Impacts of Polylactic Acid Microplastics on Performance and Microbial Dynamics in Activated Sludge System

Round 1

Reviewer 1 Report

Dear Authors,

The submitted Manuscript titled Impact of Polylactic Acid Microplastics on Performance and Microbial Dynamics in Activated Sludge System shows great significance and importance in environmental science and addresses an ever-increasing problem nowadays, that is, the issue of microplastic pollution.

The main objective of the research is clear, the applied methods are adequate and the results could greatly contribute to the field of wastewater treatment and management, and they also hold novelty to some extent.

My observations and recommendations:

- The Introduction section is really short in this form. The research paper has numerous relevant references, but the Introduction part doesn't provide enough information to get the full picture. The effects of MPs on microbial metabolism, microbial diversity; the various effects (physical, chemical) of different MPs should be elaborated further. 

- Authors claim that WAS has a hydrophobic nature, which is not necessarily true for every case - it largely depends on the overall and the microbial structure and composition (especially those of the EPS)

- The applied mathematical-statistical methods should be discussed during either the Materials & Methods part, or in the Discussion.

- The vast majority of the Figures are hard to see (the legend, measurement points are too small) and therefore results in a low comprehensibility. 

- The Tables where the results of this study and other references are shown sort of undermine the novelty of the paper in this form and a bit miselading. What does exactly 'Other setup' mean? The key differences (i.e. type of microplastics obviously) should be pointed out.

- The Conclusion section is too brief, the key findings supported by numerical data should be elaborated further. 

A thorough review and the correction of some minor typos and grammatical, semantical mistakes should be implemented. 

Author Response

Please see the attachment

Author Response File: Author Response.docx

Reviewer 2 Report

This research offers valuable insights into the nuanced impacts of PLA MPs on wastewater treatment, especially in the context of different concentrations. Not only does it provide a comprehensive evaluation of pollutant removal performance, but the study also dives deep into the roles and sensitivities of specific functional microorganisms. The revelation about PLA MPs' potential as biocarriers and slow-release carbon sources paves the way for innovative applications in microbial enrichment. Such thorough and well-presented findings are essential for the future development and optimization of wastewater treatment methodologies.

The manuscript is organized about materials and methods; results are clearly presented however I have the following comments.

1. At the method sections, How were the initial microbial populations in the SBRs standardized? Was there any pre-treatment done to ensure consistency in the initial microbial community?
2. Why were specific concentrations of PLA MPs chosen for this study? Were these based on realistic environmental concentrations or were they arbitrary?
3. How did the SBRs with no PLA MPs (control group) perform in terms of N and P removal when compared to SBRs with added PLA MPs?
4. The study mentions a significant increase in Acinetobacter abundance with PLA MPs addition. Are there any specific metabolic pathways or genes in Acinetobacter that might explain its increased abundance in the presence of PLA MPs?
5. Did the addition of PLA MPs affect other microbial populations not discussed in the study? If so, why were these not considered relevant to the study?
6. Qi et al. were referenced as stating that PLA MPs could act as biocarriers. Could you elaborate more on how PLA MPs serve as biocarriers and how this potentially influences microbial dynamics?
7. Were there any observed changes in the nitrification and denitrification rates corresponding to the changes in microbial abundances?
8. Based on your findings, what could be the potential long-term effects of increased PLA MPs in wastewater treatment systems, especially concerning nutrient removal?
9. What are the proposed hypotheses or mechanisms that could explain the depressed denitrification performance in SBRs with PLA MP exposure?
10. The study concludes by suggesting that further studies are warranted. What specific areas or aspects do you believe need more in-depth research based on your current findings?

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Dear Authors,

Thank you very much for your detailed response and corrections; I sincerely hope that my remarks and suggestions could be of help in improving the - inherently high - quality of your manuscript. No other / further issues or problem have been found. 

The use of English language and scientific formulation are fine; however a brief proofreading is advised nevertheless, just to be on the safe side.