Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This article addresses a significant and current environmental issue: the presence of microplastics in wastewater treatment plant water. Despite the importance of the topic, the work was drafted in a non-transparent manner, making it difficult to evaluate and understand the presented results. The combination of the results and discussion sections makes the interpretation of the data unclear, and the conclusions do not follow directly from the analyses conducted.

The title of the article does not fully reflect its content. The phrase "removal of microplastics" suggests an intentional process, while in reality, only the variation in their content in treated waters was studied. The main goal of the work, which is to explain the differences and causes between wastewater treatment plant waters from the mainland and island parts, should be presented in sub-objectives. This will facilitate the overall understanding of the work, as well as the implementation and description of the results and discussion sections.

More detailed comments are presented below:

1. Lines 94–111 – The goal is formulated too broadly. Specific objectives would be advisable. Furthermore, the subsequent text regarding sampling and methods is very general and essentially provides no significant information. It should be moved to the methods section.
2. Lines 113–127 – It is unclear how many samples were tested for microplastics. How many samples were collected at the inlet to the treatment plant, and how many at the outlet? This information is also missing from the rest of the article. It is also unclear why some locations were sampled in two campaigns (C1 and C2, and F1 and F2).
3. Lines 132–134 – Did filtration through cascade sieves (5 mm, 1 mm, 300 μm, 100 μm) yield as many samples as there were sieves? Was an integrated sample performed? Was this performed in the field? If so, how was the sieve rinsing with distilled water handled under difficult conditions (risk of loss of microplastic particles and fibers)? There is no information about the sieve diameter or how to prevent clogging when pouring 1,000 liters of water with a maximum load of 16,000 particles/m³.
4. Line 154 – It should be clarified that Fe(II) was prepared from FeSO₄·7H₂O.
5. Line 160 – No information about the filter diameter.
6. Line 165 – Which PerkinElmer model (with which detector) was used for the tests?
7. Figure 2 and the text of subsection 3.1 – Parameter variations should be indicated, rather than just average values. Even in the case of TSS, it is clear that parameters can differ significantly between individual measurements, making mechanical averaging of two values ​​methodologically unjustified. Averaging only makes sense when the values ​​are similar or when we have at least a dozen measurements, allowing for the presentation of both the mean and variability (e.g., standard deviation, range, or confidence intervals). In its current form, the interpretation of the results becomes unclear and can lead to erroneous conclusions. Alternatively, in addition to the figure, it is worth presenting a table of raw data that would include all the individual measurements, allowing for the assessment of parameter variability and more accurate comparisons between individual samples.
8. Figure 3 and the text of subsection 3.2 – Essential data necessary for interpreting the results of this study are missing. The number of samples tested and the types of polymers (usually designated as "n" in the text and figures) were not provided earlier in the work, as well as in subsection 3.2. Without this information, the percentage figure is meaningless and does not allow for a reliable interpretation of the results. This point also reveals the unclear way in which the sample analysis was described. The authors do not indicate whether the entire filter surface was analyzed by scanning or only specific test fields were selected, nor do they describe the filter processing and mapping method. The microplastic concentration is reported on line 315, which is surprising because it does not explain how microplastic particles were counted, what equipment was used, or what method was employed. The lack of this information prevents verification of the results and severely limits the scientific value of the presented data.
9. Figure 4 – Scale missing in the lower left corner. A description of the equipment used to obtain the image should be added.
10. Figure 5 – The text is incorrectly cited as "Figure 5a." Furthermore, the description in the article presents a completely different picture than Figure 5. For example, for objects F1–F2, the authors indicate that almost complete elimination of microplastics was achieved, while the bar on the "MP-effect" graph does not reflect this; rather, a reduction of approximately 50% is visible. This ambiguity needs to be clarified.
11. Subsection 3.4 – Using averaged values ​​of physicochemical parameters from wastewater treatment plants with different removal methods for the correlogram is questionable. It is better to divide the treatment plants according to differences in effluent water quality and only then perform the correlation. It should be noted that correlation may not be meaningful if the samples are heterogeneous. TSS can originate from various sources (organic matter, colloids, mineral particles, cellulose fibers), and the share of microplastics can vary by orders of magnitude. The current results suggest a relationship between microplastic content and the concentration of total suspended solids. However, this is not a new conclusion and stems directly from the physical properties of the suspended solids. If the authors wish to demonstrate a predictive relationship between TSS and microplastic concentration, they must conduct a series of measurements on several dozen samples from a given treatment plant type and develop local regression models.

 

For this article to be assessed positively, it requires a thorough overhaul. First, the main and specific objectives should be clarified, which should be directly linked to the obtained results and discussed in the discussion section. Next, the results should be clearly separated from the discussion, presenting the data in an orderly manner, and the interpretation should be based on comparisons with available literature, both domestic and international. The discussion should focus on comparing the microplastic content in waters from different types of treatment plants.

Author Response

Please see the attached file (for reviewer 1). 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The authors compared microplastic (MP) occurrence and removal across six wastewater treatment plants (WWTPs) in Thailand’s Eastern Economic Corridor (EEC), representing inland (oxidation ditch, aerated lagoon, stabilization ponds) and island (aerated tank; sand filtration + RO) systems. Influent and effluent streams were sampled (1,000 L per event), sieved (5 mm to 100 µm), subjected to wet peroxide oxidation (WPO), NaCl density separation, microscopic inspection, and FTIR confirmation (≥70% match). They report PE/PP dominance, fine MPs (100–300 µm) as the main fraction, removal efficiencies 86–98.5%, and a strong log–linear relationship between TSS and MP abundance (adjusted R² = 0.902), proposing TSS as a practical surrogate for routine MP monitoring.

Specific Comments

1. Abstract

• Abstract (lines 28–30): Sentence fragment: “Inland plants receiving heterogeneous municipal wastewater, including domestic inputs and agricultural runoff.” Changed to “Inland plants received heterogeneous municipal wastewater, including domestic inputs and agricultural runoff.”
• The manuscript uses MP/m³ and items L⁻¹ in some places (Supplement). Supplement Figure S1 labels items/L while the main text uses MP/m³.  Ensure MP abundance units are consistent (MP/m³ vs items·L⁻¹).

2. Introduction / Background / Rationale

• Intro (lines ~50–52): “wastewater treatment “plans” (WWTPs)” to “plants”.
• Intro (lines 96–100): “island systems would display strengthen influent characteristics” be changed to “island systems would display more consistent (or less variable) influent characteristics” — pick the intended meaning and rephrase.

3. Materials & Methods

• Section 2.4 states field and lab blanks were prepared (lines 172–180), but the manuscript does not report blank counts or how they were used to correct sample data. Please provide blank results (items per blank; polymer IDs) and state whether sample counts were blank-corrected. If airborne contamination controls / laminar flow hoods / clean-air devices were used, state specifics (Wesch et al. 2017). If not used, mention this as a limitation.

 

• Saturated NaCl cannot float high-density polymers (PET, PVC). Address this explicitly: did you detect any denser polymers? If so, explain how they were recovered; if not, acknowledge that NaCl may under-represent heavier polymers and discuss consequences for the polymer profile (e.g., PE/PP fraction may be inflated).

 

 

4. FTIR identification (lines 164–166; Supplement)

• FTIR parameters are given (transmission mode, 128 scans, 1200–4000 cm⁻¹, 4 cm⁻¹ resolution), and a ≥70% match threshold was used to classify MPs. The fingerprint region below 1200 cm⁻¹ includes many diagnostic peaks — explain why measurements started at 1200 cm⁻¹ and whether data below 1200 cm⁻¹ were available. If not, discuss the implications for misidentification risk and consider re-acquiring spectra in the full 400–4000 cm⁻¹ range for key/ambiguous particles.

• Secondly,  ≥70% match is low for automatic classification; please report the spectral library(s) used (vendor and version) and how many visually suspected particles were measured vs how many were confirmed by FTIR (N_visual and N_FTIR per sample/size class).

5. Conclusion

• Conclusion (lines 419–420): “Furthermoe” to “Furthermore” (typo).
• Author contributions (lines 435–436): author name spelled inconsistently: Suda Ittisupornrat vs Suda Ittisu-pornrat — ensure uniform spelling across the manuscript.

 

Author Response

Please see the attached file (for reviewer 2).

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript has been thoroughly revised in response to previous comments. The authors have largely incorporated the suggestions, clarifying the methodology, adding additional descriptions, and improving the overall coherence of the text. Although the "Results" and "Discussion" sections have not been completely separated, the current structure is clearer and more logical, significantly improving the readability of the manuscript.

However, a key issue remains regarding the mapping of microplastic filters. The manuscript does not clearly specify whether the entire filter or only selected filter areas were mapped. Clarifying this aspect is essential to assessing the reliability and reproducibility of the procedure; therefore, I recommend that the authors include a detailed description in the methodology section.

Author Response

Pleas see the attached file.

Author Response File: Author Response.pdf

Round 3

Reviewer 1 Report

Comments and Suggestions for Authors

All my comments, questions, and concerns have been addressed to the extent possible. Thank you very much for clarifying the issue regarding the total number of microplastic particles. The manuscript has been thoroughly revised and is now suitable for publication.