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Article
Peer-Review Record

Application of Conductive Concrete as a Microbial Fuel Cell to Control H2S Emission for Mitigating Sewer Corrosion

Water 2022, 14(21), 3454; https://doi.org/10.3390/w14213454
Reviewer 1: Jaecheul Yu
Reviewer 2: Anonymous
Reviewer 3: Anonymous
Water 2022, 14(21), 3454; https://doi.org/10.3390/w14213454
Received: 30 September 2022 / Revised: 24 October 2022 / Accepted: 26 October 2022 / Published: 29 October 2022
(This article belongs to the Special Issue Water Quality Engineering and Wastewater Treatment Ⅱ)

Round 1

Reviewer 1 Report

This study investigated conductive concrete as MFC to control H2S emission of sewer corrosion. It is interesting topic. But it is difficult to accept with present state. 

1. If the generation of sulfide ions is suppressed as much as the difference in sulfate ions between the two cements, the change in the concentration of the two ions does not seem to match. Are there other sources of sulfide ion?

2.  Are the functions and stability of the conductive concrete used in this study the same as that of conventional concrete?

3. What is the mechanism by which sulfide ion is suppressed?

4. sulfate ions affect significantly pipe corrosion, but how does H2S inhibition affect pipe corrosion inhibition?

minor comments 

- Please correct the name of Y axis in Fig. 4 

- In general, electricity increase when substrate is added. So, arrow in Fig 6. seemed to be wrong, please check and correct. 

Author Response

Response to Reviewer 1 Comments

Point 1: If the generation of sulfide ions is suppressed as much as the difference in sulfate ions between the two cements, the change in the concentration of the two ions does not seem to match. Are there other sources of sulfide ion?

 Response 1: Thank you very much for your question. Reviewer’s confusion is right. Sulfide species in bulk wastewater formed from sulfate source by sulfate reducing bacteria. Moreover, electricity producing bacteria can transform dissolved sulfide into elemental sulphur via electron pathway provided by conductive concrete in this work. In experiment of Sun earth sample, reduced sulfide was transformed to elemental sulphur found from sludge sediment on surface of anode. This is not seen from one of OPC sample. Other source of sulfide is referred from experiment and result of 2p3/2 spectrum of sulphur in the layer of sediment sludge after 66 days (Fig. 8). I changed some notes on the revised Fig 8A in the resubmission manuscript.

Point 2: Are the functions and stability of the conductive concrete used in this study the same as that of conventional concrete?

Response 2: Thank you very much for your question. Yes, they are. Because Sun earth is commercialized conductive concrete.

Point 3: What is the mechanism by which sulfide ion is suppressed?

 Response 3: Thank you very much for your question. In this study, we concentrated to confirm on three ways: adsorption of amorphous carbon, chemical oxidation, and biological process. Hydrogen sulphur generated from anaerobic environment in the sludge sediment layer without presence of oxygen. One mechanism to explain for findings in this work is the combination of adsorption capacity of conductive concrete and EPB’s activity releases electrons when organic matter and hydrogen sulfide are biologically oxidized, electron transfers from bacteria to electron acceptors, an electron pathway via to conductive concrete by Sun earth in this work to enable biological oxidation process of H2S.  H2S removal occurs in the anodic section of surface of conductive concrete. We also revised some explanation at the section of discussion in the resubmission manuscript.

Point 4: sulfate ions affect significantly pipe corrosion, but how does H2S inhibition affect pipe corrosion inhibition?

 Response 4: Thank you very much for your question. the further aim of this work is to apply for the inhibition of sewer pipes to biogenic corrosion. Sulfate ions affecting to erosion pipe corrosion and sulfate source in wastewater are different. Under anaerobic condition, sulfate mainly form to sulfide species in wastewater, and then H2S gas emits in the air phase of sewer pipe system. This sulfide will be transformed to sulfuric acid and react to normal cement materials (concrete materials as Portland cement is a calcium silicate and its hydration inescapably liberates calcium hydroxide Ca(OH)2 are highly vulnerable to moisture condensation and different acidic gases) and cause to corrosion of sewer.

Point 5: Please correct the name of Y axis in Fig. 4  

 Response 5: Thank you very much for your remark. I am sorry it was output mistake of software. ‘Sun earth’ should be ‘Electrical resistivity’. I removed it and replaced in resubmission manuscript at Line 256 in the resubmission manuscript.

Point 6: In general, electricity increase when substrate is added. So, arrow in Fig 6. seemed to be wrong, please check and correct.

 Response 6: Thank you very much for your remark. We checked and revised arrow mistake it. 

 

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript entitled ‘Application of Conductive Concrete as a Microbial Fuel Cell to Control H2S Emission for Mitigating Sewer Corrosion’ in interesting since it addresses a common problem urban sewers system which is the biogenic corrosion caused by hydrogen sulfide (H2S). The researchers herein developed a new technology in order to resolve such issue through using conductive concrete to mitigate the concentration of H2S in sewer pipe. The manuscript is organized and the results are clearly presented however I have the following comments:

1-      The manuscript English language must be enhanced, there are some grammatical mistakes for example in line 68 instead of ‘are application’ you should write ‘is the application of ….’

2-      The authors should add the composition of both ordinary Portland cement and conductive concrete

3-      Enhance the format of the manuscript: leave spaces between paragraphs and make sure that each caption is directly below its corresponding figure

4-      In the title of table 1. What is 01 L, if the authors mean 1 L then 0 must be removed

5-      In line 149, it is pKa not Pka

6-      It so important to study the effect of temperature in order to detect the efficiency of this new technology as a function of temperature fluctuation  

Author Response

Response to Reviewer 2 Comments

Point 1: The manuscript English language must be enhanced, there are some grammatical mistakes for example in line 68 instead of ‘are application’ you should write ‘is the application of ….’
Response 1: Thank you very much for your constructive comments. In this time, we revised grammatical mistakes in the resubmission manuscript. All are showed in changes marked.
Point 2: The authors should add the composition of both ordinary Portland cement and conductive concrete
Response 2: Thank you very much for your suggestion. OPC used is a normal cement in Japan, and Sun earth is commercialized conductive concrete. I appreciate your opinion. However, we will refer in further work. 
Point 3: Enhance the format of the manuscript: leave spaces between paragraphs and make sure that each caption is directly below its corresponding figure
Response 3: Thank you very much for your constructive comments. I revised them in resubmission manuscript.
Point 4: In the title of table 1. What is 01 L, if the authors mean 1 L then 0 must be removed
Response 4: Thank you very much for your kind remind, I revised it in resubmission manuscript (Table 1)
Point 5: In line 149, it is pKa not Pka
Response 5: Thank you very much for your kind remind, I revised it in resubmission manuscript (Line 150)
Point 6: It so important to study the effect of temperature in order to detect the efficiency of this new technology as a function of temperature fluctuation  
Response 6: Thank you very much for your interesting suggestion. We consider it in further work.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments:

This manuscript investigated a conductive concrete mainly with amorphous carbon as an electron transfer pathway and chemically oxidized hydrogen sulfide using oxygen near the water surface as an electron acceptor. It seems an interesting work for the potential application of H2S control in the in sewer system. I recommend that it can be published, but some issues should be addressed to improve the manuscript.

1. Since as the author wrote the hydrogen sulfide significantly decreased by using conductive concrete as a microbial fuel cell. However, it was finally confirmed that the chemical oxidation was the dominant process and biological process did not play a significant role. So I think it is not proper to use MFC to express its removal process. It should be physical absorption and electro-chemical oxidation process.

2. Since adsorption efficiency of H2S was very high, authors should considered the saturation condition of the concrete materials. How to keep its removal efficiency after H2S saturation.

3. Authors should compared the BET, element difference and maybe other properties of OPC and Sun earth, it will help to tell the superiority of this conductive concrete.

4. Since the chemical oxidation of hydrogen sulfide was mainly on the conductive carbon, please give the evidence of possible active species.  

5. In 3.4 part, authors have measured the microbial community structure in the sludge, and there were less difference in the OPC and Sun earth systems. Please supply these results and figures to support this conclusion.  

6. Figure 4, please modify the Y axis and it should be Electrical resistivity.

7.   In the references parts, the references mostly are old and please give some new references. Please use the uniform style. Some with capital letter and some was small. Please check and modify it. It is better to improve the language of this manuscript. There are small mistakes in spelling and grammer.

 

Comments for author File: Comments.pdf

Author Response

Response to Reviewer 3 Comments

Point 1: Since as the author wrote the hydrogen sulfide significantly decreased by using conductive concrete as a microbial fuel cell. However, it was finally confirmed that the chemical oxidation was the dominant process and biological process did not play a significant role. So I think it is not proper to use MFC to express its removal process. It should be physical absorption and electro-chemical oxidation process.

Response 1: Thank you very much for your comment. Reviewer’s confusion is right. All experiments in this study were conducted to confirm role of conductive concrete in the way that it can suppress H2S generation. That discovery of electron pathway in conductive concrete is an interesting finding. Even without presence of oxygen, hydrogen sulphur generated from anaerobic environment in the sludge sediment layer was oxidized. EPBs release electrons from oxidized products, electron transfers from bacteria to electron acceptors, an electron pathway via to conductive concrete by Sun earth in this work to enable biological oxidation process of H2S.  H2S removal occurs in the anodic section of surface of conductive concrete. We also revised some explanation at the section of discussion in the resubmission manuscript for easy reader.

 

Point 2: Since adsorption efficiency of H2S was very high, authors should considere the saturation condition of the concrete materials. How to keep its removal efficiency after H2S saturation.

Response 2: Thank you very much for your attention to adsorption efficiency of H2S. Related to saturation condition, we consider it in further experimental conditions.  In this work, however, we would like to concentrate the way to oxidize H2S in anaerobic condition via conductive, and adsorption capacity of concrete material is one of mechanisms contributing to minimize H2S in liquid. As explanation at response for your first question, not only adsorption mechanism of materials, but also conductive characteristics made it to enable the biological oxidation process in anaerobic condition at the bottom by electricity producing bacteria. Electron pathway in conductive concrete is the differentiate in this study. 

 

Point 3: Authors should compared the BET, element difference and maybe other properties of OPC and Sun earth, it will help to tell the superiority of this conductive concrete.

Response 3: I am agreed with you. We consider it and plan to conduct comparison with various materials to confirm superiority of conductive concrete. Not only component difference it is also for different conductive materials.

 

Point 4: Since the chemical oxidation of hydrogen sulfide was mainly on the conductive carbon, please give the evidence of possible active species. 

Response 4: Thank you very much for your interesting recommendation. We mentioned three ways to H2S control in wastewater of sewer pipe. Role of oxygen near the surface, adsorption capacity of material and assume of the way of biological oxidation to suppress H2S in anaerobic condition. From all experiments, even using layer plastic to prevent oxygen invade or using Epoxy inside conductive concrete to prevent electron pathway, we conclude that adsorption capacity of amorphous carbon contributes to remove H2S, but not main mechanism, ultimate approach to be able to biological oxidation of H2S in anaerobic condition is needed to an intermediate path to let oxygen near the surface to become an electron acceptor in H2S oxidation reaction. Conductive concrete, Sun earth, is such a way. However, to be clarifier for the reader, in this revision manuscript, we add more information in discussion area.

 

Point 5: In 3.4 part, authors have measured the microbial community structure in the sludge, and there were less difference in the OPC and Sun earth systems. Please supply these results and figures to support this conclusion.

Response 5: Thank you very much for your interesting recommendation. We expected to find the differentiate of microbial communities among samples by DGGE analysis. The results, however, showed no change and we did not put them in article manuscript. We will deeply analyse in next research. That is because in this work, we used the same anaerobic and aerobic sludge samples from Ube WTTP, Yamaguchi, Japan in pilot experiment and used the artificial wastewater.

 

Point 6: Figure 4, please modify the Y axis and it should be Electrical resistivity. 

Response 6: Thank you very much for your remark. I am sorry it was output mistake of software. ‘Sun earth’ should be ‘Electrical resistivity’. I removed it and replaced in resubmission manuscript at Line 256 in the resubmission manuscript.

 

Point 7: In the references parts, the references mostly are old and please give some new references. Please use the uniform style. Some with capital letter and some was small. Please check and modify it. It is better to improve the language of this manuscript. There are small mistakes in spelling and grammer.

Response 7: Thank you very much for your constructive comments. I revised them in resubmission manuscript. 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Author responded well to all comments.

But there is one thing, please provide the detailed physical properties of two cements used in this study. 

Author Response

Response to Reviewer 1 Comments

Point 1: Author responded well to all comments. But there is one thing, please provide the detailed physical properties of two cements used in this study.
Response 1: Thank you very much for your comment and recommendation. To clarify your concern, in this revision we added some characteristics of Sun earth and OPC (manufacturer, specific gravity, surface area…, properties of expand, shrink and crack when curing of Sun earth, a water-to-cement (W/C) ratio ) from Line 95 to 125 in the resubmission manuscript.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript can be accepted 

Author Response

Thank you very much for your constructive comments.

We added some characteristics of Sun earth and OPC (manufacturer, specific gravity, surface area…, properties of expand, shrink and crack when curing of Sun earth, a water-to-cement (W/C) ratio ) to clarify to readers from Line 95 to 125 in the resubmission manuscript. All are showed in changes marked.

 

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