Effects of Recycled Biochar Addition on Methane Production Performance in Anaerobic Fermentation of Pig and Cow Manure

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

There are many questionable results that must be explained.

1. Paper title: Delete “Investigation into the” for simplification. What is “Recycled Biochar Addition”?
2. Section “Introduction” was too comprehensive (had 83 lines) and should be shortened.
3. Table 1: Why TS was expressed in %?
4. Section 2.2 Experimental Design: The actual amounts (weights) of the cow dung, pig manure and corn straw should be shown. The term “Fecal species” should be changed into “Substrate”.
5. Lines 154-155: Why these two C/N values? Give citations.
6. Meanings-unclear: L156-159, Section 3.1 “gas production cycle”, post-initiation,
7. The characteristics of the biochar?
8. Equation 1 is questionable.
9. Figure 2: Why the curve variations were not proportional to the biochar amounts?
10. Figures 3 and 5 and Table 3: These data show that biochar additions were not effective to the biogas production. This is not reasonable because biochar was an organic and could be anaerobically degraded into biogas in 15~ days.

11. Figure 4: The daily biogas production trend for PCK was quite strange. It is not reasonable for being Up-down and up-down and up-down….

Comments on the Quality of English Language

English polishing is necessary.

Author Response

Dear Reviewer:

Thank you very much for your letter. We really appreciate the opportunity to make revisions and we also thank the reviewer for giving us many constructive suggestions that will help us improve the quality of the paper. This letter provides valuable suggestions not only for this paper but also for our further research. Taking into account all the comments from the editor and the reviewer, the manuscript has been revised. We have addressed these comments point by point and all the modified parts are highlighted in red in the revised manuscript. The response to the reviewer's comments is as follows:

评论者 #1：

Comment 1: Paper title: Remove “Investigation into the” to simplify. What is “recycled biochar added”?

Response 1: Thank you for pointing this out. “Investigation into the” has been removed. Recycled biochar refers to the biochar used in the experiment, which was recovered from purified water in the constructed wetland system and dry ground.

Comment 2: The "Introduction" section is too comprehensive (83 lines) and should be shortened.

Response 2: Thank you for pointing this out. The Introduction has been simplified.

Comment 3: Table 1: Why is TS expressed in %?

Response 3: Thank you for pointing this out. TS refers to total solids content, which includes moisture and solid matter in the compost material. The value obtained is the proportion of solid matter to total matter, expressed in %.

Comment 4: Section 2.2 Experimental design: The actual amount (weight) of cow manure, pig manure, and corn stover should be shown. The term "manure type" should be changed to "substrate".

Response 4: Thank you for pointing this out, the manuscript has been revised accordingly.

Comment 5: Lines 154-155: Why these two C/N values? Give citations.

Response 5: Thank you for your valuable feedback. The C/N ratios of 35 and 28 for cattle and pig manure were chosen based on the following considerations: Cow manure has a high C/N ratio (31.61, Table 1) due to the presence of lignocellulose, so we adjusted it to 35 to ensure sufficient carbon for microbial activity while avoiding nitrogen inhibition. The initial C/N ratio of pig manure was relatively low (23.61, Table 1), so we balanced it to 28 by adding corn stover to reduce ammonia toxicity (Reference: Effects of C/N ratio and ammonia nitrogen accumulation in a pilot-scale thermophilic dry anaerobic digester).

Comment 6: Unclear meaning: L156-159, Section 3.1 “Natural Gas Production Cycle”, after startup, Response 6: Thank you for pointing this out. The experimental reaction cycle is the delivery cycle, which has been modified to make it clearer. Added to L148-150: “The time of adding the biogas slurry is set as the start time of the experiment, and the entire experimental period is the production period of the experimental unit.

Comment 7: Characteristics of biochar?

Response 7: Thank you for pointing this out. The average carbon content of the biochar samples was above 75%. Characteristics of biochar were added.

Comment 8: Equation 1 is questionable.

Response 8: Thank you for pointing this out. Formula 1 has been modified

Comment 9: Figure 2: Why does the curve change not proportional to the amount of biochar?

Response 9: Although adding biochar is a single action, it is not a single variable. Adding biochar is also affected by (1) the positive effects of biochar as an additive; (2) the pH buffering effect brought by biochar; (3) the stimulation of biological activity (conductivity) brought by biochar; (4) the change in carbon-nitrogen ratio brought by biochar (as an external carbon source to promote nitrification and denitrification); (5) toxicity issues such as ammonia nitrogen VFA caused by starting too quickly.

Comment 10: Figures 3 and 5 and Table 3: These data show that adding biochar is ineffective for biogas production. This is unreasonable because biochar is an organic matter that can be anaerobically degraded into biogas within 15~ days.

Response 10: Thank you for pointing this out. We carefully checked Figures 3, 5 and Table 3, and Table 3 only shows the final methane production of the cow manure experimental group. Figures 3 and 5 show the total production of biogas from anaerobic co-fermentation of the cow manure experimental group and the pig manure experimental group, respectively, and the curves show an upward trend. Therefore, adding biochar is feasible.

Comment 11: Figure 4: PCK's daily biogas production trend is quite strange. Up and down, up and down, up and down is unreasonable...

Response 11: PCK is strange because the oscillation method uses indirect oscillation, and toxic substances such as VFA accumulate after biogas is produced and do not diffuse, resulting in discontinuous biogas production process. After oscillation, it can be restarted.

Comment 12: English polishing is necessary.

Response 12: Thank you for pointing this out. This article has been polished grammatically

We have tried our best to improve the manuscript and have made some changes to it. These changes will not affect the content and framework of the paper.

We sincerely thank the Reviewer for his enthusiastic work and hope that the corrections will be approved.

Thank you very much again for your comments and suggestions

Sincerely

Xu Jianling

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript (fermentation-3587139) uses cow dung and pig manure as fermentation substrates and conducts co-fermentation experiments with corn stover pre-treated with sodium hydroxide. It also introduces biochar derived from constructed wetland substrates as a conditioner to investigate the effects of different addition amounts on the pre-treated anaerobic co-fermentation process. The data in the full text are detailed, and the charts meet the requirements of the journal. Here are my suggestions: In lines 32-33, it is suggested to add a summary of this study after these lines. In lines 111-119, the hypothesis of this study should be added in this section. In lines 125-126, there should be a space before the parentheses. Please confirm that the temperature is 450℃ and cite the relevant literature. In line 135, there should be a space between TOC and (g/kg TS). In line 247, there should be a space between 21.91 and mL. Please check the whole text for similar issues. In line 365, Volatile Fatty Acids (VFAs) can be directly written as VFAs. Can the clarity of Figures 11-12 be improved? Can Figures 13 and 14 be changed to color images? In the reference section of line 629, please strictly follow the journal's requirements for modification, such as the capitalization of the first letter of each word in the title of the literature, and the abbreviation of the journal in lines 666-667.

Author Response

Dear Reviewer:

Thank you very much for your letter. We really appreciate the opportunity to make revisions and we also thank the reviewer for giving us many constructive suggestions that will help us improve the quality of the paper. This letter provides valuable suggestions not only for this paper but also for our further research. Taking into account all the comments from the editor and the reviewer, the manuscript has been revised. We have addressed these comments point by point and all the modified parts are highlighted in blue in the revised manuscript. The response to the reviewer's comments is as follows:

Reviewer #2: This manuscript (fermentation-3587139) uses cow manure and pig manure as fermentation substrates and conducts co-fermentation experiments with corn stover pretreated with sodium hydroxide. It also introduces biochar from artificial wetland substrates as a regulator to study the effects of different addition amounts on the pretreatment anaerobic co-fermentation process. The data in the full text are detailed and the figures meet the requirements of the journal.

Here are my suggestions:

Comment 1: In lines 32-33, it is suggested to add the summary of this study after these lines.

Response 1: Thank you for pointing this out. The summary section has been supplemented.

Comment 2: In lines 111-119, the hypotheses of this study should be added in this section.

Response 2: Thank you for pointing this out, it has been revised.

Comment 3: In lines 125-126, there should be a space before the bracket.

Response 3: Thank you for pointing this out. Space has been added

Comment 4: Please confirm that the temperature is 450°C and cite the relevant literature.

Response 4: Thank you for pointing this out. We confirm that the biochar synthesis was carried out at 450°C according to the cited literature.

Comment 5: In line 135, there should be a space between TOC and (g/kg TS).

Response 5: Thank you for pointing this out. Space has been added

Comment 6: In line 247, there should be a space between 21.91 and mL. Please check if there are similar issues throughout the text.

Response 6: Thank you for pointing this out. The space has been added

Comment 7: In line 365, volatile fatty acids (VFA) can be written directly as VFA.

Response 7: Thank you for pointing this out. It has been revised

Comment 8: Can the clarity of Figures 11-12 be improved? Can Figures 13 and 14 be changed to color images?

Response 8: Thank you for pointing this out. We regret to inform you that color images are not available with this submission. Nevertheless, we have improved the image quality by providing high-resolution grayscale versions.

Comment 9: In the reference section on line 629, please strictly follow the journal's requirements, such as capitalizing the first letter of each word in the article title and the journal abbreviation on lines 666-667.

Response 9: Thank you for pointing this out. It has been revised

We have tried our best to improve the manuscript and have made some changes to it. These changes will not affect the content and framework of the paper.

We sincerely thank the Reviewer for his enthusiastic work and hope that the corrections will be approved.

Thank you very much again for your comments and suggestions

Sincerely

Xu Jianling