The influence of electrode spacing on the performance of bioretention cell coupled with MFC

In order to explore the influence of electrode spacing on the performance of the enhanced bioretention system, four bioretention cells with microbial fuel cell (BRC–MFC) systems with different electrode spacing were designed, and the effect of electrode spacing on system performance was revealed by analysing its water treatment capacity and electricity production efficiency. The results showed that BRC–MFC had good water treatment capacity and could produce electricity simultaneously. Compared with other BRC–MFC systems with spacing, the BRC3 system (with an electrode spacing of 30 cm) had significant water treatment capacity under different organic loads, especially under high organic load (C/N = 10) operation, COD removal rate was as high as 98.49%, NH4+−N removal rate was as high as 97%, and it had a higher output voltage of 170.46 ± 6.17 mV. It could be seen that proper electrode spacing can effectively improve the water treatment capacity of the BRC–MFC system. This study provided a feasible method for improving the performance of the BRC–MFC system, and revealed the relevant mechanism. A proper electrode spacing with sufficient carbon sources could effectively improve the water treatment capacity of the BRC–MFC system.


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Review form: Reviewer 2 (Monika Sogani)
Is the manuscript scientifically sound in its present form? Yes

Are the interpretations and conclusions justified by the results? Yes
Is the language acceptable? Yes

Comments to the Author(s)
This study indicated ways for improving the performance of the BRC-MFC system by varying the electrode spacing on the performance of the system. The data presented seems sufficient to corroborate results. May be considered for publication in its present form.

Decision letter (RSOS-202024.R0)
We hope you are keeping well at this difficult and unusual time. We continue to value your support of the journal in these challenging circumstances. If Royal Society Open Science can assist you at all, please don't hesitate to let us know at the email address below.

Dear Dr Wang:
Title: Experimental study on the influence of electrode spacing on the performance of bioretention cell coupled with MFC Manuscript ID: RSOS-202024 Thank you for submitting the above manuscript to Royal Society Open Science. On behalf of the Editors and the Royal Society of Chemistry, I am pleased to inform you that your manuscript will be accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referee suggestions. Please find the reviewers' comments at the end of this email. I apologise it has taken longer than usual to be able to send you this decision.
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Comments to the Author: (There are no comments.) ********************************************** Reviewer comments to Author: Reviewer: 1 Comments to the Author(s) The manuscript reports the Experimental study on the influence of electrode spacing on the performance of bioretention cell coupled with MFC. Overall the experiments are performed well and also manuscript was well written, I recommend this research manuscript is appropriate to publish in this journal with minor revision mentioned below. Avoid using words such as we, he, she, them, their, etc. Language is good, but, proof reading by a native speak would avoid the minor errors. Please concrete the keywords and make it formally and academically. About the Introduction section to make the reading more clear and smooth it should be organized according to the following items: i) present state of the art; ii) literature review; iii) motivation and objective of the study proposed; iv) innovative contribution in terms of methodology developed. Authors are suggested to read the following articles and cite them in the appropriate places. Introduction must be revised to enhance readability i.e. I would also like to see following references in the revised version https://doi. Typographical errors are present throughout the manuscript. Authors are required to pay keen attention to this In the materials and methods, divide them into nice sub-sections. Provide the details of all the equipment's, instruments used, their model number, company of manufacture, country, etc. In the introduction section, write the novelty of the work and the problem statement clearly. 70% of the references should be from 2018, 2019 and 2020. Kindly do a careful literature review. The results and discussion section is ONLY to write your results and facilitate scientific / technical discussions. Provide mechanism based reactions and refer to important/recent literatures in the results and discussions Conclusions (<100 words) should be in line with the specific objectives of your work. Do not repeat the results and the methodology here Delete unwanted old references. Refer to references from the years 2017, 2018 and 2019. Figures number must rearrange in this manuscript. Authors must compare more results with previous publications mainly in characterization parts (Results and discussion) Only few previous publications in discussions. Why not authors compare the current results with previous publications. Authors are required to re-write the conclusion. Authors must check the manuscript carefully before submit to journal. Because still manuscript having grammar mistake and some words joined with some other words. Authors must show good results in abstract section to enhance the readers. Authors must use any one format throughout the manuscript for example minutes must be as "min"; "ml" must be as "mL" and hours must be as "h". Throughout the manuscript Please avoid reference overkill/run-on -do not use more than 3 references per sentence. If you need to use more, make sure you state the key relevant idea of each reference. Many sentences could be seen with our enough evidences or justifications -you appropriate references to justify your arguments. Additionally, please pay attention to revise/check the following: 1-English language throughout the manuscript. Please ensure to have a proof reading to your manuscript. 2-Similarity index shall not exceed 15% with no more than 1% from any source. Reviewer: 2 Comments to the Author(s) This study indicated ways for improving the performance of the BRC-MFC system by varying the electrode spacing on the performance of the system. The data presented seems sufficient to corroborate results. May be considered for publication in its present form.
Author's Response to Decision Letter for (RSOS-202024.R0) See Appendix A.

Are the interpretations and conclusions justified by the results? Yes
Is the language acceptable? Yes

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

Recommendation?
Accept as is

Comments to the Author(s) Accepted for publications
Review form: Reviewer 2 (Monika Sogani)

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

Recommendation? Accept as is
Comments to the Author(s) All the reviewers comments have been answered by the authors satisfactorily. The manuscript may be considered for publication.

Decision letter (RSOS-202024.R1)
We hope you are keeping well at this difficult and unusual time. We continue to value your support of the journal in these challenging circumstances. If Royal Society Open Science can assist you at all, please don't hesitate to let us know at the email address below.
Dear Dr Wang: Title: Experimental study on the influence of electrode spacing on the performance of bioretention cell coupled with MFC Manuscript ID: RSOS-202024.R1 It is a pleasure to accept your manuscript in its current form for publication in Royal Society Open Science. The chemistry content of Royal Society Open Science is published in collaboration with the Royal Society of Chemistry.
The comments of the reviewer(s) who reviewed your manuscript are included at the end of this email.
Please see the Royal Society Publishing guidance on how you may share your accepted author manuscript at https://royalsociety.org/journals/ethics-policies/media-embargo/. After publication, some additional ways to effectively promote your article can also be found here https://royalsociety.org/blog/2020/07/promoting-your-latest-paper-and-tracking-yourresults/. Dear Editor, Thank you very much for considering our manuscript for review and providing valuable comments by imminent reviewers. We have checked and revised our manuscript very carefully according to reviewer's comments. We have also checked the entire manuscript for language errors and corrected it carefully. All the changes made in our revised manuscript have been marked in YELLOW color and detailed response to reviewer's comments have been provided as given bellow. Kindly consider our revised manuscript for further evaluation and publication in Royal Society Open Science.

Zhaoyang Wang Lanzhou University Lanzhou, China 730050
Response to Reviewer's comments:

Reviewer: 1
The manuscript reports the Experimental study on the influence of electrode spacing on the performance of bioretention cell coupled with MFC. Overall the experiments are performed well and also manuscript was well written, I recommend this research manuscript is appropriate to publish in this journal with minor revision mentioned below. Response: Thank you very much for your positive and encouraging comments on our work. We sincerely appreciate your kind comments and suggestions. Please find bellow the response to each comments.

1)
Avoid using words such as we, he, she, them, their, etc. Response: Thank you very much for your kind suggestion. We have made changes in the revised manuscript. 2) Language is good, but, proof reading by a native speak would avoid the minor errors.
Response: Thank you very much for your encouraging and constructive suggestion. We have got native speakers proof reading. All the changes made in our revised manuscript have been marked in YELLOW color.

3) Please concrete the keywords and make it formally and academically.
Response: Thank you very much for your kind suggestion. We have added the keywords information in the revised manuscript. "However, Bioretention Cells (BRCs) are similar to constructed wetlands, which is a kind of ecological treatment system, using physical, chemical and biological effects to achieve removal effect, imitating the natural environment, and have good wastewater treatment effects without the need for energy input." 4) About the Introduction section to make the reading more clear and smooth it should be organized according to the following items: i) present state of the art; ii) literature review; iii) motivation and objective of the study proposed; iv) innovative contribution in terms of methodology developed. Response: Authors sincerely thank reviewer for this very important suggestion. We have revised Introduction in the revised manuscript. "Regarding water pollution, the world is facing two important challenges, one is compound pollutant pollution, and the other is energy shortage. In order to solve these two problems at the same time, a sustainable wastewater treatment technology has gradually become a research hotspot. Traditional wastewater treatment plants rely on the combined effects of physics, chemistry and biology. This process requires mechanical equipment and a large amount of energy input to achieve the removal of pollutants. However, Bioretention Cells (BRCs) are similar to constructed wetlands, which is a kind of ecological treatment system, using physical, chemical and biological effects to achieve removal effect, imitating the natural environment, and have good wastewater treatment effects without the need for energy input. Studies had shown that BRC had a significant removal effect on pollutants such as TSS, COD and heavy metals. Due to its small size, simple structure, good treatment effect, low cost, and strong sustainability, it has become a potential wastewater treatment technology. At present, the researches on BRC mainly focuses on the operation efficiency of facilities and the optimization of design parameters (Such as matrix type, gradation, hydraulic load, height of submerged layer, etc). The biogeobattery reflected a natural phenomenon that occurred at the oxidation-reduction interface of the earth's surface, it used electrons generated by microorganisms oxidizing organic carbon, sulfide and other electron donors in the anaerobic area were transmitted to the aerobic area through a "long distance" through the extracellular mediator, and then underwent a reduction reaction with electron acceptor such as oxygen. However, the operating principle of microbial fuel cells (MFCs) is similar to this. In recent years, as an emerging technology, it has great potential for simultaneous wastewater treatment and electricity generation, and it has attracted widespread attention from scholars. MFC use wastewater pollutants as fuels and convert these into electrical energy through electrochemically active microbial degradation compounds, making MFC a sustainable technology. In order to effectively play the MFC function, the anode area must be kept in an anaerobic state, while the oxygen in the cathode area combines with protons and electrons to form a circuit. The reactions that generally occur in MFC can be summarized by the following equation (Eqs.(1)-(3)): Anodic reaction: CH3COO − + H2O → 2CO2 + 2H + + 8e − (1) Cathodic reaction: O2 + 4e − + 4H + → 2H2O (2) Overall reaction: C6H12O6 + 2H2O + 6O2 → 6CO2 + 8H2O (3) There had been some early studies on independent BRC systems and MFC systems. However, the coupling of BRC and MFC is still in the initial stage of research. In this study, BRC and MFC were coupled to form a BRC-MFC system, in this system, the entire reaction column had both anaerobic and aerobic conditions. When the organic matter in the anode area was oxidized, it would produce the electrons (e-) and protons (H+), then moved to the cathode, and the external circuit transferred the electrons from the anode to the cathode through the insulated copper wire to generate current. By studying the effect of electrode spacing on the performance of BRC-MFC, on the one hand, this study could test the feasibility of the coupling system of BRC and MFC to treat wastewater and generate electricity at the same time, which laid the foundation for future research, and on the other hand to explore the influence of electrode spacing on the water treatment capacity of the BRC-MFC system, and to obtain the best electrode spacing, which provided reference for future practical application."

5)
Authors are suggested to read the following articles and cite them in the appropriate places. Introduction must be revised to enhance readability i.e. I would also like to see following references in the revised version https://doi.org/10.1016/j.fuel.2019.05.109 https://doi.org/10.1016/j.biortech.2020.123110 https://doi.org/10.1002/er.5734 https://doi.org/10.1016/j.fuel.2019.115682 https://doi.org/10.1016/j.fuel.2020.118119 Typographical errors are present throughout the manuscript. Response: Thank you very much for your kind suggestion. We have added these references and revised Introduction in the revised manuscript. For typographical errors, we have corrected and changed in the entire revised manuscript.

6)
Authors are required to pay keen attention to this In the materials and methods, divide them into nice sub-sections. Response: Thank you very much for your kind suggestion. We have add sub-sections in the revised manuscript. 7) Provide the details of all the equipment's, instruments used, their model number, company of manufacture, country, etc.

8)
In the introduction section, write the novelty of the work and the problem statement clearly. Response: Authors sincerely thank reviewer for this very important suggestion. We have corrected the novelty of the work and the problem statement in the entire revised manuscript. "There had been some early studies on independent BRC systems and MFC systems. However, the coupling of BRC and MFC is still in the initial stage of research. In this study, BRC and MFC were coupled to form a BRC-MFC system, in this system, the entire reaction column had both anaerobic and aerobic conditions. When the organic matter in the anode area was oxidized, it would produce the electrons (e-) and protons (H+), then moved to the cathode, and the external circuit transferred the electrons from the anode to the cathode through the insulated copper wire to generate current. By studying the effect of electrode spacing on the performance of BRC-MFC, on the one hand, this study could test the feasibility of the coupling system of BRC and MFC to treat wastewater and generate electricity at the same time, which laid the foundation for future research, and on the other hand to explore the influence of electrode spacing on the water treatment capacity of the BRC-MFC system, and to obtain the best electrode spacing, which provided reference for future practical application." 9) 70% of the references should be from 2018, 2019 and 2020. Kindly do a careful literature review. Response: Thank you very much for your kind suggestion. We have updated some of the references and changed in the revised manuscript.

10)
The results and discussion section is ONLY to write your results and facilitate scientific / technical discussions. Provide mechanism based reactions and refer to important/recent literatures in the results and discussions Conclusions (<100 words) should be in line with the specific objectives of your work. Do not repeat the results and the methodology here Delete unwanted old references. Refer to references from the years 2017, 2018 and 2019. Figures manuscript must rearrange in this manuscript.
Response: Thank you very much for your kind suggestions. We have added the related mechanism-based reactions and refernces and changed in the revised manuscript. For Conclusions, we have corrected and changed in the entire revised manuscript. "The main purpose of this research was to explore the optimal electrode spacing of the BRC-MFC system for wastewater treatment. The study found that when the electrode spacing was 30 cm, it had significant water treatment capacity and high output voltage. In addition, the results showed that sufficient carbon sources would also improve the water treatment capacity and electricity generation efficiency. A proper electrode spacing could effectively improve the water treatment capacity of the BRC-MFC system. Nevertheless, to make the water treatment capacity of the BRC-MFC system achieve the best effect by changing the electrode spacing, further experiments are needed." We have deleted some old references and added some new references. The figures manuscript of manuscript has been arranged.

11)
Authors must compare more results with previous publications mainly in characterization parts (Results and discussion). Only few previous publications in discussions. Why not authors compare the current results with previous publications. Response: Thank you very much for your kind query. We have added some previous publications in Results and discussion and changed in the revised manuscript.

12)
Authors are required to re-write the conclusion. Response: Thank you very much for your kind suggestion. We have re-written the conclusion and changed in the revised manuscript. "The main purpose of this research was to explore the optimal electrode spacing of the BRC-MFC system for wastewater treatment. The study found that when the electrode spacing was 30 cm, it had significant water treatment capacity and high output voltage. In addition, the results showed that sufficient carbon sources would also improve the water treatment capacity and electricity generation efficiency. A proper electrode spacing could effectively improve the water treatment capacity of the BRC-MFC system. Nevertheless, to make the water treatment capacity of the BRC-MFC system achieve the best effect by changing the electrode spacing, further experiments are needed." 13) Authors must check the manuscript carefully before submit to journal. Because still manuscript having grammar mistake and some words joined with some other words. Response: Thank you very much for your kind suggestion. We have corrected mistakes and changed in the entire revised manuscript.

14)
Authors must show good results in abstract section to enhance the readers. Response: Thank you very much for your kind suggestion. We have corrected in the revised manuscript.

15)
Authors must use any one format throughout the manuscript for example minutes must be as "min"; "ml" must be as "mL" and hours must be as "h". Throughout the manuscript Please avoid reference overkill/run-on -do not use more than 3 references per sentence. If you need to use more, make sure you state the key relevant idea of each reference. Response: Thank you very much for your kind suggestion. We have corrected format and changed in the revised manuscript. For references, we have also changed in the revised manuscript.

16)
Many sentences could be seen with our enough evidences or justifications -you appropriate references to justify your arguments. Response: Authors sincerely thank Editor for valuable suggestion. We have replaced some long sentences with references and changed in the revised manuscript.

17)
Additionally, please pay attention to revise/check the following: 1-English language throughout the manuscript. Please ensure to have a proof reading to your manuscript. 2-Similarity index shall not exceed 15% with no more than 1% from any source. 3-Citation of references with latest publications on the topic. 4-Manuscript shall have an adequate number of Figures and Tables. 5-Figures must be in high quality format.Please ensure also to submit a high quality Graphical Abstract representing a summary of your study. Response: Thank you very much for your kind suggestions. We have a proof reading to all manuscript and changed in the revised manuscript. We ensure the similarity index not exceed 15% with no more than 1% from any source. We have also updated some of the references and changed in the revised manuscript.

Reviewer: 2
This study indicated ways for improving the performance of the BRC-MFC system by varying the electrode spacing on the performance of the system. The data presented seems sufficient to corroborate results. May be considered for publication in its present form. Response: Thank you very much for your approval on our work. We are also eager to be accepted for publication in Royal Society Open Science.