Phosphorus Removal from Real Wastewater Using Biochar Derived from Sewage Sludge Pretreated with Zero-Valent Iron Nanoparticles in a Fixed-Bed Column

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Manuscript number: water-4224307

MS Type: Research Article

Title: Study of the effectiveness of biochar derived from sewage sludge with zero-valent iron nano-particle additives in retaining phosphorus from real wastewater by filtration

The authors investigated the use of sewage sludge biochar, produced with and without nZVI, for removing phosphates from real, biologically treated wastewater. Authors performed the batch and column studies for real wastewater. The work is interesting and presented with detailed analysis.

There are some comments that the authors may consider in revising their manuscript as can be seen below:

• In the title and manuscript, the term “retaining phosphorus” is confusing, in the material and methods section (2.1) alone the research hypothesis is understandable. The terminology should be revised consistently across the manuscript to improve clarity.
• The improvement achieved by nZVI addition appears to be modest (~7%). The authors should clearly justify the need for nZVI incorporation and explicitly state the advancement of this work over existing studies (e.g., DOI: 10.1007/s11356-019-04479-6). A more critical and comparative discussion is required.
• Batch and column results are presented separately but are not clearly explained in the text or figures. Ex. Section 3.1 XRF data for both types (Figure 2 for Type I, Figure 3 for Type II), but the implications of their compositional differences are not thoroughly discussed in the context of their performance, similarly section 3.3 Type I and Type II. It is suggested that restructure section3.1 into 3.1.1 Biochar Produced with Pre-Digestion nZVI Addition" and "3.1.2 Biochar Produced with Post-Digestion nZVI Addition" the findings should be compared and discussed.
• Section 3.2 governing kinetic equations may be given as supplementary text.
• In Fig 2 & 3, the user of "+/-" notation is misleading and scientifically incorrect. The authors can represent "elemental calcium" or "total calcium"
• In Fig 12, at 100x magnification, it's difficult to identify specific microorganisms, authors should recheck and revise appropriately.
• nZVI addition before pyrolysis, loses nano-properties then why nanoparticles are used for the study? A detailed discussion about pre-pyrolysis and post-pyrolysis modifications is to be included.
• In column studies without breakthrough curves how the performance was assessed?
• Error bars are shown in several graphs; however, details of the statistical analysis and error estimation are missing in the Materials and Methods section. This information should be clearly included.
• The English throughout the manuscript must be revised; the text has numerous typos and punctuation errors. Please carefully go through and correct them.

Comments on the Quality of English Language

The English throughout the manuscript must be revised; many statements are misleading, and difficult to track the flow of work. This makes it very difficult for the reader to follow the logic and draw conclusions about the effect of the nZVI addition.

Author Response

Thank you very much for the valuable comments. The responses are provided in the attached file.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Novelty of the study should be better clarified. Sewage sludge biochar for phosphorus removal has been widely studied in the literature. The authors should clearly explain: i) What is the scientific novelty of adding nZVI before pyrolysis? ii)How does this study differ from previous works on Fe-modified biochar? Currently, the manuscript does not sufficiently demonstrate the originality of the approach.

Mechanism of phosphorus removal is not sufficiently discussed. The manuscript attributes phosphorus removal mainly to Ca and Fe content in biochar. However: i) adsorption, ii)precipitation, İİİ) ligand exchange. Mechanisms are not adequately analyzed. Additional discussion should be included regarding Ca–P precipitation ,Fe–P complex formation,surface complexation mechanisms.

Characterization of the biochar is insufficient. For a high-quality journal article, additional characterization is recommended such as BET surface area, SEM / EDS, XRD, FTIR, pHpzc. Without these analyses it is difficult to understand the adsorption mechanism.

Effect of nZVI appears limited. According to the manuscript highlights, the addition of 3% nZVI increased adsorption capacity by only about 7%. This raises important questions: i) Is the addition of nZVI economically justified? ii) What is the real benefit compared to unmodified biochar? The authors should discuss this clearly.

Column experiments require additional hydraulic description. In the methodology (around page 6), the authors describe column filtration experiments. However important parameters are missing such as column porosity ,bed depth ,hydraulic loading rate, empty bed contact time (EBCT). These parameters are necessary for reproducibility.

Breakthrough curve analysis is missing. Column adsorption studies normally include breakthrough curves, Thomas model ,Yoon–Nelson model,BDST model.The manuscript reports removal efficiency but does not provide proper column modeling.

Minor Comments:

Several grammatical errors should be corrected. Some references are outdated. Units should be standardized (mg/L, m/h etc.). Figures need improved resolution and captions. Equation numbering and formatting should be checked.

Comments on the Quality of English Language

The manuscript is generally understandable; however, the English language requires improvement to enhance clarity, readability, and academic style. Several sentences are grammatically incorrect or overly complex, which makes some sections difficult to follow. In particular, the Introduction, Materials and Methods, and Discussion sections would benefit from careful language editing to improve sentence structure, terminology consistency, and scientific precision.I recommend that the authors have the manuscript carefully revised by a professional English editing service or a person to ensure that the research findings are communicated clearly and effectively.

Author Response

Thank you very much for the valuable comments. The responses are provided in the attached file

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Summary and general comments

This work involves the study of sewage sludge biochar performance in removing phosphate from aqueous solution. The current version needs significant improvements across all sections of the manuscript before publication. There are some major concerns where the kinetic data is limited and do not provide proper kinetic analysis, and column experiment usually involve Ct/C0, rather than qt. Please see the specific comments for details.

 

Specific Comments

1. Title is not accurate. This is a column adsorption experiment.
2. Affliation numbering is wrong. The “number” refer to the affiliation, not number to label the author.
3. The highlight should include the adsorption capacity value to give an accurate sense of performance.
4. Abstract: Quoting the sentence “…confirmed by an “in batch” experiment during which…”. The sentence is too wordy and can be written more concisely. i.e. Batch adsorption experiment conducted and… It is noticed in the later methodology section that this work involves only the column adsorption study, which is a dynamic study of adsorption, and should not be labelled as a batch study.
5. Language should be improved. Many long sentences contain redundant words.
6. The introduction needs to focus on the adsorbent. The authors note high phosphate levels in sewage sludge, but still use sewage sludge biochar as an adsorbent. This required more explanation of why sewage sludge is still chosen.
7. The introduction should discuss whether similar work has already been published in the literature and highlight the novelty of this study.
8. Authors need to be mindful of the rules of using abbreviations in scientific writing. Currently, the rules are not followed. Do look online for the guide.
9. The methodology section needs to be improved.
   1. The methodology should include a list of reagents, their grades, and manufacturers.
   2. Lines 141-142. How did the authors achieve the limited O2 condition without N2 gas? What is the heating rate of the furnace?
   3. Lines 150 to 154 can be written concisely as a single sentence, avoiding repetition.
   4. Lines 170-172. It is not clear what +/- represents. If the authors mean the ionic form, they should follow the IUPAC convention for representing ions.
   5. Lines 175-176. What are the heating temperature and time for the BET analysis?
10. Observation of mixed in-text citation formatting at Line 181, and other parts of the manuscript. In some cases, rather than using the square bracket [number], switch to the (author, year) format, as observed in line 188. Authors should recheck the entire manuscript for similar errors. These errors should be avoided at all times.
11. Line 198. What is the adsorbent bed height?
12. What is used for keeping the adsorbent in the column? Silica wool?
13. Lines 202-203. It is unclear what is m/h. Is it supposed to be mL/h?
14. The experimental procurement for the determination of phosphate phosphorus, ammonium nitrogen and nitrate nitrogen should be briefly described. The reagents involved, the waiting time for colour development, and the UV/vis wavelengths at which they are measured.
15. Both “ml” and “mL” are observed. The unit should be consistent.
16. Figure 2. It is not clear what 1, 2, 3, and 4 in the legend mean. Authors should label it as NZVI x%. The figure’s caption needs to be sufficiently informative.
17. Rather than adding redundant words to label the adsorbent as “sampled marked no.1 3% nZVI”, or in another part of the manuscript as No.3, I suggest that the author use “3% nZVI” directly. Achieving clarity is important in scientific writing. This should be applied to the rest of the manuscript.
18. Recheck table 2 values. 0,99000? What does this value mean?
19. The switch of time unit from h to min from section 3.2 onwards. It is important to maintain consistency in scientific writing.
20. Figure 4. There are only 4 data points for the kinetic studies? How did the authors get the kinetic data? From the column data?
21. Figures 7, 9. It is unclear what the primary concentration is.
22. Table, Have authors tested whether any of these elements leached from the prepared biochar?
23. In the methodology, the authors mentioned measuring nitrogen-nitrate and ammonium nitrogen; however, there is no data or discussion on these measurements.
24. Usually, column studies use Ct/Co on the y-axis rather than qt.
25. This work uses column studies; however, there is no modelling using any of the column model such as yoon-nelson or Thomas models.
26. No explanation of the mechanism of phosphate retention

Comments on the Quality of English Language

See specific comments

Author Response

Thank you very much for the valuable comments. The responses are provided in the attached file.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

Dear Authors, after reviewing your manuscript, I have the following comments:
Lines 58–60: This is a rather debatable statement, as there is no direct relationship indicating that an increase in population necessarily leads to increased water consumption. It is well known that there are effective measures to reduce water use, including pricing policies, installation of water meters, and the use of water-saving household appliances and sanitary equipment in bathrooms and toilets.
In the Introduction, the authors should also clearly explain the fundamental difference between primary and secondary sludge. Is it always possible to use both types of sludge in agriculture?
Lines 67–72: The authors should provide the typical composition of biogas. It is also important to explain under which conditions CO₂ and H₂S are formed in higher or lower amounts. In its current form, this part of the text appears disorganized and lacks logical consistency.
Line 73: Why do the authors consider zero-valent iron to be a promising material? Why specifically nanoparticles rather than larger particle sizes? This material is relatively expensive. In water treatment, its main role is typically associated with redox reactions. Therefore, this choice should be properly justified and supported with relevant literature.
The authors focus only on biochar as a sorbent, while other potential sorbent materials are not discussed. The research objective is not clearly formulated, and both the practical significance and scientific novelty are lacking.
In the Introduction, the authors describe results obtained in the study; however, this information does not belong in this section.
The conclusions are not linked to the stated objectives, scientific novelty, practical significance, or even to the Introduction in general.

In my opinion, the manuscript resembles a report of laboratory experimental work and requires substantial revision to be considered a scientific article. While experimental results are presented, there is almost no discussion, particularly in terms of comparison with existing studies. This is especially important, as the topic is not new in the field of municipal wastewater sludge utilization.
One issue concerns the comparison between Type I and Type II biochars. The difference between them is defined as adding nZVI before or after digestion. However, the experiments were carried out using different sludge samples and separate digestion processes. This means that not only the timing of nZVI addition changes, but also the properties of the sludge itself. Therefore, it is not correct to state that the observed differences are caused only by when nZVI was added. This comparison should be treated more carefully.

Another issue relates to the explanation of phosphorus removal mechanisms. The manuscript states that Ca₃(PO₄)₂, hydroxyapatite, and Fe–P compounds are formed. However, the methods used (XRF, BET, SEM/EDS, microscopy) cannot confirm this directly. They only suggest possible mechanisms. Therefore, the conclusions should be written more cautiously (e.g., “suggest” instead of “confirm”).

Finally, there is a mismatch between material properties and performance. The 3% nZVI biochar has the highest surface area and pore volume, so it should work much better. However, in the column tests, the improvement is small (about 7%). This suggests that surface area is not the main factor, and that chemical composition (for example, calcium content) may be more important. This point should be explained more clearly.

Author Response

Thank you very much for the valuable comments. The responses are provided in the attached file.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The comments have been satisfactorily addressed, and I recommend the paper for publication in its current form.

Comments for author File: Comments.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The authors have clarified and responded sufficiently to the comments.

Reviewer 4 Report

Comments and Suggestions for Authors

I agree