Optimization of Hydrolysis-Acidogenesis Phase of Swine Manure for Biogas Production Using Two-Stage Anaerobic Fermentation
, Chyi-How Lay 1,2,5,*, Chin-Chao Chen 6, Chun-Yi Lee 2 and Pau Loke Show 4,*Round 1
Reviewer 1 Report
Manuscript ID processes-1301089 is an interesting and valuable scientific material. It corresponds to the Processes journal profile. In my opinion, it may be published after making changes and additions. My remarks below:
Please prepare a graphical abstract.
Please do not use abbreviations (RSM) in Keywords
Cultivation is not the right word for writing methane fermentation processes.
Why in the methodology the authors do not use the commonly used technological parameter of the methane fermentation process, Organic Load Rate, which is described in gCOD /dm3 x day or g VS/ dm3 x day.
Due to the fact that the experimental condition was carried out in triplicate in Figure 1 and in Table 1, the values of the standard deviation should be marked and given.
The manuscript lacks a well-prepared discussion chapter, in which the results obtained by the authors would be shown against the background of the existing knowledge. It needs to be completed.
Most of the literature items cited by the authors are old and out of date. It needs to be changed and supplemented.
Author Response
Manuscript ID processes-1301089 is an interesting and valuable scientific material. It corresponds to the Processes journal profile. In my opinion, it may be published after making changes and additions. My remarks below:
Please prepare a graphical abstract.
Response: The graphical abstract has been updated.
Please do not use abbreviations (RSM) in Keywords
Response: The full name of RSM, response surface methodology, has been added.
Cultivation is not the right word for writing methane fermentation processes.
Response: “Cultivation” has been revised.
Why in the methodology the authors do not use the commonly used technological parameter of the methane fermentation process, Organic Load Rate, which is described in gCOD /dm3 x day or g VS/ dm3 x day.
Response: The organic loading rate was fixed at 4.0 ± 0.3 g COD/L-d in L170.
Due to the fact that the experimental condition was carried out in triplicate in Figure 1 and in Table 1, the values of the standard deviation should be marked and given.
Response: The standard deviation has been added.
The manuscript lacks a well-prepared discussion chapter, in which the results obtained by the authors would be shown against the background of the existing knowledge. It needs to be completed.
Response: More discussions have been added.
Most of the literature items cited by the authors are old and out of date. It needs to be changed and supplemented.
Response: Thank you for your feedback. We have updated the work with more up to date literatures.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The authors of the reviewed article raises a very important issue related to the possibility of energy use of agricultural waste through the production of biogas. In their research, they focused on optimizing this process by using two-stage fermentation. The methodology adopted in the research was applied correctly (apart from minor issues requiring further clarification, which I write about below), and the RSM analysis used in the work allowed for proper determination of the parameters of the first stage of fermentation. Figure 7 for the Venn diagram is also noteworthy, although I think it can be discussed more. The conclusions presented at the end of the work summarize the results achieved in the work well.
Below are some comments, the inclusion of which should improve the quality of the article:
- Please provide information whether the amounts of methane produced, presented in the article include the amounts produced by the inoculum itself. In other words, was there a control for the inoculum itself?
- Please explain the application of this particular method of measuring methane production. Has it been used by anyone before?
- Line 121-129 - the description of the experiment is somewhat unclear. Please clearly state that you have prepared three series of samples. At the moment, it is not known exactly how Effects of solid content was defined. The description suggests that everything ended up in one vial. It seems to me that the information on how the production and composition of biogas was studied should be found here, not just in line 177.
- Line 145 - please explain how many samples there were with this different pH (4-10).
- Lines 177-178 - Please explain how the volume of gas in the syringe was converted to the volume of methane produced. Have changes in pressure in the vials due to taking samples for analysis been taken into account in the calculations?
- Figure 2 - no error bars on the charts
- Figure 2 - why are the results shown in ml and not in ml / gVS?
- Figure 3 - figure b - I think it should be methane production yield
- Line 306 - The title does not quite fit the content of this section. I propose to change it.
- Table 5 - I think the use of the parameter name: CH4 composition is wrong.
Author Response
The authors of the reviewed article raises a very important issue related to the possibility of energy use of agricultural waste through the production of biogas. In their research, they focused on optimizing this process by using two-stage fermentation. The methodology adopted in the research was applied correctly (apart from minor issues requiring further clarification, which I write about below), and the RSM analysis used in the work allowed for proper determination of the parameters of the first stage of fermentation. Figure 7 for the Venn diagram is also noteworthy, although I think it can be discussed more. The conclusions presented at the end of the work summarize the results achieved in the work well.
Response: Thank you for your feedback. We appreciate the comments a lot.
Below are some comments, the inclusion of which should improve the quality of the article:
Please provide information whether the amounts of methane produced, presented in the article include the amounts produced by the inoculum itself. In other words, was there a control for the inoculum itself?
Please explain the application of this particular method of measuring methane production. Has it been used by anyone before?
Response: Thank you for your feedback. This method of measuring methane production has been used by several researchers.
‘Biogas volume was determined by a gas tight syringe at room temperature (20 °C) and pres-sure (760 mm Hg) [14-18].’
References:
[14] S. Lopez, C.J. Newbold, in: H.P.S. Makkar, P.E. Vercoe (Eds.), Meas. Methane Prod. From Ruminants, Springer Netherlands, Dordrecht, 2007, pp. 1–13.
[15] C.R. Soliva, H.D. Hess, in: H.P.S. Makkar, P.E. Vercoe (Eds.), Meas. Methane Prod. From Ruminants, Springer Netherlands, Dordrecht, 2007, pp. 15–31.
[16] P. Deuri, N. Sood, M. Wadhwa, M.P.S. Bakshi, A.Z.M. Salem, Agrofor. Syst. 94 (2020) 1455–1468.
[17] A. Melesse, H. Steingass, M. Schollenberger, J. Holstein, M. Rodehutscord, Agrofor. Syst. 93 (2019) 135–147.
[18] J. Filer, H.H. Ding, S. Chang, Water 11 (2019) 921.
Line 121-129 - the description of the experiment is somewhat unclear. Please clearly state that you have prepared three series of samples. At the moment, it is not known exactly how Effects of solid content was defined. The description suggests that everything ended up in one vial. It seems to me that the information on how the production and composition of biogas was studied should be found here, not just in line 177.
Response: Thank you for your comment. In the last line of this paragraph, it was stated that each experimental condition was carried out in triplicate.
Line 145 - please explain how many samples there were with this different pH (4-10).
Response: Thank you for your comment. Three samples were carried out follow by the experimental design in Table 2.
Lines 177-178 - Please explain how the volume of gas in the syringe was converted to the volume of methane produced. Have changes in pressure in the vials due to taking samples for analysis been taken into account in the calculations?
Response: Biogas volume was determined by a gas tight syringe in the batch experiment and wet gas meter (RITTER TG1, Germany) at room temperature (20 °C) and pressure (760 mm Hg). The composition of product gas in the batch enrichment assays and continuous systems was measured with a CHINA Chromatography 8700T GC. The methane production (mL) is calculated by biogas volume (mL) multiplied by methane content (%).
Figure 2 - no error bars on the charts
Response: The standard deviation has been added.
Figure 2 - why are the results shown in ml and not in ml / gVS?
Response: This result is referring to the cumulative methane production from various sources, therefore is shown in mL. The methane yield with the unit of ml/gVS is calculated by dividing the cumulative methane production by the mass of volatile solid.
Figure 3 - figure b - I think it should be methane production yield
Response: Yes, we agree that Figure 3b is the contour plot for methane production yield.
Line 306 - The title does not quite fit the content of this section. I propose to change it.
Response: Thank you for your feedback. The title of the subsection has been changed from ‘Reactor performance’ to ‘Methane production yield in single and two stage system’.
Table 5 - I think the use of the parameter name: CH4 composition is wrong.
Response: Thank you for your feedback. We agree that CH4 content is a better term.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
The authors revised the manuscript. In my opinion, it can be accepted in current form.