Effect of Addition of Zero-Valent Iron (Fe) and Magnetite (Fe3O4) on Methane Yield and Microbial Consortium in Anaerobic Digestion of Food Wastewater
Round 1
Reviewer 1 Report
This manuscript studies the improvement in biogas production and changes in microbial population in food waste anaerobic digestion with the addition of conductive materials. The authors focus their justification on the DIET, when other effects of the addition of magnetite or zero-valent iron may also be improving methane production, as reported in previous papers. A part of this comment, other improvements are needed in the manuscript before being suitable for publication.
Page 2, line 89-91. To better define the state of the art, reference to these studies is needed, for different conductive materials and organic waste substrates. The authors should notice that apart from sewage sludge and food waste, also studies with feedstock manure have been published. Also, some of these works have dealt with microbial changes. The authors can see:
Barrena, R., Vargas-García, M. del C., Capell, G., Barańska, M., Puntes, V., Moral-Vico, J., Sánchez, A., Font, X., 2021. Sustained effect of zero-valent iron nanoparticles under semi-continuous anaerobic digestion of sewage sludge: Evolution of nanoparticles and microbial community dynamics. Sci. Total Environ. 777, 145969. https://doi.org/https://doi.org/10.1016/j.scitotenv.2021.145969
Cerrillo, M., Burgos, L., Ruiz, B., Barrena, R., Moral-Vico, J., Font, X., Sánchez, A., Bonmatí, A., 2021. In-situ methane enrichment in continuous anaerobic digestion of pig slurry by zero-valent iron nanoparticles addition under mesophilic and thermophilic conditions. Renew. Energy 180, 372–382. https://doi.org/10.1016/j.renene.2021.08.072
Wu, S. Zheng, A. Ding, G. Sun, M. Yang, 2015. Performance of a zero valent iron-based anaerobic system in swine wastewater treatment, J. Hazard Mater. 286 1-6, https://doi.org/10.1016/j.jhazmat.2014.12.029.
Y. Yang, F. Yang, W. Huang, W. Huang, F. Li, Z. Lei, Z. Zhang, 2018. Enhanced anaerobic digestion of ammonia-rich swine manure by zero-valent iron: with special focus on the enhancement effect on hydrogenotrophic methano-genesis activity, Bioresour. Technol. 270 172e179,https://doi.org/10.1016/j.biortech.2018.09.008.
Page 2, line 92-94. The authors should specify which is particular of Korean food waste to justify a specific study on DIET for food wastewater in Korea. The authors should justify which is the novelty of this work with respect to previous ones.
Page 3. Equations 3 and 4 should be renumbered to 1 and 2.
Page 3, Line 116-117. “The batch reactor used for the BMP had a serum bottle...” should it be “was” instead of “had”? I don’t know if I correctly understand the set-up. In line 129 it is stated that the batch reactor was stirred once a day in an anaerobic reactor. Could the authors clarify this description? Were not glass vials used?
Page 3, Line 122. In order to compare with other works, it would be better to express the dosages of conductive materials with respect to organic matter (SV) concentration of the substrate, since using % or g/L is not very indicative. Please, the authors could see:
C. Lizama, C.C. Figueiras, A.Z. Pedreguera, J.E. Ruiz Espinoza, 2019. Enhancing the performance and stability of the anaerobic digestion of sewage sludge by zerovalent iron nanoparticles dosage, Bioresour. Technol. 275 352-359,https://doi.org/10.1016/j.biortech.2018.12.086.
Page 7, Table 4. The heading of the table has to be corrected, since it says “Zero-valent iron concentration (%)” and it should be “Magnetite concentration”.
Other comments: a review of the grammar and use of English would improve the quality of the manuscript.
Author Response
We have revised the manuscript as the reviewer’ comments. And finally, this manuscript was checked by a native English-speaking colleague.
Thank you so much.
Comment 1. Page 2, line 89-91. To better define the state of the art, reference to these studies is needed, for different conductive materials and organic waste substrates. The authors should notice that apart from sewage sludge and food waste, also studies with feedstock manure have been published. Also, some of these works have dealt with microbial changes.
Response: Thank you. As pointed out, an introduction was revised to better define the state of the art by the additional references.
Comment 2. Page 2, line 92-94. The authors should specify which is particular of Korean food waste to justify a specific study on DIET for food wastewater in Korea. The authors should justify which is the novelty of this work with respect to previous ones.
Response: Thank you. Reviewer pointed out the important part. Korea has high grain consumption and low meat consumption, so food wastes have characteristics of high starch and cellulosic organic matter content, and low protein and fat content. Therefore, since the content of easily biodegradable organic matter is high in food waste, acid fermentation proceeds rapidly in the early stages of anaerobic digestion, which often causes operation problems in anaerobic digestion of food waste. Therefore, DIET research can be a technical alternative for the stabilization of food waste anaerobic digesters in Korea.
Comment 3. Page 3. Equations 3 and 4 should be renumbered to 1 and 2.
Response: Thank you. These are corrected.
Comment 4. Page 3, Line 116-117. “The batch reactor used for the BMP had a serum bottle...” should it be “was” instead of “had”? I don’t know if I correctly understand the set-up. In line 129 it is stated that the batch reactor was stirred once a day in an anaerobic reactor. Could the authors clarify this description? Were not glass vials used?
Response: Thank you. As pointed out, the text was corrected. Also, we used glass serum bottles, and the mixing of the serum bottles was conducted manually during the fermentation period. The text has been corrected to avoid confusion.
Comment 5. Page 3, Line 122. In order to compare with other works, it would be better to express the dosages of conductive materials with respect to organic matter (SV) concentration of the substrate, since using % or g/L is not very indicative.
Response: Thank you. We calculated the dosages of conductive materials on the basis of VS content(mg/g-VS), and the text was revised as pointed out.
Comment 6. Page 7, Table 4. The heading of the table has to be corrected, since it says “Zero-valent iron concentration (%)” and it should be “Magnetite concentration”.
Response: Thank you. We have a mistake. The text was revised as pointed out.
Young-Man Yoon
Author Response File: 
Author Response.docx
Reviewer 2 Report
ZVI and magnetite are two electron transfer mediators to achieve DIET. In this study, the author investigated the effects of them on ethane production and microbial consortium. Thought this is a very interesting study, there are some drawbacks and the author need to make major revisions.
1. Ln192-199, the published data should not appear in the R&D. These data can be placed in M&M as base data of food wastewater.
2. I can confirm the values of parameter about table 2 undergo a significant difference over time. Hence, the author should replenish these data when the experiments are end. Especially, the author emphasizes pH can facilitate or inhibit methane production.
3. What are the differences in methane yield and reaction kinetics between the ZVI and magnetite treatment group? The author should discuss why there is a significant difference or not.
4. Line 311-314, please explain the reason that Methanomicrobiales increased and Methanosarcinales decreased with ZVI concentration.
5. It is confused that the abundance of Methanosarcina decreased when ZVI concentration increased from 0.25% to 1%. However, when the ZVI concentration is 1.5%, why the abundance of Methanosarcina is more than 40%? (Figure 3).
6. In figure 4, in magnetite treated group, when magnetite concentration is 1.5%, the abundance of Methanomicrobiales and Methanoculleus is zero, and the abundance of Methanosarcinales and Methanosarcina have increased, please give reasonable explanation.
Author Response
We have revised the manuscript as the reviewer’ comments. And finally, this manuscript was checked by a native English-speaking colleague.
Thank you so much.
Comment 1. Ln192-199, the published data should not appear in the R&D. These data can be placed in M&M as base data of food wastewater.
Response: Thank you. Chemical composition data of food wastewater were placed in the M&M section. The concerned text was revised.
Comment 2. I can confirm the values of parameter about table 2 undergo a significant difference over time. Hence, the author should replenish these data when the experiments are end. Especially, the author emphasizes pH can facilitate or inhibit methane production.
Response: Table 2 indicates the chemical composition of the substrate. In the BMP assay, generally, batch-type anaerobic reactors are prepared as a mixture of substrate and inoculum. Therefore, the alkalinity provided by the inoculum has the role of buffer during the fermentation period and the pH of the reactor is maintained stable. Based on these experimental characteristics, the chemical composition of the substrate and the chemical composition of the digestate have a low correlation, so the chemical composition of the digestate at the end of the experiment is not often analyzed. In addition, the methane yield and methane concentration data obtained during the fermentation period give enough information for the interpretation of anaerobic reaction characteristics.
Comment 3. What are the differences in methane yield and reaction kinetics between the ZVI and magnetite treatment group? The author should discuss why there is a significant difference or not.
Response: Thank you. Reviewer pointed out a very important part. The results of the BMP assay only show the phenomenon caused by the ZVI treatment, but do not provide information on the reason. Therefore, we analyzed the change in community characteristics of anaerobic microorganisms according to ZVI treatment, and the answers in Comment 3 were summarized in the response for Comment 4.
Comment 4. Line 311-314, please explain the reason that Methanomicrobiales increased and Methanosarcinales decreased with ZVI concentration.
Response: Thank you. We also had the same question about the reason pointed out by the reviewer. So, after the end of BMP, the solubilized Fe content was analyzed in ZVI and Magnetite treatment groups. And, based on the result of solubilized Fe content analysis (Table 5), the reason that Methanomicrobiales increased and Methanosarcinales decreased with ZVI concentration was explained and the text was revised.
Comment 5. It is confused that the abundance of Methanosarcina decreased when ZVI concentration increased from 0.25% to 1%. However, when the ZVI concentration is 1.5%, why the abundance of Methanosarcina is more than 40%? (Figure 3).
Response: I think there is some confusion. At ZVI 0, 0.25, 0.50, 1.00 and 1.50%, Methanosarcina was 12.80, 4.00, 2.20, 1.00, and 1.10%. Methanoculleus is more than 40% in ZVI of 1.5% (Figure 3).
Comment 6. In figure 4, in magnetite treated group, when magnetite concentration is 1.5%, the abundance of Methanomicrobiales and Methanoculleus is zero, and the abundance of Methanosarcinales and Methanosarcina have increased, please give reasonable explanation
Response: I think there is some confusion. In the 1.5% magnetite treatment group, the microbial abundance of Methanomicrobiales and Methanoculleus was 23.10% and 23.10%, respectively, and the microbial abundance of Methanosarcinales and Methanosarcina was 12.60% and 11.60%, respectively. Changes in their community characteristics did not show significant trends.
Young-Man Yoon
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
The authors have answered all of my questions.
