Bioelectrochemical CO2 Reduction to Methane: MES Integration in Biogas Production Processes
Round 1
Reviewer 1 Report
I read with much interest this research manuscript reporting bioelectrochemical CO2 reduction to methane: MES integration in biogas 2 production processes. Overall, I found the study only partially interesting and bringing limited new knowledge about such conversion of CO2. Overall, the work lacks depth and novelty required for a research paper in Applied Science. As such there are no fundamental drawbacks in the manuscript. However, the manuscript as it is written now, the work needs major improvements. Also state of art needs to be defined better and some key references are missing as indicated below.
1) My main concern is about the novelty and usefulness of the manuscript. The work describes bioelectrochemical conversion of CO2 into methane. There lies a fundamental question about this type of conversion as it is economically not feasible. At the current market price the cost of pure CO2 is higher than methane. So why would someone who can sell pure CO2 after bottling it will convert to a lower value chemical using a not so efficient process is beyond my understanding.
2) The bioelectrochemical processes has been used for CO2 conversion to much more useful chechias like Volatile fatty acids and alcohols as well (as shown here recently- 1/2017. Bioelectrochemical Conversion of CO2 to Chemicals: CO2 as Next Generation Feedstock for the Electricity-driven Bioproduction in Batch and Continuous mode. Faraday Discussions 202, 433-449; 2/ 2017. In situ acetate separation in microbial electrosynthesis from CO2 using ion-exchange resin. Electrochimica Acta, 237, pp.267-275), so why not choose them instead of methane. Explain in light of these papers.
3) The third important aspect considered by authors is MES integration into biogas grid. This was discussed extensively recently but the authors have not cited it- 2018. An overview of microbial biogas enrichment. Bioresource Technology 264, 359-369.
4) In the title, authors mention about “Bioelectrochemical CO2 reduction to methane” but in the abstract they mention something about “MES is able to produce biogas with over 90% methane when fed with reject water obtained from local wastewater treatment plant”. So which is it ‘CO2’ or ‘Reject water’ as they are two different things.
5) Introduction, Para 1- Authors start with the increasing role of electrochemical conversion of CO2. A very recent paper discusses its current industrial scale. You may check and mention it- 2019. Recent advances in industrial CO2 electroreduction. Current Opinion in Green and Sustainable Chemistry, 16, 47-56.
6) It is not clear from the results whether replicate reactors were operated or is the data based on single run of one experiment only. Were the experiments repeated?
Author Response
Thank you for the expeditious and thorough review. The Reviewers’ comments have been used to revise and improve the manuscript considerably as described below. We also discovered a few ‘language errors’ during our revision, which we corrected without further comments.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This article investigated bioelectrochemical CO2 reduction from reject water by MES. This attempt is very important and interesting. Unfortunately, this experiment had not set a control condition, which is an open circuit operation to evaluate methane fermentation performance except for MES reaction. Therefore, this experiment is very difficult to distinguish between commonly anaerobic methane fermentation and methane fermentation from MES. For this problem, the worth of this article is still not sufficient for publication.
It seems that autoclaving the reject water was not very effective in reducing the heterotrophic methanogenesis activity. Experiments of the bioelectrochemical system (BES) such as this experiment needs an open circuit condition operation to evaluate the effect of the BES. Otherwise, it was required that the open circuit condition was temporarily operated in the R2 operation.
What does the electron recovery to methane (ERM) evaluate? The high percentage of the commonly anaerobic methane fermentation occurred in this experiment. In this case, if the commonly anaerobic methane fermentation is high, the ERM becomes high. On the other hand, if the current of MES is low, the ERM also becomes high. Please explain the meaning of the ERM.
The author explains that the low CO2 concentration was occurred by CO2 reduction by the MES. Is it correct? In this experiment, effluent pH was above 8.2, so it was an alkaline condition. CO2 easily dissolves in an alkaline solution. It is possible that the low CO2 concentration was caused by it. The result of low CO2 concentration is not sufficient to prove the occurrence of the CO2 reduction by MES. I recommend that the author provide more information about it.
Author Response
Thank you for the thorough review. The Reviewers’ comments have been used to revise and improve the manuscript considerably as described below. We also discovered a few ‘language errors’ during our revision, which we corrected without further comments. We have added two new sections, one in materials and methods and another in results and discussion considering the valuable and major revision suggested by the reviewer. The sections are about 'control experiments' are described in sections 2.2.5 and 3.5.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
I was very happy to read this revised version of the manuscript and was glad to notice that the authors have carried out the revisions suggested to them. Indeed the revised manuscript looks much better than the original submission. Also the authors have answered very clearly all the concerns raised by me. For my comments, I have no further remarks to make on this manuscript and I am satisfied with the current revised version.
Reviewer 2 Report
The article has been almost corrected in the right way.
Currently, the worth of this article is still sufficient for publication.
