Pollutant removal from municipal sewage by a microaerobic up-flow oxidation ditch coupled with micro-electrolysis

The development of efficient and low-cost wastewater treatment processes remains an important challenge. A microaerobic up-flow oxidation ditch (UOD) with micro-electrolysis by waterfall aeration was designed for treating real municipal wastewater. The effects of influential factors such as up-flow rate, waterfall height, reflux ratio, number of stages and iron dosing on pollutant removal were fully investigated, and the optimum conditions were obtained. The elimination efficiencies of chemical oxygen demand (COD), ammonia nitrogen (NH4+-N), total nitrogen (TN) and total phosphorus (TP) reached up to 84.33 ± 2.48%, 99.91 ± 0.09%, 93.63 ± 0.60% and 89.27 ± 1.40%, respectively, while the effluent concentrations of COD, NH4+-N, TN and TP were 20.67 ± 2.85, 0.02 ± 0.02, 1.39 ± 0.09 and 0.27 ± 0.02 mg l−1, respectively. Phosphorous removal was achieved by iron–carbon micro-electrolysis to form an insoluble ferric phosphate precipitate. The microbial community structure indicated that carbon and nitrogen were removed via multiple mechanisms, possibly including nitrification, partial nitrification, denitrification and anammox in the UOD.


Recommendation? Accept with minor revision (please list in comments)
Comments to the Author(s) Zhao et al. designed a microaerobic up-flow oxidation ditch (UOD) coupled with microelectrolysis for water purification. Development of cost-effective and environmentally sustainable methodologies for purification of municipal sewage water is an important research topic. The present work is impressive to warrant a publication in R. Soc. Open. Sci. journal.
Authors should include the mechanism figure which describes the chemical reaction scheme leads to removal of nitrogen and phosphorous from municipal water.

2.
Include the Comparison table showing the efficiency of present process over the reported methods in terms of cost effectives and green chemistry metrics.

Review form: Reviewer 2
Is the manuscript scientifically sound in its present form? Yes

Recommendation?
Major revision is needed (please make suggestions in comments)

Comments to the Author(s)
A) The authors consider the TN as the following TN = NH3-N + NO2-N + NO3-N What about organic nitrogen. The TN is calculated as the following: TN= Organic N + NH3-N + NO2-N + NO3-N B) Could you compare your method with other methods such as: 1-Combining UASB and the "fourth generation" down-flow hanging sponge reactor for municipal wastewater treatment, Water Sci Technol, 2006;53(3):209-18. doi: 10.2166/wst.2006 The editor assigned to your manuscript has now received comments from reviewers. I apologise that this has taken longer than usual.
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Once again, thank you for submitting your manuscript to Royal Society Open Science and I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. ********************************************** RSC Associate Editor: Comments to the Author: The research presented in this draft is original and of interest to RSOS audience, however some clarification regarding the mass balance of nitrogen is needed in addition to a comparison of your method with some others. Please read carefully each of the comments from the reviewers and address each of them.
Major issues 1. Authors should include the mechanism figure which describes the chemical reaction scheme leads to removal of nitrogen and phosphorous from municipal water.

Do you have any ethical concerns with this paper? No
Have you any concerns about statistical analyses in this paper? No

Recommendation?
Accept with minor revision (please list in comments) Figure 9 needs to be clear.

Decision letter (RSOS-201887.R1)
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Decision letter (RSOS-201887.R2)
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Dear editor and reviewers, we would like to thank the reviewers for their time and comments on our work for publication in this journal. They have provided a lot of thoughtful comments and suggestions. Our responses to their comments are provided below, and all changes in the revised manuscript are highlighted. In this file, the italic font indicates comments from reviewers, our responses are in blue font. The revised manuscript and figures with highlighted changes have been uploaded.

Reviewers' Comments to Author:
Reviewer: 1 Comments to the Author(s) Zhao et al. designed a microaerobic up-flow oxidation ditch (UOD) coupled with micro-electrolysis for water purification. Development of cost-effective and environmentally sustainable methodologies for purification of municipal sewage water is an important research topic. The present work is impressive to warrant a publication in R. Soc. Open. Sci. journal.
Major issues 1.Authors should include the mechanism figure which describes the chemical reaction scheme leads to removal of nitrogen and phosphorous from municipal water.
According to the suggestion from the review, the possible mechanism was proposed in Figure 10. The revised parts have been highlighted by red color font in the revised manuscript (on Page 7).

2.Include the Comparison table showing the efficiency of present process over the reported methods in terms of cost effectives and green chemistry metrics.
According to this suggestion, we have compared the efficiency of pollutant removalin this work and that reported in the literature. The corresponding comparisions were summarized in Table 2. Please see our further discussions on Page 6. The revised parts have been highlighted by red color font in the revised manuscript.

Reviewer: 2 Comments to the Author(s)
A) The authors consider the TN as the following TN = NH3-N + NO2-N + NO3-N What about organic nitrogen. The TN is calculated as the following: TN= Organic N + NH3-N + NO2-N + NO3-N Thanks for the comment from reviewer. The influent concentration of TN were detected by a multi-parameter portable colorimeter according to APHA method on Page 3. The description have been highlighted by red color font in the manuscript.
In a biochemical reaction (anaerobic-aerobic alternation), organic nitrogen can be converted to ammonia nitrogen. In addition, organic nitrogen is also present in the solids, and organic nitrogen in the effluent is also related to suspended solids. Due to a precipitation stepwith low-speed circulation (0.5 m/h) in the MUOD reactor, the activated sludge forms a dense layer, and suspended solid is effectively intercepted. Moreover, the sludge swells slightly, and the drain has a specific T-shaped head. Under these conditions, the specific concentration of SS in the effluent is very low. Therefore, we presumed that the organic nitrogen content in the effluent is close to zero and can be ignored. In this case, the TN of effluent can be approximately estimated by the equation of TN = NH3-N + NO2-N + NO3-N.
To further reply this question, we added the above discussion on Page 3. Please see the highlights by red color font in the revised manuscript.
B) Could you compare your method with other methods such as: 1-Combining UASB and the "fourth generation" down-flow hanging sponge reactor for municipal wastewater treatment, Water Sci Technol, 2006;53(3):209-18. doi: 10.2166/wst.2006.095. 2-The feasibility of using non-woven fabric as packing material for wastewater treatment. Desalination and Water Treatment. 2018;111:94-100. 3-Integration of UASB and down flow hanging non-woven fabric (DHNW) reactors for the treatment of sewage water. Desalination and Water Treatment. 2019;164:48-55 According to the review's suggestion, the comparisons of efficiency of pollutant removal with that reported in the literature were discussed (Page 6) and the comparison results were provided in Table 2. The revised parts have been highlighted by red color font in the revised manuscript.
C) The resolution of the figures is very low. The font size is very small in some figures.
Figures have been revised with higher resolution and larger font size.

D) Why did you neglect the BOD parameter in your work? This parameter measures the biodegradability of the organic loads in wastewater.
According to the suggestion, BOD has been added in Tables 1 and 2. E) What about the growth of sludge during the study?
After the reactor was started, the continuous growth and accumulation of biomass would reach an equilibrium state. Especially, after the reactor has been run over 2 years, we observed that the total amount of sludge was not changed that much. During our test period, no excess sludge was dischargedfrom thereactor. Therefore, we expected that the sludge age was long enough to occur the self-degradation in the reactor, resulting in the insignificant growth of sludge. In this case, the growth of sludge was presumed to be ignored in the reactor. So the growth of sludge was not tested.
To reply thisquestion, we have added the above discussion on page 6. The revised parts have been highlighted by blue color font in the revised manuscript.

F) What is the number of samples analyzed during the study?
To optimize the performance, totally, 60 samples were collected for each experimental condition, and a total of 378 samples were collected.We have added the sample numbers on page 3. The revised parts have been highlighted by blue color font in the revised manuscript.