Mitigating Nitrous Oxide Emission from a Lab-Scale Membrane-Aerated Biofilm Reactor

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript discuss about the correlation between the MABR operating parameter and nitrous oxide emission. Overall, the flow of knowledge of the paper is good and well written. Nevertheless, there is some questions and comments related to the manuscript.

1. What is the innovative element of the lab scale MABR? There is publication of N2O emission from the full scale MABR. For example, Nerea Uri-Carreño, Per H. Nielsen, Krist V. Gernaey, Carlos Domingo-Félez, Xavier Flores-Alsina (2024) Nitrous oxide emissions from two full-scale membrane-aerated biofilm reactors, Science of The Total Environment,908.

Therefore, the reviewer would like to know the motivation/strength of using lab scale MABR .

2. Figure 2. The nitrification efficiency and nitrogen removal efficiency looks good. Is there any benchmark with similar published results related to MABR?

3. page 14. The discussion on “effect of mass transfer” and “Effect of substrate concentration profiles” is not scientifically convincing. The authors are suggested to described in details with a diagram or support with published papers.

 

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The submitted manuscript demonstrates substantial technical merit and addresses an important topic in wastewater treatment. The article contributes valuable insights into mitigating N₂O emissions in MABRs and offers practical strategies for optimization. However, the study's limited focus on lab-scale systems and synthetic wastewater constrains its applicability. Future research addressing biofilm dynamics, microbial community interactions, and real-world validation would significantly enhance the impact and relevance of these findings.

For the authors to consider.

Introduction
1.    The article addresses a critical issue in wastewater treatment mitigating nitrous oxide (N₂O) emissions, a potent greenhouse gas. This aligns with global climate change mitigation efforts and emerging wastewater treatment technologies. However, more detail on its environmental impact and regulatory frameworks is needed to strengthen the argument for the study's significance.
2.    The authors clearly articulate their goal of investigating the operational factors influencing N₂O emissions in an MABR system. However, the literature review is comprehensive but does not sufficiently highlight the gaps addressed by the study.

Materials and Methods
1.    The authors allow three months for biofilm maturation but do not provide evidence of microbial community stability or diversity. These factors are crucial for understanding biofilm performance. The authors need to provide more clarification about their biofilm maturation assumptions.
2.    The use of synthetic wastewater limits the applicability of findings to real-world conditions, where wastewater composition is more complex and variable. The authors need to clarify why they chose to use synthetic over real wastewater.

Results and Discussion
1.    The study provides detailed data on nitrogen removal efficiency, N₂O concentrations, and their relationship with operational variables such as COD:N ratios and airflow rates. However, the correlation analysis between N₂O emissions and bulk liquid concentrations (e.g., ammonium, nitrite) is weak and lacks depth. This raises questions about the reliability of bulk measurements in capturing biofilm dynamics.
2.    The transient experiments (COD and ammonium spikes) offer valuable insights but lack a deeper exploration of microbial response mechanisms.
3.    The discussion highlights the role of biofilm heterogeneity and its impact on N₂O production and consumption.
4.    While the authors note the complexity of biofilm interactions and highlight the role of biofilm heterogeneity and its impact on N₂O production and consumption, they do not investigate or quantify the microbial community composition, which could provide critical insights into N₂O dynamics.

Discussion
1.    The discussion delves into potential mechanisms driving N₂O production and emission, considering factors like oxygen penetration and substrate gradients. The study also acknowledges the dual role of biofilm as both sinks and sources of N₂O, emphasizing the importance of biofilm structure and microbial stratification. However, Some conclusions, such as the role of COD:N ratios in reducing N₂O emissions, may not be universally applicable across different wastewater types or MABR designs. In addition, the absence of predictive modeling to simulate N₂O dynamics limits the ability to generalize findings or predict behavior under untested conditions.

 

Recommendation

The article’s English grammar is generally clear and appropriate for a technical audience, with accurate use of scientific terminology and well-structured content. However, some areas require improvement to enhance readability and precision. Sentences are occasionally overly complex or verbose, and inconsistent tense usage disrupts the flow, particularly when switching between past and present tense unnecessarily. Articles ("a," "an," and "the") are sometimes omitted or misused, and preposition choices could be refined for smoother phrasing.

Conclusions
1.    The conclusions effectively summarize the key findings and provide actionable insights for practitioners and researchers. The focus on airflow rate management and COD:N optimization provides relevant guidance for wastewater treatment operators. However, the conclusions are based on a controlled lab-scale system with synthetic wastewater, making it challenging to apply the findings to full-scale or real-world systems.
2.    The authors acknowledge the complexity of biofilm dynamics but do not outline specific directions for future studies.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The topic taken is very interesting, current and practical. Therefore, it should be presented in a reliable way. In my opinion, the article is correct in terms of content, and the methodology used is appropriate. However, it requires some supplementation.
1. Take care of the correct and uniform notation of units, please. In some places, the degree Celsius is missing the degree symbol.
2. The Authors describe key chemical processes in the introduction and in my opinion this description lacks 1. appropriate chemical reactions (fragment from lines 47-54) 2. and/or a diagram illustrating the described mechanisms 3. Literature references that would support the reliability of the reports (especially in fragments from 47 to 54).
3. Not all materials and elements have been described in full, e.g. the manufacturer of the membrane and details regarding its operation in technological processes are missing.
4. Complete the manufacturer's data as required by the journal.
5. The introduction of colors and increasing the font size on diagrams would significantly improve their readability. On the other hand, the data contained in tables is introduced excessively large - improve the aesthetics of data presentation.
6. The brief reports of the graphs in points: Ammonium spike results, COD spike results, Step loading changes should be enriched with a brief explanation of the causes of the phenomena. Despite the discussion, the addition of the type "what was caused by..." should take place. Especially since the Authors describe chemical processes that may be unknown to some Readers.
7. I understand that you have attempted to systematize the knowledge acquired - based on the results obtained - in the following sections: Effect of mass transfer, Effect of substrate concentration profiles, Change of production by autotrophs, etc. However, I do not know whether it would not be better to intertwine this knowledge with the results report. However, if you want to maintain the adopted form, you should expand the descriptions of phenomena in terms of content and necessarily support - in all points - with literature knowledge. Please supplement this.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Mitigating nitrous oxide emission from a lab-scale membrane aerated biofilm reactor
I want to extend my sincere appreciation to the authors for thoughtfully addressing the comments provided in my peer review and for their efforts to enhance the quality of the manuscript. The revisions have significantly improved the clarity and depth of the article, ensuring that its scientific insights are communicated more effectively. By refining the discussions on operational factors influencing nitrous oxide emissions and incorporating clearer interpretations of the findings, the authors have made the study more accessible and impactful.
This article makes a valuable contribution to wastewater treatment and greenhouse gas mitigation, addressing a critical aspect of sustainable environmental management. Its focus on the practical optimization of membrane aerated biofilm reactors (MABRs) advances understanding in this area and offers actionable solutions for minimizing greenhouse gas emissions for real-world applications. This work represents a meaningful step forward in both the academic study and practical implementation of innovative wastewater treatment technologies, and I commend the authors for their significant contribution to this important research area.