Removal of Heavy Metals during Primary Treatment of Municipal Wastewater and Possibilities of Enhanced Removal: A Review

Round 1

Reviewer 1 Report

The manuscript's quality has increased since previous submission. Therefore, it is now suitable for publication.

Author Response

Thank you for taking your time to review our work and for your valuable input throughout the review process.

Reviewer 2 Report

The article is well-organised and written. The authors put a lot of work to review and summarize the most important elements of heavy metals occurrence and removal from wastewater.

I have few comments to improve the article:

 

Section 1. Introduction

Page 1, lines 36-39, there is no need to indicate after itself two times ref. [2]

Page 1, lines 31-39, I suggest to add to the first paragraph of Introduction that heavy metals occur in the aquatic environment, as wastewater due to the next paragraph is about wastewater. The text should be consistent, and one paragraph follows from the other.

Page 2, line 45, all shortcuts (including elements) should be explained when used first time.

Lack of section “Material and methods”. Despite the fact that this is the review paper, some methods were used during the state of the art analysis.  Authors should add this section to the article.

 

Section 2. Occurrence of heavy metals in wastewater and sludge

What was the criteria to include in the review elements from Tables 1 and 2, and silver (Ag)?

Well-organised Tables 1 and 2. Why silver (Ag) is not listed there? (and in Figures 1, Table 5). If there is no limited values for it, it should be clear from the text under.

 

Section Technologies for Enhanced Heavy Metal Removal

Well-organised section, incl. Table 4.

Page 13, line 437, 100% should be in one line.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

This article describes heavy metals removal from raw municipal wastewater in primary settlers to reduce heavy metals concentration in effluent and sludge.

The purpose of the study is important. But there are many comments to be considered.

First, English should be edited thoroughly.

Many old papers were referred. The article should be changed to a mini review paper referring to the latest papers.

It is unclear why you focused only on Cd, Cr, Cu, Pb, Zn and Hg.

Throughout the article, contents should be divided into smaller parts and subtitle should be added.

Line 42 to 44 should be modified.

Reviewer 2 Report

The manuscript is scientifically sound and valid, and reviews the topic in a comprehensive way. It could be reorganized to obtain a better structure that would enhance overall quality. tables' quality should be improved as they are sometimes hard to understand. Particular attention should be payed to figures. Please refer to my comments and remarks listed below.

1. Introduction is too lengthy and verbose. Part of it could be moved to other sections or constitute a section of their own.
2. L102: “more strict” should be changed in “stricter”,
3. Table 3: box-plot graphs may have better represented the data.
4. 8: “Based on the studies available…. …. Fig. 1”. I cannot see the correlation from the graph. The authors should provide a statistical analysis or value to justify their statement.
5. 2: R2 values are too low and therefore not significant of any relation of trend between the data points. A clear relation is seen in Figure 3, and probably an equation could be easily derived to describe the process.

 

Reviewer 3 Report

This review paper discusses reducing heavy metal concentrations in those wastewater components (water and sludge) that can be reused as valuable resources. In other words, this manuscript tends to converge the concepts of wastewater treatment plants (WWTPs) and water resource recovery facilities (WRRFs). The trends in research and practice in waste management has recently shifted toward resource recovery rather than ‘refuse’ management and control. It is because the circular economy has been globally identified as a sustainable solution for water crisis and climate change. Therefore, this paper can attract wide readership from those researchers who are generally interested in resource recovery and those who are in particular interested in heavy metal removal from wastewater. So, I recommend the publication of this paper in MDPI Water. The paper is well-structured, the information flows well, and the language quality is high enough to result in an enjoyable read. However, before publication, the paper should undergo a minor revision to address a few questions and concerns. The detailed comments are provided in the following: 

1. L205-207: Some information sound missing in this sentence, as the sentence does not seem complete: “They found that most of dissolved organic matter (<0.45 μm) in the effluents could be found in the truly dissolved fraction”. Does it mean a larger fraction of organic matter concentration was associated with the size distribution of <1 kDa? Please check and correct (if needed). Also, is there any value to quantify it? What do the authors mean by “most of dissolved organic matter”? Any percentage reported in your source?

In addition, about this part “The association of heavy metals to different size fractions”, what does it exactly mean? Association of heavy metals to different size fractions of heavy metals or organic matters? Please clarify.

Moreover, it seems that in this section (L206-211), you were trying to link size distribution of heavy metals and organic matter in wastewater, to suggest that efficient removal of heavy metal can happen through removing very small particulate organic matters. If that was the case, there is a missing connection between organic matter and heavy metal size distributions in this part. Further explanation can help resolving the issue.   

2. L211-213: Please specify how “biological treatment of wastewater containing low fractions of dissolved heavy metals may though contribute to desorption or solubilization of particle bound metals”
3. Table 3: You have only 11 studies cited, while you mentioned “12 studies” in the Table. Please correct. Also, what was the basis to calculate the percent values (dissolved heavy metals/total heavy metals? And mass percent?)
4. L260-261: This sentence is vague: “The negative correlation remained when one study at a time was left out, with exception for Zn.”
5. L269-283: in this paragraph, the authors are talking about the fractionation of dissolved heavy metals between colloidal and truly dissolved portions. However, in the opening sentence, they are justifying the measured values for these fractions with the “the assess for the design of enhanced removal”. What does it exactly mean? Are you trying to correlate the measured values of these fractions with the quantification method? Please clarify. The opening sentence should be re-phrased to match the rest of the paragraph.
6. Please define Suspended solids (SS) at its first appearance in the manuscript (I could not find any).
7. L293-295: This statement is a bit misleading: “Information on pH, HRT and SS removal is though often missing in published literature which makes it hard to fully generalize the findings (Cantinho et al., 2016).” This is not completely true as many wastewater-related literature sources report pH, HRT, TSS/VSS concentrations of the influent and effluent of the process which they have focused their study on. So, in the way that this sentence is written, it sounds like all the wastewater treatment sources miss all the above details – which is not correct. In addition, you just used one reference to point out to a general issue in a literature. I recommend that the authors try to diversify their citation. Please find at least 4-5 other references related to heavy metal removal in the primary sedimentation (in particular) or the whole WWTP (in general), which lack the suggested information. In this way, the readers are provided with sufficient supporting information to be able to judge for themselves. This will also increase the credibility of your work.    
8. L303-307: First of all, too long sentence. Separating it into 2-3 sentences helps the reader to follow easily. Also, provide a brief explanation about how the changes in redox potential may affect heavy metal removal.
9. L336: Neutral and low pH can both be characterized as < 8, rather than just neutral pH.
10. L350-353: why did you claim that the researchers “did not consider the possible influence of organic matter concentrations on K_P”? Immediately after that, you claimed that the researcher found an effect of biodegradable COD on heavy metal removal capability of the system. Biodegradable COD is one form of organic matter in the wastewater field. Please clarify or re-phrase your sentence to prevent confusion.
11. L364: what were those three compartments? Please define them.
12. Table 4, the last row: Guo et al. (2008) should be listed as one of the references in the last cell of this row.
13. L463: I presume a better term is “steady sorption performance”, rather than reproducibility. Reproducibility is mainly used for data analysis, and its use here can be confusing (at first, it brings “regeneration” of the sorbent to the reader’s mind).
14. L474: This term “and in simultaneous precipitation” does not fit into this sentence. If you insist keeping it, please provide additional information.
15. Figure 4: why the y-axis of Figure 4(g) is different from the y-axes of the other graphs in this Figure?
16. L655 & 659: what is AB system? I cannot recall AB is defined through the manuscript so far. What is A-stage?
17. L612-660: This section is more like the future outlook, because it provides recommendations for the future research and delineates why the recommended studies are required. As such, I recommend including it in the final section, which can be titled “Conclusions and outlook”. The information in Sections 6 and 7 can be merged (the repetition can be removed) and the combined information can be provided in this new section “Conclusions and outlook”.
18. L664-668: here, you should summarize that this new separation step has resulted into characterizing the heavy metals into three categories: heavy metals present in the particulate, colloidal, and dissolved phases (rather into just two phases of particulate and dissolved which were previously used before this new separation step was introduced into the analytical procedure).
19. L572-612 & 668-691: This section (L572-612) has been written with a very simplified and thus generalized assumption, i.e., 100% removal efficiency of adsorption and coagulation/flocculation processes for dissolved and particulate heavy metals, respectively. As a result, this part of the conclusions (L668-691) is not realistic and may not be fully correct. Although you have pointed out the uncertainties arising from this simplistic assumption in this section, I don’t think it is sufficient to publish this piece of information as it is. It is not desirable to introduce such biased conclusion into the ongoing discussion among the researchers. There must be some papers available in the current literature which worked on the performance and efficiency of these processes (adsorption and coagulation/flocculation) in heavy metal removal (and I assume those papers might be published by researchers from the chemical engineering discipline). So, I recommend the authors attempt to find those resources. Consulting with those resource, you should be able to find and incorporate the removal efficiencies of these processes for different types and fractions of heavy metal into your data analysis, to make it more convincing.
20. A few language errors (including missing articles “the” and proper punctuations) could be detected. So, a thorough proofread is strongly recommended before publication, with the help of an editor. Some examples include: L33: a group of high density; L81: primary settling; L218: contribute to; L234-235: This sentence does not make much sense “…when only sampling known to not include return liquors is considered…”. It can be written a simpler way to avoid confusion; L294: the published literature; L296: the optimization; L298: This is because…; L318: “to have a high importance” is not a very comment combination; L338: dependent on; L350: the experimental data; L374: according to(?!); L387: presumed not be relevant; etc.