Harnessing Digestate Potential: Impact of Biochar and Reagent Addition on Biomethane Production in Anaerobic Digestion Systems

Round 1

Reviewer 1 Report

1. The authors mentioned that the raw digestate demonstrated  higher methane production of 547.7 NmL and  addition of 1% biochar to the raw digestate resulted in similar methane production of 550.5 NmL. There is no significant difference in the methane content when biochar was added. besides, increasing biochar content, is reducing the methane production.

When there is no significant methane production, then there is no point in adding biochar to the digestate.

2. It would be good if the authors include the porosity of spruce BC, oak wood BC and chicken manure digestate BC. If this data is included in the manuscript, then authors my justify their result with more emphasis.

3. In the results mentioned in 3.1, the authors showed the production of 547.7 NmL of methane from raw digestate. In 3.2, the authors mentioned the production of 782 NmL of methane from raw  digestate. In both these cases, raw digestate is a control. 3.1 dealt with concentration and 3.2 dealth with type of biochar. These two factors are nothing to do with control. Can the authors explain the factor/reason behind improvement of methane content from raw digestate? 

4. The authors need correct "870,2" in the table 3

5. Authors need to check "," "." and other punctuation marks throughout the manuscript. Grammer should be checked throughout the manuscript

NIL

Author Response

Please see the attachment 

Author Response File: Author Response.pdf

Reviewer 2 Report

General comments: This paper aims at investigating the effects of digestate and co-substrates input with different biochar concentrations and different types of biochar on biomethane production during the anaerobic digestion (AD) process. The equipment (Automatic Methanisation Potential Test System II) used for conducting the experiments is appropriate. 

Specific comments:

1.  The methodology should include measuring the characteristics of the cattle manure (from the Castel Metha Unit) and the co-substrates (comprised of manure, manure slurry, and maize silage from the SAS Biogaz-IFF Unit).  This would be helpful towards discussion of the results.

2.  (Page 3, section 2.2):  Was each experimental treatment done in replicates or triplicates using the AMPT II?  And (Page 5, Figure 1): What do the error bars represent?

3.  The experimental design is inadequate. 

a) The digestate (also called "raw digestate" in the paper) was derived from AD of cattle manure.  Hence, biomethane production from AD of the digestate also needs to be compared to that from AD of the cattle manure itself.  It is not appropriate to compare the biomethane production from the digestate with that from the co-substrates because the characteristics of the cattle manure and the co-substrates should be different.

b) Table 1:  The three types of biochar (BC1, BC2, BC3) have different source materials for production (using pyrolysis or gasification). And take pyrolysis for example, the pyrolysis operating conditions (temperature and residence time) were different.  It will be useful to also make biochar "BC1" samples with a pyrolysis temperature of 400 ^oC, and residence time of 30 minutes, so a proper comparison can be made between BC1 and BC2 in terms of AD performance.

4. There are errors and insufficient descriptions in the Results and Discussion section of the paper.

a) Lines 172-174:  Do you mean "the methane production level of 550.5 NmL is slightly lower than 299.9 NmL"?

b) Lines 118 and 163:  What is the last day of experiments? 15 days or 9 days?

c) Page 5 Figure 1:  Figure caption says "biochar 1".  But the legend in Figure 1(a) and 1(b) indicates "BC2" and "BC 1", respectively.

d) Line 162 and Figure 1:  Are the biochar doses of (1%, 2%, and 4%) based on the mass of dry matter (DM) of the AD substrate, e.g. does "1% dose" mean "0.01 g biochar per gram of DM"?

e) Biomethane yield can also be expressed in [NmL/g VS, where VS stands for volatile solids or organic matter].  Were the VS contents of the substrates measured?

f) Figure 2 and Table 2:  Are there any similar data for the co-substrates.

5. The Conclusion is inadequate. Also, in view of the above-mentioned shortcomings of the paper, the ranking of BC1 vs. BC2 and BC3 would not be valid. 

Author Response

Please see the attachment 

Author Response File: Author Response.pdf

Reviewer 3 Report

This work deals with a current issue in the field of anaerobic digestion, such as the use of certain additives (charcoals, iron oxides …) as possible process improvers. In general terms, this article discusses the cause-effect relationship of the results achieved and conclusions are endorsed over experimental evidence. However, he hypothesis that BC serves as a host for improved anaerobic microbial communities is not supported experimentally. Besides, it would be convenient to explore or discuss a possible effect of biochar as an in-situ CO2 adsorbent, as a possible explanation or not of the improvement in methane production. Also, the increase in COD during the experiments needs to be better explained.

Although the use of English is quite correct, the text still has a small opportunity for improvement in this regard.

Author Response

Thank you for your valuable feedback on our work. We appreciate your positive assessment of the article's relevance to a current issue in the field.

You raised a valid point about the hypothesis regarding biochar (BC) serving as a host for improved anaerobic microbial communities. We acknowledge that this aspect requires further experimental validation, and we will address this limitation in future research. Additionally, we agree with your suggestion to explore or discuss the possible effect of biochar as an in-situ CO2 adsorbent and its potential role in the enhancement of methane production. We incorporate this aspect into the discussion to provide a comprehensive analysis of our findings. (line 257-275). Your feedback prompted us to thoroughly revise the manuscript, providing a more detailed explanation of the observed changes in COD and their implications on the anaerobic digestion process (line 242-248).  I'm glad to hear that the use of English in the text is correct. I appreciate your keen eye for detail and your willingness to help me improve further. If you have any specific suggestions or areas where you think the text can be enhanced, I would be more than happy to hear them and make the necessary adjustments. Constructive criticism is always valuable in refining the quality of my work, so feel free to share any insights you may have.

Round 2

Reviewer 1 Report

Nil

Author Response

Thank you for reviewing our study. We appreciate your time and consideration. If you have any further questions or require additional information, please feel free to let us know. We value your feedback and are grateful for your assessment of our work.

Reviewer 2 Report

The authors have addressed most of my review comments in the revised manuscript.  

However, the statements (Lines 12-13 and Lines 167-174) that I have highlighted in the revised paper and shown below can be revised or deleted.

Lines 12-13 (Abstract): "Raw digestate demonstrated higher methane production compared to the co-substrates, indicating the influence of feedstock composition and degradability."

Lines 167-174 (Section 3.1): "Specifically, the raw digestate demonstrated higher methane production (around 548.0±49.6 NmL) compared to the co-substrates (341.5±16.5 NmL). The observation that raw digestate produced more methane than co-substrates could be attributed depending on the composition and characteristics of the feedstock use; in terms of complexity and degradability, the co-substrates with its multiple organic materials would generally be considered more complex and potentially harder to degrade compared to digestate, which primarily consists of raw cattle manure."

The authors conducted lab-scale anaerobic digestion (AD) experiment using raw digestate as the feedstock in the first set of tests (Set 1). Here, the raw digestate was obtained from the full-scale AD Unit Castel Metha.  However, the authors did not conduct additional lab-scale AD experiment using raw cattle manure as the feedstock in the Set 1 tests.

Also, the authors conducted lab-scale experiment using co-substrates as the feedstock in the second set of tests (Set 2). Here, the co-substrates (comprised of cattle/pig/chicken manure, manure slurry, and maize silage) were sourced from the full-scale SAS Biogaz-IFF Unit.  However, the authors did not conduct additional lab-scale AD experiment using raw digestate of the co-substrates as the feedstock in the Set 2 tests.

Next, the authors compared the methane production from the raw digestate (Set 1 tests) with that from the co-substrates (Set 2 tests). This comparison will not lead to meaningful conclusion since the characteristics of the raw cattle manure and the co-substrates (which contain not just cattle manure but a more complex mixture with a fixed mixing ratio) should be quite different, though measurements of these characteristics were not made in their study.

Therefore, the statements made in Lines 12-13 and Lines 167-174 are inappropriate and they shall be revised or deleted from the paper.

Author Response

We express our gratitude for your valuable feedback concerning our research on lab-scale anaerobic digestion (AD) experiments. Our study revealed that raw digestate demonstrated superior methane production in comparison to the co-substrates, suggesting a substantial impact of feedstock composition and degradability on the anaerobic digestion process.

In response to your recommendations, we have taken necessary actions to enhance the clarity and accuracy of our findings. As advised, we have removed the sentences in line 12-13 and have made appropriate revisions to the sentences in line 167-174 to address any potential confusion.

Thank you once again for your constructive comments, which have contributed to the improvement of our paper.