Biogas Production from a Solar-Heated Temperature-Controlled Biogas Digester

Response to Reviewer 1 Comments

1. Summary

Thank you very much for taking the time to review this manuscript. Please find detailed responses below and the corresponding revisions/corrections highlighted/in track changes in the re-submitted files.

2. Questions for General Evaluation

Reviewer’s Evaluation

Response and Revisions

Does the introduction provide sufficient background and include all relevant references?

Yes/Can be improved/Must be improved/Not applicable

Are all the cited references relevant to the research?

Yes/Can be improved/Must be improved/Not applicable

Is the research design appropriate?

Yes/Can be improved/Must be improved/Not applicable

Are the methods adequately described?

Yes/Can be improved/Must be improved/Not applicable

Are the results clearly presented?

Yes/Can be improved/Must be improved/Not applicable

Are the conclusions supported by the results?

Yes/Can be improved/Must be improved/Not applicable

3. Point-by-point response to Comments and Suggestions for Authors

Comments 1: [The study lacks a detailed explanation of the methods used for measuring the biogas composition, particularly methane concentration. It would benefit from including a clear description of the measurement instruments and procedures. This information could enhance reproducibility and credibility.]

Response 1: Thank you for pointing this out. We agree with this comment. Therefore, we have….[included a detailed description of the measuring instrument, its technical parameters, its technical specifications for biogas measurements and the principle of operation as well as how the measurements were done.] see page 12 paragraph 1, lines 417- 430 of the corrected manuscript.

Comments 2: [ There is insufficient discussion regarding the potential effects of seasonal variations on biogas production. While the paper mentions that the experiments were conducted in summer, it should suggest how different seasons might influence the results. This would provide a more comprehensive understanding of biogas production.]

Response 2: Agree. We have, accordingly, inserted a subtitle on the potential of seasonal effects on biogas production to emphasize this point. [we have included a new subtitle on the potential effects of seasonal variations attending to temperature variability and variations in sunshine hours between summer and winter months.] See page 20, paragraph 1, lines 584-595 of the corrected manuscript.

Comments 3: [The paper does not adequately address potential sources of measurement error in the temperature control system. Including a discussion on sensor calibration, positioning, or response times would strengthen the reliability of the findings. This is essential for accurate temperature management in biogas production.]

Response 3: Thank you for pointing out this observation. We agree with this fact, and we have included this. [We have included the description of the DS18B20 temperature sensor used its specifications and temperature response. The on/off nature of the temperature control system was highlighted on page 11, paragraph 2, lines 411-420 of the corrected manuscript. The position of the temperature sensors inside the digesters is shown in Figure 5.

Comments 4:[The limitations of using cow dung as the sole substrate are not discussed in detail. It would be beneficial to compare the performance of co-digestion with other substrates, which could enhance biogas yield. This would also provide a broader context for the paper's findings.]

Response 4: Thank you for that crucial observation, however the focus of the paper was enhancing biogas yield using process parameters  optimization. [We have however included a discussion on “Techniques of enhancing biogas yield” in the literature review section(page 10, paragraph 3 to page 11, paragraph 4, lines 388-441) and combining different ways for future research in the recommendation section(page 23 paragraph 4, lines 697-699).]

Comments 5: [The authors do not sufficiently explain the rationale behind the selection of a 10-minute stirring duration every four hours. Discussing the basis for this choice and comparing it with other stirring frequencies could help optimize mixing strategies. More insights into the effects on temperature uniformity and biogas yield would also be valuable.]

Response 5: We agree with the observation. The selection of 10 minutes for every 4 hours was based on recommendations from literature and that the microorganisms could have chance to digest the nutrients undisturbed. [We have included a close to explain the basis of this choice and cited some research which support the choice in the Material and Methodology section lines 449-456 in the corrected manuscript]

Comments 6: [The influence of pH fluctuations on the biogas production process is mentioned but not deeply analyzed. Including more data on pH trends during different stages of the retention period could help understand microbial activity better. This could lead to recommendations for maintaining optimal pH conditions.]

Response 6: Thank you for pointing out that point. Some information in this regard has been added in the relevant section. [We have rearranged the sentencing to highlight the explanations and added extra information on lines 645, 646, 648 and 649o of the corrected manuscript]

Comments 7: [There is limited analysis on the environmental impact of the process, particularly concerning greenhouse gas emissions. Incorporating a discussion on the carbon footprint or emission reductions achieved by using biogas would highlight the sustainability aspect of the research. This would also align with the environmental significance stated in the introduction.]

Response 7: Agreed. [We have inserted a sub-section in the literature review on the environmental benefits of biodigesters to address this shortcoming. This appears on page 7, paragraph 3, lines 245-256 of the corrected manuscript]

Comments 8: [The discussion on heat exchanger efficiency lacks technical details, such as heat transfer rates or system thermal conductivity. Including these metrics could provide a better understanding of the heat exchanger's performance. Moreover, exploring alternative heat exchanger designs could offer insights into system optimization.]

Response 8: Thank you for highlighting that point. This was deliberately left out because of the focus of the paper but we[ have inserted extra information concerning the conductivity of the heat exchanger material, the dimensions of the copper pipes and the maximum flow rate.]

Comments 9: [ The study does not explore potential cost implications of scaling up the temperature-controlled biogas digester. A cost-benefit analysis comparing conventional systems with the proposed solution could help validate its economic feasibility. This would be particularly useful for decision-makers considering large-scale deployment.]

Response 9: Thank you for pointing out point. We have addressed this issue under results and discussion. [We have inserted an explanation of the extra initial costs and the associated benefits basing on the results of the research and commended on the balance between cost and payback period] See pages 22-23 last paragraph, lines 708-719.

Comments 10: [There is no mention of microbial analysis or monitoring of microbial communities involved in the digestion process. Detailing microbial species and their behavior could help explain variations in biogas yield and quality. Such insights could also inform adjustments to operating conditions for better results.]

Response 10: Thank you for the observation. This point was left out deliberately due to the focus of the research paper which is not much into microbiology.[ However, the nature of microorganisms responsible for the digestion process were mentioned and described in the literature review under the 4 processes. See page 3 paragraph 2 to page 4 paragraph 1, lines 106 to 152 of the corrected manuscript.]

Comments 11: [The effect of insulation quality on the thermal stability of the digester is not thoroughly addressed. Discussing the choice of insulation materials and their thermal properties would add depth to the temperature control aspect. This information could guide improvements in digester design for various climatic conditions.]

Response 11: We agree and for this reason [we have included a discussion on the effect of the insulation properties of clay soil since the digester was underground and the type of soil was clay. This is shown on page 16, paragraph 4, lines 578-588]

Comments 12: [The paper does not discuss the potential for automatization improvements beyond the current system. Exploring the integration of advanced control algorithms or machine learning for predictive temperature management could enhance biogas production efficiency. Including suggestions for future developments in automation would make the study more forward-looking.]

Response 12: Thank you for the observation. This idea was mentioned in the recommendations, and we had only mentioned automating the loading rate. [We have included the reason why future digesters should be fully automated and automating all measured parameters such as loading rate, temperature control, pressure, stirring mode, volume of gas and composition] See page 23,paragraph 4, lines 736-743 of the corrected manuscript.

4. Response to Comments on the Quality of English Language

Point 1: Yes,     Can be improved

Response 1: [A thorough proof reading was done to correct the omissions and grammatical errors in the corrected manuscript]

5. Additional clarifications

[ Where additions are supposed to be added, it is beneficial to the authors if the reviewers and editors point out in which section of the manuscript should the additions be made. This will assist new authors significantly]

Response to Reviewer 2 Comments

1. Summary

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections highlighted/in track changes in the re-submitted files.

2. Questions for General Evaluation

Reviewer’s Evaluation

Response and Revisions

Is the work a significant contribution to the field?

Is the work well organized and comprehensively described?

Is the work scientifically sound and not misleading?

Are there appropriate and adequate references to related and previous work? 

Is the English used correct and readable?        

3. Point-by-point response to Comments and Suggestions for Authors

Comments 1:  [INTRODUCTION is not structured quite correctly. The purpose and objectives of the study should be at the end of this section after the review. Biogas production issues should be moved to the section above. It would be very important to indicate in the INTRODUCTIO the existing experience in regulating the biogas production process, reactor operation, and the influence of various parameters on the stability of fermentation.]

Response 1: Thank you for pointing this out. We agree with this comment. Therefore, we have….[restructured the introduction section including the process parameters influencing reactor stability, reactor operation and present means of regulating process parameters. The purpose and objectives of the study were relocated to the appropriate position] See page 1 paragraph 1 lines 38 to page 2 line 83.

Comments 2: [The literature review should be under point 2]

Response 2: Agree. We have, accordingly, revised the section to emphasize this point. [ The aspects on biogas production were removed from introduction to literature review under point 2. More aspects on importance of biogas as a sustainable energy source were added including economic and environmental benefits.] see page 2 last paragraph line 92- page 3 paragraph 1line 100 of the corrected manuscript]

Comments 3: [There are inaccuracies in the design of subsections, for example, periods (2.4.1, 2.4.2, etc.). The design of subsections should be done in a single style, because there are periods, italics and upright letters, etc.]

Response 3: Thank you for pointing out this point. It was an oversight on our party we have [ changed all italics to upright letters in the whole corrected manuscript]

Comments 4: [ In Fig. 4, the designations are not clear, the letters are at different levels]

Response 4: We agree, and effort has been made correct this [labels have been redone to point at the referred positions] See Figure 4 page 20 of the corrected manuscript]

Comments 5:  [The image in Fig. 5 is not clear, the lines are broken.]

Response 5: Thank you for that observation [We have made the line bold and continuous for clarity. See the said Figure in the corrected manuscript on page 12.]

Comments 6: [The axis designation in Fig. 11 is not accurate. The dimensions must be moved. Make the axis labels in a single style not clear]

Response 6: Thank you for pointing out that. We agree. [We adjusted the labelling of the axis to have one style and dimensions were moved. See Figure 11 on page 17 of the corrected manuscript]

Comments 7: [After Fig. 11, the text is interrupted. Look, controlled digester increases significantly.]

Response 7: Its true, thank you for the observation. [we realigned the text with the rest of document. See pages 17 to 23]

Comments 8: [ In Table 2, the column captions need to be corrected.]

Response 8: Agreed and for this reason we have [redrawn the table with distinct columns and captions. See Table 3 on page 20 of the corrected manuscript]

Comments 9: [ In Fig. 15, the scale can be reduced, there is a lot of empty space.]

Response 9: Thank you for pointing out that. [We have adjusted the scale to eliminate empty spaces. See Figure 16 on page 21 of the corrected manuscript]

Comments 10: [On the graphs with the results, the scale captions, legends, and dimensions can be made brighter.]

Response 10: It’s a good observation however the brightness used is the maximum for the software used. For this reason, we are maintaining the brightness since the captions are well legible.

Comments 11: [ In paragraph 5.1. Conclusions, it is stated that "The regression of biogas yield and retention time showed a greater R2 value for the temperature-controlled digester than the uncontrolled digester". At the same time, there are no calculations or condemnation of this value in the results above. Where did it come from, how was it calculated, this information must be provided in the Results paragraph.]

Response 11: Thank you for the observation, we agree with the fact therefore we [have included the regression graphs showing the referred determination constants. See Figure 17 on page 22 of the corrected manuscript.]

Comments 12: [The article practically lacks a comparison of the obtained results. It is necessary to provide a comparison with the data of other authors. A table or comparison graphs can be provided.]

Response 12: We agree with your observation and we [ have made some comparisons of the findings of this research with what other authors found about pH and volume of gas produced on page 24 lines 775-778 of the corrected manuscript]

Comments 13: [ In paragraph 5.0. Conclusions and Recommendations states that "This research paper used a multi-disciplinary approach to automatically control and optimize slurry temperature to enhance biogas production". In general, from the Results section it is not entirely clear what approaches were used in the study, since data are provided for 2 fermentation modes, the graphs are standard. It is also stated that "The water flow control system was controlled by the average reactor temperature and the temperature control system. The mixing system was programmed to operate for 10 minutes every 4 hours", but it is also unclear from the text how the temperature change affected the parameters of the biogas reactor. Therefore, in the results of the study it is necessary to discuss these approaches more precisely, for example, how the temperature change and other important parameters affect the stability of the plant, as was stated in the conclusions of this study.]

Response 13: Thank you for the point. We do agree with your observation and we [ have inserted an explanation regarding the effect on temperature on the parameters of the reactor including a graph (Figure 11) showing the temperature variations for the two scenarios heated and unheated. The multi-disciplinary approach referred to electronics controlling electric circuits which in turn operated mechanical systems. See page 17, paragraph 1 and Figure 11 of the corrected manuscript]

4. Response to Comments on the Quality of English Language

Point 1: Fine

Response 1: Thank you [ A thorough proof reading was done to correct the English]

5. Additional clarifications

[none]

Response to Reviewer 1 Comments

1. Summary

Thank you very much for taking the time to review this manuscript. Please find the detailed responses below and the corresponding revisions/corrections highlighted/in track changes in the re-submitted files.

2. Questions for General Evaluation

Reviewer’s Evaluation

Response and Revisions

Does the introduction provide sufficient background and include all relevant references?

Yes/Can be improved/Must be improved/Not applicable

Are all the cited references relevant to the research?

Yes/Can be improved/Must be improved/Not applicable

Is the research design appropriate?

Yes/Can be improved/Must be improved/Not applicable

Are the methods adequately described?

Yes/Can be improved/Must be improved/Not applicable

Are the results clearly presented?

Yes/Can be improved/Must be improved/Not applicable

Are the conclusions supported by the results?

Yes/Can be improved/Must be improved/Not applicable

3. Point-by-point response to Comments and Suggestions for Authors

Comments 1: [I suggest accepting this paper now]

Response 1: [Thank you ]