Integrated Coagulation–Disinfection Using Aluminium Polychloride and Sodium Hypochlorite for Secondary Wastewater Treatment: Operational Advantages and DBP Mitigation

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Review of the manuscript water-3839852

Entitled “Integrated Coagulation–Disinfection Using Aluminium Polychloride and Sodium Hypochlorite for Secondary Wastewater Treatment: Operational Advantages and DBP Mitigation ”

The manuscript investigated the integration of coagulation and disinfection units into a simultaneous dosing in one unit using PACl as a coagulant and NaClO as disinfectant. This process has been applied to secondary wastewater effluent through extensive experimental work, resulting in a comprehensive database that can be useful for evaluating the process. The results indicate some benefit of this integrated approach. However, the data presentation requires adjustments to conclude the benefits of this integrated process accurately. The work is generally of good quality; some points need to be addressed before publication.

General comments:

1. The most critical flaw in this work is data presentation, especially regarding ANOVA and Design-Expert output charts. Since no measured data are presented in these charts, the representation of factors that are neglected in the model with a slope of zero, precisely because they are neglected in the model, is misleading. Additionally, the ANOVA table for each factor is not presented. Therefore, conclusions such as independence of NaClO dose and Fecal coliform inactivation are not supported with any data but only with a model that is prone to error. I recommend that the authors restructure their data presentation as per comments 7 and 8 below.
2. At times, the manuscript appears to have been prepared with insufficient attention to detail. For example, the title of the work in the main article and supporting information is different. Information on Template usage is also still in the main manuscript, although it is clearly stated that the “How to Use This Template” paragraph should be deleted.

 

More detailed comments:

1. Line 56: Salt is a very general and vague term here. Please rephrase.
2. Line 61: The sentence stating “Numerous studies…” should be supported with references..
3. Line 74-76: Aluminum in PACl is already in the highest oxidation state and cannot be further oxidized. Therefore, it cannot happen in a PACl and NaClO system and thus cannot be an argument to choose/study this system for phosphorus removal.
4. Table 2: A list of abbreviations in the main text of the article does not seem to be necessary. I suggest moving this to SI.
5. Line 240: “Multivariate statistical analysis”, no ANOVA summary table is presented, and the sentence indicates that it had only one response variable.
6. Line 278: Readers may find it helpful to see these pH shifts. Maybe add to the supporting information.
7. Figure 3: This shows the dependence of the model on each parameter. In that, Design-Expert excluded, for example, Al (Panel B) after ANOVA. This chart indicates that there is no dependency between the model and the factor, which is obvious, to say the least. In a way, these charts are redundant in their current form, as the slope of a factor is initially set to zero and then the chart shows it to be zero. For that reason, the software gives an error that this factor is not in the model. It will make sense to present how the actual measured data is relative to the model data. So either the actual vs predicted values, or an overlay of the measurement over the current charts or .... Additionally, please add the ANOVA results table to the paper or SI.
   The same can be said about Figures 4, 5, 6, 7, and 8.
8. Figure 5: This seems extremely unlikely to me, as there is no dependency between disinfectant dose and fecal inactivation. That’s another reason that measured data should also be included in the figure. It is mentioned that the chlorine was terminated in the samples taken from the pilot scale. Was residual chlorine present in the sample before termination? Was the residual chlorine measured?

Author Response

General comments:

1. The most critical flaw in this work is data presentation, especially regarding ANOVA and Design-Expert output charts. Since no measured data are presented in these charts, the representation of factors that are neglected in the model with a slope of zero, precisely because they are neglected in the model, is misleading. Additionally, the ANOVA table for each factor is not presented. Therefore, conclusions such as independence of NaClO dose and Fecal coliform inactivation are not supported with any data but only with a model that is prone to error. I recommend that the authors restructure their data presentation as per comments 7 and 8 below.

 

Response 1: We appreciate the reviewer’s careful assessment of this issue. We would like to note that the complete set of measured data was already provided in tabular form in the Supporting Information (SI). However, we agree that presenting the results in additional formats will improve clarity. Therefore, in the revised submission, we have:

1. Added ANOVA summary tables for all models to the SI.
2. Included “actual vs. predicted values” plots in the SI to illustrate the agreement between measured and modeled data.

We believe these additions address the reviewer’s concerns while maintaining a concise main manuscript.

 

2. At times, the manuscript appears to have been prepared with insufficient attention to detail. For example, the title of the work in the main article and supporting information is different. Information on Template usage is also still in the main manuscript, although it is clearly stated that the “How to Use This Template” paragraph should be deleted.

 

Response 2: We thank the reviewer for this observation. We have carefully reviewed the manuscript to ensure consistency and attention to detail. Specifically:

• The titles in the main article and supporting information have been corrected to match.
• The “How to Use This Template” text, which was mistakenly left in the submitted version before the Introduction, has now been completely removed.

 

 

More detailed comments:

Comment 1. Line 56: Salt is a very general and vague term here. Please rephrase.

Response 1: Thank you for the observation. We have revised this sentence to specify “Salt”. The revised sentence now reads “Coagulation is often conducted using either aluminum or iron-based coagulants such as polyaluminum chloride (PACl), alum, ferric chloride, or ferric sulfate.”

 

Comment 2: Line 61: The sentence stating “Numerous studies…” should be supported with references.

 

Response 2: We agree and have added several references [2–7] to support this statement. References added include:

1. Yan et al. (2008). DOI: https://doi.org/10.1016/j.watres.2007.12.006
2. Ji et al. (2024). DOI: https://doi.org/10.3390/w16111470
3. Wei et al. (2015). DOI: https://doi.org/10.1016/j.cjche.2015.02.003
4. Liu et al. (2019). DOI: https://doi.org/10.1016/j.jenvman.2018.08.101
5. Sharafi et al. (2017). DOI: https://doi.org/10.5004/dwt.2017.20227
6. Nunes et al. (2024). DOI: https://doi.org/10.3390/antibiotics13010068

 

Comment 3: Line 74-76: Aluminum in PACl is already in the highest oxidation state and cannot be further oxidized. Therefore, it cannot happen in a PACl and NaClO system and thus cannot be an argument to choose/study this system for phosphorus removal.

Response 3: We thank the reviewer for pointing out this important clarification. We revised the text to correct this. The section now reads:

“While oxidants can influence hydroxide formation for some metals (e.g., Fe-based salts), aluminum in PACl is already present in its highest oxidation state (+3) and cannot be further oxidized by NaClO. Thus, in PACl–NaClO systems, improvements in phosphorus removal are more likely to arise from synergistic interactions with organic matter and microbial inactivation, rather than enhanced hydroxide formation.”

 

Comment 4: Table 2: A list of abbreviations in the main text of the article does not seem to be necessary. I suggest moving this to SI.

Response 4: We thank the reviewer for this helpful suggestion. As recommended, we have moved Table 2 (list of abbreviations) from the main text to the Supporting Information file.

 

Comment 5: Line 240: “Multivariate statistical analysis”, no ANOVA summary table is presented, and the sentence indicates that it had only one response variable.

Response 5: We thank the reviewer for pointing this out. We have now included the complete ANOVA summary tables for each response variable in the Supporting Information (SI). This addition provides transparency regarding the statistical analysis and addresses the absence noted by the reviewer.

 

Comment 6: Line 278: Readers may find it helpful to see these pH shifts. Maybe add to the supporting information.

Response 6: We thank the reviewer for this suggestion. We measured pH in some of the samples and observed minor changes, which is why we mentioned the possibility of subtle pH shifts contributing to the reduced coagulation performance. However, pH was not systematically measured across all experiments, and therefore, we cannot provide supporting data in the SI. To avoid over-interpretation, we have revised the text to clarify that this explanation remains a possible mechanism rather than a confirmed result. The section now reads: “…the data suggest that NaClO induced subtle changes in particle surface chemistry, and possibly minor pH variations, that marginally reduced floc formation. Since pH was not systematically measured across all experiments, this explanation should be considered tentative and will be investigated further in future work.”

 

Comment 7: Figure 3: This shows the dependence of the model on each parameter. In that, Design-Expert excluded, for example, Al (Panel B) after ANOVA. This chart indicates that there is no dependency between the model and the factor, which is obvious, to say the least. In a way, these charts are redundant in their current form, as the slope of a factor is initially set to zero and then the chart shows it to be zero. For that reason, the software gives an error that this factor is not in the model. It will make sense to present how the actual measured data is relative to the model data. So either the actual vs predicted values, or an overlay of the measurement over the current charts or .... Additionally, please add the ANOVA results table to the paper or SI. The same can be said about Figures 4, 5, 6, 7, and 8.

Response 7: We thank the reviewer for this valuable suggestion. While all measured data were already included in the SI tables, we have now added ANOVA summary tables and “actual vs. predicted values” plots in the SI to better illustrate the relationship between experimental data and model predictions. These additions address the reviewer’s concerns, while keeping the main manuscript concise.

 

Comment 8: Figure 5: This seems extremely unlikely to me, as there is no dependency between disinfectant dose and fecal inactivation. That’s another reason that measured data should also be included in the figure. It is mentioned that the chlorine was terminated in the samples taken from the pilot scale. Was residual chlorine present in the sample before termination? Was the residual chlorine measured?

Response 8: We thank the reviewer for this helpful comment. Although all measured data were already presented in SI tables, we have now added an “actual vs. predicted values” plot for this dataset in the SI to facilitate comparison between measured and modeled results. Regarding residual chlorine, we confirm that both free and total chlorine were measured after treatment and before termination in all pilot-scale tests, and residual chlorine was consistently present in the samples.

Reviewer 2 Report

Comments and Suggestions for Authors

This paper investigated the integrated coagulation–disinfection for secondary wastewater treatment. However, there is a lack of discussion on the mechanisms involved. In addition, the overall format, figures, and tables are not well prepared. The specific comments are as follows:

 1. Abstract section: It is necessary to add some hard numbers. The summary content needs to be more concise, as there are currently too many words. The tense of some sentences is not very accurate and needs to be modified. 

2. Introduction section: The logic between paragraphs needs to be enhanced, as each paragraph is currently very short. The tense of some statements needs to be modified.

3. There are too many figures in the text, it is recommended to keep 6-7 figures. Put the rest in the supplementary materials section. 4. Line 390-394: The format is confusing.

5. Results section: Many paragraphs are too short, such as line 398-400, line 438-440, and line 476-479. The logic of the entire paragraph needs to be reorganized. 

6. Practical+Implications: This section is too long. 

7.Figures: The information in the image is insufficient. What do the black dots represent? What does the blue area represent? What does the red dashed line represent? What does the red text in Figure 3 mean? 

8. There are too many tables that need to be reorganized, with some placed in supplementary materials.

 

Comments on the Quality of English Language

 The tense of some sentences is not very accurate and needs to be modified. 

Author Response

Comment 1: Abstract section: It is necessary to add some hard numbers. The summary content needs to be more concise, as there are currently too many words. The tense of some sentences is not very accurate and needs to be modified. 

Response 1: We appreciate the reviewer’s comment. The abstract has been revised to be more concise, with corrections to sentence tense, and now includes key quantitative results to better summarize the study’s findings.

 

Comment 2: Introduction section: The logic between paragraphs needs to be enhanced, as each paragraph is currently very short. The tense of some statements needs to be modified.

Response 2: Thank you for the suggestion. We have restructured the Introduction to improve logical flow, merging shorter paragraphs and clarifying transitions. The background discussion now progresses from (i) sustainability challenges to (ii) coagulation, (iii) phosphorus removal, (iv) disinfection and microbial risks, and (v) DBPs. We have also revised verb tense throughout the section to distinguish between general knowledge (present tense) and reported findings (past tense).

 

Comment 3: There are too many figures in the text, it is recommended to keep 6-7 figures. Put the rest in the supplementary materials section.

Response 3: We appreciate the reviewer’s feedback. In preparing the revised manuscript, we have already reduced and consolidated figures where possible, and we moved a substantial portion of the original figures into Supplementary Information (SI). At present, the figures remaining in the main text are those we consider essential for clarity and for readers to follow the discussion without needing to constantly refer to the SI. We are concerned that moving additional figures would disrupt the flow and reduce accessibility of the key findings. Therefore, we respectfully kept the current set of figures in the main text while ensuring that supporting information is provided in the SI.

 

Comment 4: Line 390-394: The format is confusing.

Response 4: We thank the reviewer for pointing this out. We agree that the original format was difficult to follow. We have revised the paragraph for clarity by breaking it into a structured list.

 

Comment 5: Results section: Many paragraphs are too short, such as line 398-400, line 438-440, and line 476-479. The logic of the entire paragraph needs to be reorganized. 

Response 5: We thank the reviewer for noting that several short paragraphs disrupted the flow of the Results section. We have revised the section by merging short stand-alone paragraphs (e.g., lines 398–400, 438–440, and 476–479) into surrounding text. This restructuring improves logical continuity: descriptions of validation and graphical techniques are now presented within the main results narrative, and the discussion of future work on toxicity is combined with the preceding context on DBP health risks. These changes strengthen the overall coherence of the section.

 

Comment 6: Practical+Implications: This section is too long. 

Response 6: We thank the reviewer for this observation. We carefully revisited this section and made all the adjustments for clarity and shortened the text. However, we decided to retain some level of detail because this section directly connects our findings to full-scale application, sustainability benefits, and regulatory implications. Given that one of the aims of this study is to provide practical guidance for adoption in real systems, we believe that maintaining this detail is important for practitioners and stakeholders.

 

Comment 7: Figures: The information in the image is insufficient. What do the black dots represent? What does the blue area represent? What does the red dashed line represent? What does the red text in Figure 3 mean? 

Response 7: We thank the reviewer for pointing this out. In the figures generated using Design-Expert software, the black dots represent the range of actual measured data points, the blue shaded area shows confidence interval of the model prediction, and the red dashed line indicates the center point of the factor in the design. The red text (“Warning! Factor not in model”) appears when the factor plotted was not included in the statistical model for that response. We have clarified these points in the figure captions to improve clarity.

 

Comment 8: There are too many tables that need to be reorganized, with some placed in supplementary materials.

Response 8: We appreciate this suggestion. To streamline the manuscript, we have reduced the number of tables by moving a non-essential table to Supplementary Information. We have retained the remaining tables in the main text because they contain key data and model outputs that are critical for readers to follow the discussion without repeatedly referring to supplementary materials.

 

Reviewer 3 Report

Comments and Suggestions for Authors

1. Introduction: expand discussion of microbial risks, chlorine kinetics, and DBP regulation to balance background.
2. The description of the experimental setup is missing some key details, like how long the water was held, how many times the tests were repeated, and how they handled the variability. Please consider to accomodate this issue.
3. The figures and tables do a decent job of showing the results, but they're missing error bars and some of the backup data. Likewise, some of the statistical models didn't explain much of the data (they had a low R²), but the text still talks about them very confidently.
4. Methods: add details on when/how often samples were collected; note variability, justify as rapid-mix phase, or add rationale for why such short times were chosen, specify replicate count for each condition; clarify whether data are averages, provide calculated HRT and G values, and state explicitly that chlorine residual was always measured post-quenching.
5. Results: add error bars or replicate statistics to all plots, low R² models not contextualized (explicitly state model limitations; avoid overgeneralizing), and weakens applied significance of DBP reductions (add stronger discussion on health/environmental relevance of DBP reductions).
6. The conclusions seem to overstate how new the findings are and how much they contribute to sustainability. These claims would be stronger if they were backed up with numbers and if the study’s limitations were also mentioned.
7. The reference list is mostly fine, but a few of the spots for citations are still blank, and it doesn't cover as much of the literature on regulatory frameworks and DBP toxicity as it should.
8. The manuscript itself could use a bit of a polish. There are some leftover words from the template, a few mislabeled captions, and some clunky sentences that could be smoothed out with a little editing.

Author Response

Comment 1: Introduction: expand discussion of microbial risks, chlorine kinetics, and DBP regulation to balance background.

Response 1: We have expanded the Introduction to include more detail on microbial risks, chlorine kinetics, and DBP regulations. Specifically, we added the following text:

“Microbial risks, including bacteria, viruses, and protozoa, necessitate reliable disinfection to safeguard public health. The effectiveness of chlorine disinfection depends on both concentration and contact time (CT), and its inactivation kinetics vary depending on the target organisms and water quality conditions. However, chlorine also reacts with effluent organic matter (EfOM) to form disinfection by-products (DBPs), including trihalomethanes (THMs) and haloacetic acids (HAAs). These DBPs are regulated due to health concerns…”

 

Comment 2. The description of the experimental setup is missing some key details, like how long the water was held, how many times the tests were repeated, and how they handled the variability. Please consider to accommodate this issue.

Response 2: We thank the reviewer for this helpful comment. We have revised the Methods section to provide additional details. Specifically, the wastewater used in each test was collected fresh on the day of the experiment and was not stored prior to testing. Samples were analyzed immediately after mixing: general water quality parameters were measured directly, and aliquots were sent for DBP and microbial analysis. To ensure reproducibility, the bench-scale experiments were repeated on five separate days using independently collected wastewater samples. For the pilot-scale experiments, each operating condition was tested in duplicate. These clarifications have been added to Section 2.3.

 

Comment 3: The figures and tables do a decent job of showing the results, but they're missing error bars and some of the backup data. Likewise, some of the statistical models didn't explain much of the data (they had a low R²), but the text still talks about them very confidently.

Response 3: We thank the reviewer for this comment. The raw measured (actual) data was already provided in the Supporting Information tables. To further strengthen the data presentation, we have now added the full ANOVA output tables as well as actual vs. predicted value plots for all of the models and figures discussed in the manuscript (see SI Tables S7–S19 and Figures S6–S18). In the main text, we have included references to these additions in the relevant results sections. These additions provide the underlying statistical details and better illustrate the quality of the model fits.

 

Comment 4: Methods: add details on when/how often samples were collected; note variability, justify as rapid-mix phase, or add rationale for why such short times were chosen, specify replicate count for each condition; clarify whether data are averages, provide calculated HRT and G values, and state explicitly that chlorine residual was always measured post-quenching.

Response 4: We thank the reviewer for this comment. Detailed information regarding the experimental setups, including mixing times, mean velocity gradient (G) calculations, G·t values, hydraulic retention times, and sample collection procedures, is provided in Fallah et al. (2025) [21]. This reference covers all aspects requested, including rationale for rapid-mix times, replicate counts, and measurement of chlorine residuals post-quenching. We have added a citation to this work in the Methods section to guide readers to these detailed specifications.

 

Comment 5. Results: add error bars or replicate statistics to all plots, low R² models not contextualized (explicitly state model limitations; avoid overgeneralizing), and weakens applied significance of DBP reductions (add stronger discussion on health/environmental relevance of DBP reductions).

Response 5: We thank the reviewer for these comments.

• Error bars / replicate statistics: The figures are generated by the software and do not include error bars. However, all raw measured (actual) data, including replicates, are provided in the Supporting Information. This allows readers to evaluate variability and replicate data directly.
• Low R² models: We acknowledge that some models have low R² values. These models are presented as indicative of trends rather than precise predictions, avoiding overgeneralization. Full ANOVA output and actual vs. predicted plots are provided in the SI.
• DBP reduction significance: We have expanded the discussion in the Results and Discussion section (Section 3.3) to emphasize the health and environmental relevance of the observed DBP reductions, highlighting their potential benefits for regulatory compliance and reduced exposure risk.

 

Comment 6: The conclusions seem to overstate how new the findings are and how much they contribute to sustainability. These claims would be stronger if they were backed up with numbers and if the study’s limitations were also mentioned.

Response 6: We thank the reviewer for this comment. We revised the Conclusions section and added clarifying information to avoid overstatements about novelty and sustainability, providing a more balanced and accurate summary of the study’s findings.

 

Comment 7: The reference list is mostly fine, but a few of the spots for citations are still blank, and it doesn't cover as much of the literature on regulatory frameworks and DBP toxicity as it should.

Response 7: We thank the reviewer for this comment. All missing citation spots have been addressed, and we have added nine additional references, including three specifically focusing on DBP toxicity, to better cover the literature on regulatory frameworks and the health and environmental relevance of DBPs.

 

Comment 8: The manuscript itself could use a bit of a polish. There are some leftover words from the template, a few mislabeled captions, and some clunky sentences that could be smoothed out with a little editing.

Response 8: We thank the reviewer for this comment. The manuscript has been thoroughly revised to correct leftover template text, fix mislabeled captions, and improve sentence clarity and overall readability.

 

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript has been revised carefully. It can reach the level of the Journal.

Reviewer 3 Report

Comments and Suggestions for Authors

Congratulations accepted for publication.