A Novel Layered and Advanced Nitrogen Removal Filter with Gravel and Embedded Bio-Organic Carrier Based on Autotrophic and Heterotrophic Pathways
Round 1
Reviewer 1 Report (New Reviewer)
The authors of the paper entitled "A novel layered and advanced nitrogen removal filter with gravel and embedded bio-organic carrier based on autotrophic and heterotrophic pathways" constructed an up-flow layered nitrogen removal filter and evaluated its performance in treating a synthetic mixture of fresh sewage and secondary effluent, with a focus on evaluating the different proportions of mixtures. In general, it is an interesting work that aims to answer a problem that deserves attention, seeking answers that can contribute to developing strategies for treating sewage, particularly for removing nitrogen. Still, as a general impression, the manuscript needs careful revision, the following comments are for authors to consider:
1. Define what is "GB18918-2016".
2. In this study, a bio-organic carrier is produced and used. However, this only has an emphasis on methodology. What is the relevance of having done everything described in topic 2.3? Doesn't this deserve a background that justifies it in the introduction? How is it expected to impact results?
3. Topic 3.4: It needs to be clarified why you have chosen the sample from day 133, in phase III, for sequencing. Is this community distribution representative since it does not include the other phases?
4. The abstract overview should emphasis the outlines. That is, what is the problem industrially? Why nitrogen removal is necessary? etc. Just like that in introduction.
5. It is necessary to indicate a little more about how the experimental part was made in the abstract, for example, type of wastewater, scale of the experiment, what technique was used to identify microorganisms, etc.
6. The introduction does not clearly mention the objective of the article. It must be pointed out precisely.
7. Subsection 2.5.4 Nitrogen Conversion Pathways requires a more detailed explanation of what is done and for what purpose.
8. In CONCLUSIONS, the enlightments were not prominent enough. It is suggested to polish the language and highlight the key points.
9. Manuscript writing can be improved. Please review the English and punctuation.
10. Lines 353-357: The descriptions of the secondary sedimentation tank and sand settling tank in section 3.5 are recommended to be consistent with other places.
11. Check that all abbreviations in the manuscript are explained accordingly.
12. There are some writing errors, such as line 14, 118 and 221.
13. Lines 12 and 101: Check for ambiguities in the expressions regarding denitrification.
Overall, English is good. Minor editing of English language required, including some typos.
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report (New Reviewer)
This study constructed a novel layered nitrogen removal filter using inorganic and organic carriers. By introducing a portion of raw water as a carbon source to treat secondary effluent, advanced nitrogen removal in wastewater treatment plants can be achieved with low energy consumption. The authors demonstrate their knowledge of the different nitrogen transformation mechanisms involved in the biological removal of this element. The paper presents interesting results that can be used for other research works on the subject. It also presents the novelty of including a filtering material prepared by the authors. This is an interesting paper. However, more analyses are needed before this can be accepted for publication. The following issues need clarification:
1) why choose the material based on corncob? The introduction on organic carriers is somewhat inadequate and it is suggested to reorganize the logic and rewrite the introduction on carriers in the introduction.
2) I suggest mentioning earlier in the article, for example, in the abstract, that the study was conducted with synthetic sewage and justifying that this was done to test the proportions of the feed components.
3) line 12: The “denitrification” should be “nitrogen removal”, check for the same errors in the manuscript.
4) Define "AOB and NOB" and check that all abbreviations in the manuscript are explained accordingly.
5) line 221: Without data on the microbial fraction, it is not sufficient to infer the enrichment of nitrifying bacteria only by the increase in nitrification.
6) line 223: Please provide the relevant references for "while the denitrification performance slightly decreased with the increase of C/N, possibly due to the decrease in carbon release capacity of middle organic carriers ".
7) line 273: Please explain the reason why the removal efficiency of middle layer NO3--N increases while the C/N of the influent decreases in phase â…£.
8) Line 303: What caused the significant increase in ANAMMOX nitrogen removal contribution compared to the previous two stages, please provide more explanation.
9) Line 305: “Throughout the experiment, SD accounted for about 95% of the NO2--N produced by PD”. There is an ambiguity in this expression, whether SD consumes 95% of the middle layer or 95% of the entire reactor.
10) Line 311: Since only mid-layer SAA was measured, there is not enough evidence to say that organic carriers promote the enrichment of AnAOB. Further explanation is needed at the microbial level.
Author Response
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Author Response File: Author Response.pdf
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 paper investigated the nitrogen removal filter for wastewater. This is an interesting paper. However, more analyses and additional experiments are needed before this can be accepted for publication. The following issues need clarification:
1. A major concern is that the synthetic wastewater was used instead of the real wastewater in this manuscript. As mentioned in section 2.4, many chemicals were used to simulate the mixture of urban sewage and secondary effluent; however, how to confirm that the synthetic wastewater was identical to the real wastewater? How would the aim to provide a new energy-efficient removal process in “practical application” as the authors stated in introduction section be achieved?
2. In section 2.5.2, the manuscript stated that the “High-throughput sequencing was carried out to investigate the distribution of microbial species by the Bio-engineering (Shanghai) co, LTD.” It seems like that the samples were analyzed in another city. Considering the long transportation distance, would the samples especially microbial species not change since that they could not be analyzed immediately after collection?
3. It is unreasonable that the NO3-N removal efficiency of the top layer was negative.
4. Five pathways of nitrogen conversion were considered in this study, i.e. Anammox, PD, PN, SD, and SN, and the results were discussed in section 3.3; however, only the contribution of Anammox to nitrogen removal was shown in Figure 6. The data for other 4 pathways should be shown in Figures for convincing and comparing.
5. In section 2.2, the authors said that gravel and poly-caprolactone (PCL) were purchased from Taobao. However, Taobao is an online shopping portal, more detailed supplier information and some available parameters (such as grade level) are needed for clarification. Please also provide the grade level for the polyvinyl alcohol (PVA) and sodium alginate (SA). In addition, boric acid and CaCl2 were mentioned in section 2.3, lot of other chemicals were used in section 2.4; however, their names, supplier information, and grade level were not mentioned in section 2.2. All chemicals used in this manuscript should be included in an experimental material section.
6. Please define the COD the first time it was used.
7. When two or three numbers with the same unit close together, the first and the second units can be omitted. For example, page 1, line 17, “77 mg·L-1,10 mg·L-1 and 8 mg·L-1” can be ““77, 10, and 8 mg·L-1”. Please make changes accordingly throughout the manuscript.
Typo:
1. “4. Discussion” should be “4. Conclusions”
2. There are two section 3.5, one of them should be section 3.4.
Author Response
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Author Response File: 
Author Response.docx
Reviewer 2 Report
Comments
1. There is no novelty in this paper. The results were not sufficient.
2. There is no conclusion in this paper. Poor presentation.
3. Figure 1. The experimental setup unit's first letter should be capita. Check all the figures for such kinds of errors.
4. Figure 2. Should be redrawn. Poor quality also presentation quality should be increased.
5. Check all the equations formats.
Author Response
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Author Response File: Author Response.docx
