter, or organic dry matter.
- When applicable, data normalization was performed to ensure comparability across different studies, taking into account substrate characteristics such as moisture content and organic composition.
- The experimental conditions referenced in the literature generally adhere to anaerobic digestion standards, including mesophilic (35-42°C) and thermophilic (55°C) conditions, as well as standard retention times in controlled fermentation processes.

Coherence and Strength of Arguments

Q.22. Section 2.1 is disorganized. Short and isolated paragraphs (174-175, 176-178) should be reformatted for better readability, particularly regarding the number of animals per farm and their implications.

Response to Reviewer Comment

We appreciate the reviewer’s suggestion to reformat the text for better readability, particularly in relation to the number of animals per farm and their implications. However, reformatting the section in the suggested manner is not feasible due to the following reasons:

Focus of the Section: The section primarily discusses the composition and biogas potential of different manure types rather than the number of animals per farm. Introducing farm size data in this context would diverge from the core focus of the paragraph.
Data Availability and Relevance: While data on livestock numbers per farm is available elsewhere in the manuscript, its direct integration into this segment would not provide additional clarity or improve readability. Instead, it could introduce unnecessary complexity and disrupt the logical flow of information.
Structural Integrity: The current structure efficiently conveys the key message regarding dry matter content and organic composition, which are the primary factors influencing biogas potential. Any major restructuring risks shifting the emphasis away from these critical aspects.

Q.23. Use "Adapted from" instead of "Adopted by" (line 183), and ensure all exponents are in superscript format.

Response to Reviewer:

We appreciate the reviewer’s attention to detail and their suggestion regarding terminology and formatting.

We have replaced "Adopted by" with "Adapted from" on line 183, as suggested. These revisions have been implemented accordingly.

Q.24. Table 2 should be compared with other sources, and values in the last column should be presented as a range rather than fixed.

Response to Reviewer:

We appreciate the reviewer’s suggestion to compare Table 2 with additional sources and to present the values in the last column as a range rather than fixed numbers.

We have completely revised all tables, including Table 2, ensuring that the biogas yield values are cross-referenced with multiple sources to improve the robustness and reliability of the presented data.

We initially expressed values as fixed numbers for the following reasons:

Standardization and Comparability:

Fixed numbers were used to ensure consistency across different data sources, as some studies report single representative values rather than ranges.
This approach aimed to facilitate direct comparisons between different manure types without introducing unnecessary variability.

Q.25. The discussion lacks a critical evaluation of limitations. Adding a "Study Limitations" subsection would strengthen credibility.

Response to Reviewer:

In this work, not all available biomass materials that occur in the defined territory were considered. The reason why not all resources were taken into consideration is: very small quantities of certain raw materials, the absence of continuous sources of certain raw materials, as well as the impossibility of organized collection. That is why the authors opted for the available raw materials that were taken into consideration in the work

Q.26. Some claims (e.g., on methane yield potential) need numerical comparisons to similar studies to emphasize novelty.

Response to Reviewer:

We appreciate the reviewer’s suggestion to provide numerical comparisons to similar studies regarding methane yield potential. In our manuscript, we have supported our claims with appropriate citations that align with findings from similar investigations. However, it is important to note that direct numerical comparisons are challenging due to the substantial variability in substrate composition, which is influenced by factors such as feedstock type, regional differences, seasonal variations, and operational conditions of anaerobic digestion systems. Given these inherent variations, methane yields can differ significantly across studies, even when assessing similar substrates. Nevertheless, our results fall within the range reported in the literature, confirming the reliability and consistency of our findings. To further enhance clarity, we can provide additional context on the factors contributing to variability in methane yields if necessary.

Presentation of Empirical Results

Q.27. Tables 4-6 should be unified and analyzed together rather than presented in isolation. The same applies to Tables 7-9.

Response to Reviewer:

We appreciate the reviewer’s suggestion to unify and analyze Tables 4-6 and Tables 7-9 together rather than presenting them in isolation.

We have completely revised all tables, including Tables 4-6 and Tables 7 and 8, ensuring they are now integrated and analyzed collectively to improve coherence and readability.

Q.28. Table 10 is unnecessary as it does not contribute significant information.

Response to Reviewer:

We appreciate the reviewer’s feedback regarding Table 10 and its relevance to the manuscript.

After careful consideration, we removed Table 10 as suggested, since it did not provide significant additional information beyond what was already discussed in the text.
Any relevant data from Table 10 that contributed to the discussion have been integrated into the main text where necessary.

Q.29. Lines 295-321 cite the census but do not provide meaningful discussion. This section should summarize findings, integrate relevant discussion, and ensure all values are expressed in SI units.

Response to Reviewer:

We appreciate the reviewer’s feedback regarding this section. However, we respectfully maintain that the statistical data provided are essential for contextualizing the availability of agricultural residues for biogas production.

Standard Agricultural Reporting Format:
- The data presented are sourced from the 2022 Agricultural Census of Serbia, which reports land area in hectares (ha) and tree counts per species, following standard agricultural and statistical conventions. This information serves as a foundation for assessing the potential availability of biomass for biogas production.
Necessity of Statistical Data:
- While we agree that excessive discussion of raw statistical data may not be necessary, summarizing key figures provides essential context for evaluating the biogas potential of the region.
- The revised section now focuses more explicitly on how agricultural residues can serve as viable feedstocks for biogas production, ensuring alignment with the study’s objectives.
- Regarding SI units: While we maintain that hectares (ha) and tree counts are the standard reporting units in agricultural statistics, we acknowledge that "ha" is the accepted abbreviation for hectares in scientific writing. Therefore, we have replaced "hectares" with "ha" throughout the manuscript for consistency and readability.

Q30. The discussion of results remains too superficial. Rather than summarizing findings, provide a deeper analysis of trends, patterns, and comparisons with existing literature.

Response to Reviewer:

We appreciate the reviewer’s feedback regarding this section. In response, we have enhanced our discussion to provide a more in-depth analysis of the results, focusing on trends, patterns, and comparisons with existing literature. We have highlighted significant trends in biogas production by rewriting the tables to present a clearer overview of potential yields for various substrates. For instance, the revised tables now illustrate the biogas yields and methane content from different types of organic waste and energy crops, emphasizing their efficiency in methane generation.

Q.31. Improve table and figure legends by specifying data sources and ensuring they provide enough detail for independent interpretation.

Response to Reviewer

As before

Q.31. Table 11: Define "Accepted value" to clarify its meaning.

Response to Reviewer:

We appreciate the reviewer’s feedback regarding the need to clarify the term "Accepted value" in Table 11, which presents the available energy potentials for biogas production at the wastewater treatment plant (WWTP) in the Čačak municipality. In the context of this table, "Accepted value" refers to the standard or benchmark values established through previous research and industry standards that define the expected energy potentials from various substrates used in biogas production. These values serve as a reference point against which the measured energy potentials from our study can be compared. By including these accepted values, we aim to provide context for our findings and demonstrate the feasibility of the energy potentials identified in the Čačak area. We will ensure that this definition is clearly stated in the table's caption to enhance understanding for readers.

Q.32. Figure 1 requires a wider map for better understanding by international readers. Include essential elements such as scale, color meanings, north direction, and positioning system.

Response to Reviewer

We appreciate the reviewer’s suggestion to improve Figure 1 for better clarity and comprehension by international readers. The figure has been created at a scale of 1:30,000, ensuring sufficient detail while maintaining readability. Essential elements are already incorporated as follows:

Color Coding: The map distinguishes key infrastructure components—green represents the location of the transfer station for waste collected from the defined territory, purple indicates the wastewater treatment plant (WWTP), and gray marks the composting plant.
Site Positioning: The planned biogas plant site is located below the wastewater treatment plant and composting plant.
Orientation and Scale: A north arrow, scale bar, and positioning system can be included to enhance spatial understanding for a broader audience.

Q.33. Figures 2 and 3 should be created by the authors. Figure 4 is unnecessary as it presents basic information.

Response to Reviewer

We appreciate the reviewer’s feedback regarding the figures. In response to the suggestion, Figures 2 and 3 have now been created by the authors to ensure originality and consistency with the study. Additionally, Figure 4 has been removed, as it presented basic information that was not essential to the manuscript. These revisions improve the clarity and relevance of the visual content while maintaining a focus on the key aspects of our research.

Q.34. Ensure unit and terminology consistency throughout the manuscript (e.g., format “m³/day” uniformly).

Response to Reviewer:

We appreciate the reviewer’s comment regarding unit and terminology consistency throughout the manuscript. After a thorough review, we confirm that we have not utilized the unit "m³/day" in our manuscript. Instead, we have adhered to the appropriate units relevant to our study, such as "Nm³/t" and "Stm³/t," as these are more applicable to our biogas production analysis.

However, we recognize the importance of maintaining consistency in terminology and units throughout the document. We will carefully review the manuscript to ensure that all units are formatted uniformly and that the terminology aligns with industry standards. This will improve clarity and enhance the overall readability of the paper.

Q.35. The results section should compare findings to current industry challenges to emphasize their impact. Consider adding a comparative analysis against recent studies to contextualize relevance and limitations.

Q.36. Define abbreviations early (e.g., CHP in line 481 should be introduced earlier).

Response to Reviewer:

We appreciate the reviewer’s constructive feedback regarding the definition of abbreviations. In response, we have revised the manuscript to ensure that all abbreviations, including "CHP" (Combined Heat and Power), are defined earlier in the text. This change aims to enhance clarity and ensure that readers can easily understand the terminology used throughout the paper.

Q.37. The literature review includes relevant sources but lacks engagement with recent studies. To better align with the latest advancements, incorporate post-2023 publications.

Response to Reviewer:

We appreciate the reviewer’s insightful feedback regarding the engagement with recent studies in our literature review. We would like to clarify that most of the cited literature in our manuscript is no older than five years, ensuring that our discussion reflects contemporary research and advancements in the field. However, we acknowledge the importance of incorporating the most current studies to enhance the relevance of our review.

38. Ensure all in-text citations match the reference list and verify missing or redundant references.

Response to Reviewer:

We appreciate the reviewer’s comment regarding the need to ensure that all in-text citations match the reference list and to verify any missing or redundant references. In response, we have conducted a thorough review of the manuscript to ensure that all citations are accurately represented in the reference list.

We have revised the reference list to address any discrepancies and have ensured that all cited works are included and correctly formatted. Additionally, we have removed any redundant references to streamline the list.

Reviewer 3 Report

Comments and Suggestions for Authors

The comments are palced in the attachment.

Comments for author File: Comments.docx

Author Response

Dear Editor and reviewer,

We look forward to hearing your response.

Strategies for sustainable food systems in a local municipality in Serbia was studied in this manuscript. This manuscript was a carefully done and the findings are of considerable interest. This manuscript could be accepted for publication after modification. The following points should be considered.

Q.39. Although the article discusses methane production from different substrates, it does not provide a detailed analysis of the environmental impacts of these substrates, such as greenhouse gas emissions and other pollutants. How can we assess the impact of different types of organic waste (e.g., food waste, livestock manure, etc.) on the greenhouse gas (e.g., methane) emissions during the production process?"

Response to Reviewer:

We appreciate the reviewer's comment and would like to clarify that the manuscript already discusses the environmental impact (L49,58,86,89,94,96,114,311,436,643) of different substrates used for methane production, particularly through the analysis of their biogas yields and composition. The methane yield variations among substrates directly influence greenhouse gas emissions, as higher biogas production efficiency translates to improved waste utilization and reduced environmental impact. Additionally, the study examines different organic waste sources, such as manure, food industry residues, and municipal waste, highlighting their potential to contribute to sustainable energy generation and waste management. By assessing the substrate properties, the manuscript inherently addresses their role in mitigating emissions compared to conventional waste disposal methods.

We hope this explanation clarifies our approach. However, if the reviewer suggests including specific emission quantifications (e.g., CO₂-equivalent values), we would be open to adding relevant references or a brief discussion in the manuscript.

Q.40. Has the research in the article taken into account the potential impact of climate change on substrate characteristics (such as moisture content, nutritional composition, etc.)?

Response to Reviewer:

We appreciate the reviewer’s question regarding the potential impact of climate change on substrate characteristics, such as moisture content and nutritional composition, in the context of our research on biogas production. This question is indeed reasonable and highlights an important consideration in the sustainability and efficiency of biogas production systems. Climate change can significantly affect agricultural practices, crop yields, and the availability of organic waste, which may alter the characteristics of substrates used in anaerobic digestion. In response to this concern, we will include a discussion in our revised manuscript addressing how climate change may influence the properties of substrates over time. Specifically, we will explore:

Moisture Content Variability: Changes in precipitation patterns and temperature fluctuations can affect the moisture content of organic materials, which is crucial for optimal digestion conditions. We will reference studies that have examined how extreme weather events may impact the availability and quality of substrates.

Nutritional Composition: Climate change can affect the nutrient profiles of crops and organic waste, influencing their suitability for biogas production. For example, shifts in plant growth due to increased CO2 levels or altered growing seasons may lead to changes in the nitrogen and carbon content of substrates.

Adaptation Strategies: We will discuss potential strategies for adapting biogas production practices to account for these changes, such as selecting more resilient substrate sources or employing co-digestion techniques that enhance biodegradability and methane yield in the face of fluctuating substrate quality. By incorporating this analysis, we aim to provide a more comprehensive understanding of the factors influencing biogas production and emphasize the importance of considering climate change impacts in future research and practical applications.

Q.41. In different substrate combinations, which are the best co-digestion strategies to enhance biogas production?

Response to Reviewer

We appreciate the reviewer’s insightful question regarding optimal co-digestion strategies for enhancing biogas production. To maximize biogas yield from available raw materials, the most effective approach is to utilize individual substrates at defined time intervals whenever feasible. However, if this is not possible, co-digestion should be conducted by strategically mixing different substrates, ensuring that the most readily available resources dominate during each defined period. This approach optimizes microbial activity, balances nutrient composition, and enhances process stability, ultimately improving biogas production efficiency.

Q.42. Besides energy production, can the biogas production process effectively reduce the environmental risks of waste (such as water pollution, soil contamination, etc.)?

Response to Reviewer

Thank you for your insightful comments regarding the environmental implications of biogas production. As outlined in Directive 2009/28/EC, it is crucial to develop sustainable biofuel practices that do not compromise biodiversity or food production while significantly reducing greenhouse gas emissions.

The biogas production process indeed provides considerable benefits beyond energy production, particularly in mitigating environmental risks associated with waste management. By utilizing available raw materials, such as agricultural residues and manure, biogas production plays a pivotal role in reducing water pollution. This is primarily because these materials are prevented from entering waterways, where they could contribute to contamination.

In summary, biogas production contributes significantly to reducing the risks of water pollution and soil contamination, while aligning with the requirements outlined in Directive 2009/28/EC regarding sustainable biofuel development. We have elaborated on these issues further in Response 39, where we explore the benefits of utilizing fermentation residues over raw manure in detail.

Q.43. When selecting biogas substrates, have long-term sustainability factors (such as soil health, crop rotation, etc.) been considered?

Response to Reviewer

Thank you for your valuable feedback regarding the selection of biogas substrates and the consideration of long-term sustainability factors such as soil health and crop rotation.

In our substrate selection process, we prioritized the long-term sustainability of the available raw materials for biogas production within the defined territory. Several critical factors were taken into account, including the impact of these substrates on soil health, nutrient cycling, and overall agricultural practice sustainability. By choosing substrates that contribute positively to soil fertility and structure, we aim to support resilient agroecosystems.

Additionally, we have carried out the project "Innovative Composting Practices in a Public Utility Company (PUC-Čačak, Serbia): A Case Study of Effective Microorganisms in Urban Waste Management." This initiative provides valuable insights into the sustainable management of urban waste, further informing our approach to substrate selection. The findings from this case study help us understand the benefits of utilizing specific organic waste in a way that promotes environmental health and supports effective nutrient management.

We are also aware of the potential risks associated with substrate selection. By integrating resources from the territory and responding to local conditions, we can enhance the viability of our biogas production while safeguarding environmental health.

This topic has been further elaborated in Response 40, where we delve deeper into the specific criteria for selecting substrates and their alignment with sustainable agricultural practices.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The revised version has been substantially improved

Author Response

Cover Letter

Dear Editors,

We would like to thank you for the opportunity to revise our manuscript titled:

“Assessing Biogas Production Potential from Organic Waste and Livestock Byproducts in a Serbian Municipality: Implications for Sustainable Food Systems”,

submitted to Sustainability.

We appreciate the insightful and constructive feedback provided by the reviewers, which helped us to improve the clarity, depth, and real-world relevance of our manuscript. In response to the comments received, we have carefully revised the manuscript and addressed all suggestions and concerns in the attached document titled "Response to Reviewers' Comments."

We believe that the revisions have significantly improved the manuscript and made it more valuable to the journal’s readership. We hope the revised version meets your expectations and the standards of Sustainability.

Thank you once again for your consideration.

Sincerely,

Dr Dragan Milićević

Question 12: "Sentences are too long; abbreviations can improve readability."

While the response suggests that the current sentence length is appropriate, a few sections in the manuscript still contain rather long sentences, which could make comprehension more challenging for readers. To improve readability, the authors might consider breaking up complex sentences where possible and incorporating abbreviations strategically to avoid redundancy. A brief review of the introduction and discussion sections with this in mind could enhance clarity while keeping the scientific rigor intact.

Response to Reviewer Comment

We respectfully do not fully agree with this comment. In our view, the sentence structures used throughout the manuscript are logically constructed, contextually appropriate, and support the clarity and continuity of the scientific argument. Nevertheless, we recognize the importance of readability and are open to improvements.

To ensure targeted and effective revisions, we kindly ask the reviewer to indicate specific sentences or sections that they consider excessively long or difficult to follow. Such guidance would be valuable and allow us to adjust the phrasing in a way that enhances clarity without compromising scientific accuracy.

We appreciate the reviewer’s efforts and are committed to making improvements where necessary.

Question 20: "Control conditions and comparative basis must be clarified. Include statistical methods and sensitivity analysis." The additional explanation regarding control conditions and data normalization is much appreciated. However, one area that could benefit from further refinement is the sensitivity analysis, which is not yet fully addressed in the manuscript. Since variability in biogas yield can significantly influence conclusions, it would be helpful to include a short explanation of how such variability was assessed. If a specific statistical approach was used to evaluate potential fluctuations, mentioning it explicitly would strengthen the methodological section.

Response to Reviewer Comment

We thank the reviewer for this constructive and insightful comment. We agree that addressing variability in biogas yield is important for strengthening the robustness of our conclusions.

To clarify, while our study primarily relied on published yield coefficients and region-specific input data, we have now included a brief sensitivity discussion in the Methodology section to acknowledge the potential variability of biogas yield due to factors such as substrate composition, moisture content, and operating conditions.

Additionally, we have expanded the relevant paragraph to include reference to scenario-based comparisons and range-based variation analysis to illustrate how yield fluctuations might influence the projected energy outputs. Although a full statistical sensitivity model (e.g., Monte Carlo simulation) was beyond the scope of this study, we have noted this as a limitation and a potential direction for future work.

Question 25: "The discussion lacks a critical evaluation of study limitations."A brief discussion on study limitations would provide a more balanced perspective and further strengthen the credibility of the findings. Recognizing challenges such as data availability, logistical constraints, or variations in substrate composition would not diminish the impact of the results but rather enhance their scientific validity. Including a short subsection on study limitations could also provide useful insights for future research directions.

Response to Reviewer Comment

We thank the reviewer for this valuable comment. We agree that acknowledging the limitations of the study contributes to its scientific transparency and enhances the credibility of the results.

In response, we have added a short subsection on study limitations at the end of the 3. Optimizing Biogas Production: Case from Practice, highlighting key constraints such as data availability, generalization of biogas yield values from literature, and the lack of on-site measurement or pilot testing. These limitations do not detract from the core findings but rather provide important context and offer guidance for future research and validation efforts.

Question 26: "Some claims (e.g., methane yield potential) need numerical comparisons with similar studies."Since methane yield can vary widely depending on substrate type and experimental conditions, including a table or a short paragraph summarizing how the reported values compare with those in existing literature would help contextualize the study’s contributions. If direct numerical comparisons are challenging due to variability, highlight general trends or ranges from related studies.

Response to Reviewer Comment

We thank the reviewer for this valuable comment. The results obtained in our study are supported by appropriate and relevant references already included in the manuscript. However, as the reviewer correctly notes, methane yield values vary significantly depending on substrate type, composition, treatment methods, and digestion conditions. For this reason, direct numerical comparisons with existing studies can be challenging and potentially misleading.

Nevertheless, we have taken care to discuss our findings critically, emphasizing the context-specific nature of the data and clearly explaining the reasons behind observed differences. Where possible, we also referenced general trends and ranges from the literature to ensure our results are appropriately positioned within the broader scientific context. These values are consistent with those reported in biogas assessments conducted in Serbia and other European regions, and align with the estimates used in our analysis.

We believe this approach offers a balanced interpretation that reflects the inherent variability in biogas research while maintaining scientific rigor.

Question 35: "The results section should compare findings to current industry challenges to emphasize their impact."One way to further highlight its real-world significance would be to connect the findings to current industry challenges explicitly. For example, discussing how the study’s results address key issues such as substrate efficiency, process optimization, or operational feasibility in biogas facilities could make the practical implications even more transparent.

Response to Reviewer Comment

We appreciate the reviewer’s valuable comment and fully agree that linking our findings to current industry challenges enhances the real-world relevance of the study. In the revised manuscript, we have carefully addressed this aspect by incorporating additional discussion that connects our results to practical concerns in the biogas sector—such as substrate availability and efficiency, feedstock variability, and the operational feasibility of small- to medium-scale biogas facilities.

These additions emphasize how the identified waste streams could contribute to more stable and optimized biogas production, especially in rural or decentralized settings. The discussion also highlights how aligning organic waste management with biogas utilization supports sustainability objectives and resource efficiency. We believe this strengthens the practical impact of the study and responds directly to the reviewer’s suggestion.

Belgrade 27.03.2025.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The revisions have strengthened the study in many aspects, and the additional clarifications provided are appreciated. However, there are still a few areas where further refinements could enhance the clarity, methodological transparency, and overall impact of the paper:

Question 12: "Sentences are too long; abbreviations can improve readability."
While the response suggests that the current sentence length is appropriate, a few sections in the manuscript still contain rather long sentences, which could make comprehension more challenging for readers. To improve readability, the authors might consider breaking up complex sentences where possible and incorporating abbreviations strategically to avoid redundancy. A brief review of the introduction and discussion sections with this in mind could enhance clarity while keeping the scientific rigor intact.

Question 20: "Control conditions and comparative basis must be clarified. Include statistical methods and sensitivity analysis."
The additional explanation regarding control conditions and data normalization is much appreciated. However, one area that could benefit from further refinement is the sensitivity analysis, which is not yet fully addressed in the manuscript. Since variability in biogas yield can significantly influence conclusions, it would be helpful to include a short explanation of how such variability was assessed. If a specific statistical approach was used to evaluate potential fluctuations, mentioning it explicitly would strengthen the methodological section.

Question 25: "The discussion lacks a critical evaluation of study limitations."
A brief discussion on study limitations would provide a more balanced perspective and further strengthen the credibility of the findings. Recognizing challenges such as data availability, logistical constraints, or variations in substrate composition would not diminish the impact of the results but rather enhance their scientific validity. Including a short subsection on study limitations could also provide useful insights for future research directions.

Question 26: "Some claims (e.g., methane yield potential) need numerical comparisons with similar studies."
Since methane yield can vary widely depending on substrate type and experimental conditions, including a table or a short paragraph summarizing how the reported values compare with those in existing literature would help contextualize the study’s contributions. If direct numerical comparisons are challenging due to variability, highlight general trends or ranges from related studies.

Question 35: "The results section should compare findings to current industry challenges to emphasize their impact."
One way to further highlight its real-world significance would be to connect the findings to current industry challenges explicitly. For example, discussing how the study’s results address key issues such as substrate efficiency, process optimization, or operational feasibility in biogas facilities could make the practical implications even more transparent.

Author Response

Cover Letter

Dear Editors,

We would like to thank you for the opportunity to revise our manuscript titled:

“Assessing Biogas Production Potential from Organic Waste and Livestock Byproducts in a Serbian Municipality: Implications for Sustainable Food Systems”,

submitted to Sustainability.

Thank you once again for your consideration.

Sincerely,

Dr Dragan Milićević

Question 12: "Sentences are too long; abbreviations can improve readability."

Response to Reviewer Comment

We appreciate the reviewer’s efforts and are committed to making improvements where necessary.

Response to Reviewer Comment

We thank the reviewer for this constructive and insightful comment. We agree that addressing variability in biogas yield is important for strengthening the robustness of our conclusions.

Response to Reviewer Comment

We thank the reviewer for this valuable comment. We agree that acknowledging the limitations of the study contributes to its scientific transparency and enhances the credibility of the results.

Response to Reviewer Comment

We believe this approach offers a balanced interpretation that reflects the inherent variability in biogas research while maintaining scientific rigor.

Response to Reviewer Comment

Belgrade 27.03.2025.

Author Response File: Author Response.pdf