Review Reports - Adsorption of Phosphonates to Iron- or Aluminum-Based Flocculants in Wastewater Treatment

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript entitled “Adsorption of phosphonates to iron- or aluminum-based flocculants in wastewater treatment” investigated the impact of varying iron (Fe) and aluminum (Al) contents on the adsorption of phosphonates to activated sludge. After reading the manuscript, some comments can be explained so that the authors can implement them for the improvement of this manuscript as below:

The article should emphasize some essential background information on the importance of studying. The introduction should be strengthened to highlight the research gap, review existing solutions, and underscore the study's originality and societal significance.
What about the physical and chemical composition of the adsorbent? The author did not characterize it using SEM-EDS, FTIR, XRF, XRD, or acid digestion analysis.
Since the authors did not study the cyclic regeneration stability of the adsorbent in Section 3, the ion leaching from the adsorbent during the adsorption process should somehow be indicated.
What do the authors think about the possibility of physical adsorption and chemical adsorption processes as the mechanism of adsorption of phosphonates to iron- or aluminum-based flocculants since the authors did not study the effect of pH and temperature? What about the ion exchange involved besides those physical and chemical adsorptions?
Please consider comparative analyses to validate the effectiveness of the adsorbent materials in comparison to existing methods or commercial products.

Author Response

1.The article should emphasize some essential background information on the importance of studying. The introduction should be strengthened to highlight the research gap, review existing solutions, and underscore the study's originality and societal significance.

Response: Phosponates are a main portion of the so called non-reactive phosphorus. They are difficult to eliminate by conventional chemical processes. Up to now, it was unclear, why some treatment plants eliminate a large portion of the phosphonates but others not. In special, it was not clear, if the organic sludge components are able to adsorb phosphonates. In this study, we found, that this is nearly negligible.

To clarify this, we tried to improve the introduction.

2.What about the physical and chemical composition of the adsorbent? The author did not characterize it using SEM-EDS, FTIR, XRF, XRD, or acid digestion analysis.

Response: This study aimed to identify the sites that are responsible for adsorption of phosphonates to activated sludge. Therefore, we measured the content of metals in the flocs using acid digestion and MP-AES. SEM-EDS analyses have been done also. Results are presented e.g. in Figures 4 and 5. A complete elemental composition is given in the supplement. We analyzed al individual sludge samples also. However, to present all these results would not increase the information of this paper.

3.Since the authors did not study the cyclic regeneration stability of the adsorbent in Section 3, the ion leaching from the adsorbent during the adsorption process should somehow be indicated.

Response: During waste water treatment, the activated sludge is not primary used as an adsorbent. Adsorption is an effect that can be measured. Therefore, activated sludge treatment does not contain any adsorbent regeneration. The aim of this study was to identify the adsorbing part in the sludge flocs.

The adsorbent leaching is shown as an example in Fig. 2. In all other adsorption experiments the leaching of metals due to complexation is measured and taken into account.

This information has been added in the methods section.

4.What do the authors think about the possibility of physical adsorption and chemical adsorption processes as the mechanism of adsorption of phosphonates to iron- or aluminum-based flocculants since the authors did not study the effect of pH and temperature? What about the ion exchange involved besides those physical and chemical adsorptions?

Response: As the aim of this study was to identify the adsorbing sites inside of activated sludge flocs, we did not study the adsorption mechanisms. However, from other studies it can be concluded that chemical sorption is dominant. This has also been described e.g. by Cheng et al. (https://doi.org/10.1016/j.jwpe.2017.07.002)

5.Please consider comparative analyses to validate the effectiveness of the adsorbent materials in comparison to existing methods or commercial products.

Response: The aim if this study was not to provide an efficient adsorbent. The aim of the study was to identify the sites of adsorption within activated sludge.

Reviewer 2 Report

Comments and Suggestions for Authors

Thank you for your well-structured and valuable study. The experiments are clearly designed, and the results support your conclusions. The paper contributes useful insights into phosphonate adsorption in activated sludge systems.

I have only a few minor suggestions to help improve the manuscript:

Some sentences are quite long and could be simplified to make the text easier to read.

A few figures and tables could be presented more clearly, especially by improving labeling or spacing.

The conclusion successfully summarizes the main findings; however, it would benefit from additional clarification of the practical implications of the results for wastewater treatment operations. Including a brief note on how the findings may affect the use of iron-based coagulants in real systems, as well as acknowledging the limitations of the study (e.g., laboratory scale, selected phosphonates only), would strengthen the overall impact and completeness of the conclusion.

The description of the sludge enrichment procedure may benefit from a slightly clearer explanation or shorter sentences.

See the attached file.

These are small points and can be corrected easily. Overall, the manuscript is strong, and I encourage you to address these minor issues to further enhance the clarity of your work.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

The English in the manuscript is generally clear, but a few sentences are long and could be written in a simpler way to improve readability. A light language edit is recommended to make the ideas flow more smoothly.

Author Response

Thank You for carefully reviewing our manuscript and for your helpful comments.

Here our response to your remarks:

Some sentences are quite long and could be simplified to make the text easier to read.

We checked the text again with a native speaker and tried to shorten some sentences.

A few figures and tables could be presented more clearly, especially by improving labeling or spacing.

According your remarks, Fig. 9 has been completely reorganized. In other figures, we rearranged the legends and have improved the information in the figure description. We hope that this makes the content more clearly.

Thanks for the helpful recommendations. We added some more information about limitation and practical application of the results. The basic outcome of this study was to clarify the information about the site for the adsorption of phosphonates in the activated sludge process. The main conclusion for practical application is that simultaneous chemical phosphorus elimination should be more efficient than a separated precipitation/flocculation step.

The description of the sludge enrichment procedure may benefit from a slightly clearer explanation or shorter sentences.

We tried to clarify the procedure in the method section.

Reviewer 3 Report

Comments and Suggestions for Authors

The article "Adsorption of phosphonates to iron- or aluminum-based flocculants in wastewater treatment" is devoted to a relevant and interesting topic.

The paper is written in a very good scientific style and is presented logically.

The article is recommended for publication after minor revision.

Comments are described below:

The value of the work would be increased if the authors added information on the characteristics of the sludges.

Authors may provide sludge volume index (SVI), morphological characteristics (dense or loose floccules, etc.), surface charge, etc. Sludges can be very different.

The adsorption mechanism also depends on this.

Author Response

The value of the work would be increased if the authors added information on the characteristics of the sludges.

Authors may provide sludge volume index (SVI), morphological characteristics (dense or loose floccules, etc.), surface charge, etc. Sludges can be very different.

The adsorption mechanism also depends on this.

Thank You for reviewing our paper.

We completely agree, that the sludge characteristics can be significantly different. However, we tried to use sludge with comparable morphology. We have added the sludge description protocols as supplements Fig. S2 to S4.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have improved the manuscript well.

There are occasional grammatical errors and typographical errors; please check again for the overall manuscript.

L. 56: "makes it increasingly difficult to consistently comply" -> "makes consistent compliance with discharge standards increasingly difficult."
L. 288: "in al technical" -> "in all technical."

The results must be compared with those obtained by others so as to show the advantages of the proposed research.

Author Response

1. There are occasional grammatical errors and typographical errors; please check again for the overall manuscript.

We would like to thank the reviewer for the thorough review and helpful suggestions. According to the recommendation, we checked again spelling and grammar and eliminated several mistakes.

2. The results must be compared with those obtained by others so as to show the advantages of the proposed research.

We checked again the discussion. In our opinion, all relevant studies from other researchers have been considered. The results of our work were compared with those of the other authors. Similarities and differences were evaluated.

To our knowledge, no study has yet identified the components of activated sludge that are responsible for the adsorption of the phosphonates. This significant gap in knowledge about the elimination of phosphonates in WWTPs has been recently highlighted by a panel of experts from the DWA also.