Pilot Study of Oxic–Anoxic Process under Low Dissolved Oxygen for Nitrogen Removal from Low COD/N Tropical Wastewater

Round 1

Reviewer 1 Report

The manuscript presents the results of research on the oxic-anoxic process of biological nitrogen removal from tropical wastewater, with low DO concentration, in a pilot installation operating in conditions close to natural. In this aspect, the paper undoubtedly brings new knowledge that can contribute to the implementation of this technology on a full technical scale.

The research was conducted in a way that did not raise my objections, the results are presented in an understandable way, and their interpretation is logical and adequate to the subject of the paper.

However, after a careful analysis of the presented manuscript, I would like to make a few remarks that require clarification by the authors:

1) the keywords include "aeration energy", which in my opinion is not justified by the content of the paper, because the research does not directly refer to this aspect; the process itself is undoubtedly energy-saving (lower aeration energy), but this was not the subject of the presented research; I suggest considering another keyword which would better reflect the subject of the paper (e.g. “tropical wastewater” or so?)

2) lines 254-259: the authors inform that the process conducted at low DO concentration was characterized by high TP removal efficiency - how can this effect be explained in the absence of a separate anaerobic phase in the SBR cycle?

3) Figure 2 presents the results obtained on selected (single) days of the experiment - what is the justification for choosing these specific days?

4) in section 3.2 possible SND is discussed, but authors’ thesis is only supported by references to other studies; would it not be advisable in this case to conduct research on the composition of the biocenosis of activated sludge in order to demonstrate the co-existence of nitrifiers and denitrifiers in AS flocs?

5) to what extent (according to the authors) the presented process depends on the temperature of the wastewater and whether it could be applied in other climatic conditions, where the average temperature of the wastewater is e.g. 15 degrees Celsius?

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The present manuscript ' Pilot Study of Oxic-Anoxic Process under Low Dissolved Oxygen for Nitrogen Removal from Low COD/N Tropical Wastewater' studied the biological nitrogen removal   of low COD/N tropical municipal wastewater by a pilot SBR reactor under low-DO condition. I think that this is a good paper and will serve the wastewater treatment community well. The manuscript is well written and structured and it is clear, however it may be accepted for publication subject to the following modifications:

1)There are some typing errors in line 126,129,135.(Error! Reference source not found.)?.

2) line 274, the authors stated that “suggesting that the nitrification and denitrification occur concurrently during the oxic phase”. How to prove that SND has occurred and why not conduct microbial community analysis to verify this?

3) The authors mentioned that ”PSS in low COD/N tropical wastewater is feasible to be utilized for the biological nitrogen removal process”. How long will it take for the PSS to be hydrolyzed into rbCOD? 

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

This study investigated the BNR at low DO condition (< 1.0 mg/L) to further reduce energy consumption, which is meaningful for WWTP DO control. The authors proposed that SND process happened under low DO condition. However, some important information was missing, and the figures need to be improved. As such, major revisions to address the comments below are suggested prior to further consideration for publication.

1. Please provide a table about the operating parameters of different stages.

2. Why 0.6 mg/L and 2.0 mg/L DO were chosen as the typical level.

3. What is the slowly-biodegradable COD and readily-biodegradable COD? Please give more details? What is difference between the rbCOD and sbCOD, and how about the test method? Which is new for readers.

4. DO is most important regulating factor, which should be provided in Fig. 1.

5. Fig. 1e, why such a increase of sludge volume at the beginning of Phase I?

6. Fig. 2: no error bars? Only test once?

7. Fig. 3: The symple name does not match the Y-axis title. NO3-N or NOx-N? What about the variation of both nitrate and nitrite?

8. As the DO concentration decreased from 2.0 mg/L to 0.6 mg/L, how about the energy saving? So for engineering application, whether the saving of aeration operation cost is considerable？

9. Too many Ref. cited errors were founded in the whole manuscript.

Line 126, Line 129~~~~

Grammatically errors are present throughout the manuscript.

Author Response

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Author Response File: Author Response.pdf

Reviewer 4 Report

Title: PILOT STUDY OF OXIC-ANOXIC PROCESS UNDER LOW DISSOLVED OXYGEN FOR NITROGEN REMOVAL FROM LOW COD/N TROPICAL WASTEWATER

Manuscript ID: water-2379575-peer-review-v1

 

The study concerns a pilot-scale experiment to improve, also in terms of energy savings, the biological removal of nitrogen in wastewater with a low COD/N ratio.

The research paper has been well conceived and is clearly the result of a demanding study, in particular the results are presented in a clear and complete way.

Minor revisions are recommended. Suggestions and general comments are provided below.

 

·        The length of the abstract is adequate, presenting less than 200 words as required by the journal.

·        The keywords are also relevant, equal to 5 and not already present in the title.

·        Among others, for example in the introduction, these studies on alternative external carbon source for denitrification could be cited:

–       Collivignarelli, M. C., Abbà, A., Caccamo, F. M., Carnevale Miino, M., Durante, A., Bellazzi, S., ... & Bertanza, G. (2021). How to produce an alternative carbon source for denitrification by treating and drastically reducing biological sewage sludge. Membranes, 11(12), 977.

–       Collivignarelli, M. C., Carnevale Miino, M., Caccamo, F. M., Baldi, M., & Abbà, A. (2021). Performance of full-scale thermophilic membrane bioreactor and assessment of the effect of the aqueous residue on mesophilic biological activity. Water, 13(13), 1754.

·        Line 121: In materials and methods, more technical characteristics of WWTP should be added, such as equivalent population, treated wastewater only civil or also industrial, water treatment chain.

·        Line 126, Line 129, Line 135: Probably some problem with inserting references “(Error! Reference source not found.)”

·        The number of references is suitable for this type of research article.

·        The conclusions are well structured and summarize the salient points of the study (in particular the results) in an organic and integrated way.

 

Comments for author File: Comments.pdf

Minor editing required.

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

No

No