Anaerobic Co-Digestion of Oil Sludge with Corn Stover for Efficient Biogas Production
Round 1
Reviewer 1 Report
Comments to the Authors
This paper addresses anaerobic co-digestion process to assess the influence of different raw material ratios and inoculum volume. The authors selected corn stover for co-digestion with oils sand sludge. The authors claim that they have determined the optimum ratio (as 4:1). In my opinion, the experiment is not designed properly to determine the optimum ratio. If the ratio is increased to 5:1 ---6:1 the gas yield may increase. The authors have only evaluated the feasibility of co-digestion, which is not a new concept or a novel finding.
The experimental design is not adequate to determine optimum values The overall papers lacks novel innovation. The VSS of the inoculum is not mentioned. It is not appropriate to characterize the inoculum by volume. It is one of the drawbacks of this study. Comparison with prior similar research works is missing
Other comments
Title should be modified with the term co-digestion. In addition, Oil Sludge Sand does not look the right phrase. It should be Oil Sand Sludge Line 9 : Volume is misspelt Line 47: A table from previous studies should be provided (not from this study). The same is true with the figure. Line 47: Where is Figure 1? Line 69: Mention the name of the country Section 2.1.3: What is the VSS of the inoculum? Not that the any one should be able to replicate your methods. Line 112: Why is the reactor not continously stirred? How did you decide to shake twice a day for 10 min. Section 3.1.1: Methane composition (%) in the gas is not mentioned. Therefore, it is difficult to talk about biogas as far as feasibility is concerned. The authors are monitoring only gas production Line 136: fitness should be replaced with feasibility Line 142: How do you conclude that the sludge has lower organic components from Table 1? Are you referring to degradable organics? Line 162: Gas production (yield) should be reported in rates as mL per VS or any other appropriate rate and compare these results with previous similar studies. Comparison with prior similar research works that used oil sand sludge or co digestion other substrate with corn stover is required. Section 3.2: It is difficult to understand the amount of innoculum added. It should be expressed in terms of VSS so that one can be able to replicate your study for example. Line 183: I am curious to know what will happen with the ratio of corn stover and oil sand sludge at 4:1 because you identified 4:1 as the optimum ratio. Line 221: 4:1 is not optimal. Because if the ratio is increased to 5:1 ---6:1 the gas yield may increase.
Author Response
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Author Response File: 
Author Response.docx
Reviewer 2 Report
The article (sustainability-692589) entitled “Study on gas production performance of anaerobic digestion of oil sludge sand” should be reconsidered after major revisions.
The aim of the study is the reuse of oil sludge sand through the anaerobic digestion and its optimization, as this material contains hazardous compounds and this can affect the biological process. However, the issue of the study is that there are reported only the results of biogas performances, without any more analyses that can help to explain the findings obtained. Moreover, the statistical analysis is not reported in “materials and methods”, however there are discussed the results in terms of correlation and significant differences. The results must be supported adequately by the bibliography.
Other considerations are reported in the detailed responses below.
Abstract:
Lines 14-15. Please refer only to the cumulative gas production volume or and volumetric gas production.
Introduction:
Line 39. It is not necessary to mention both livestock and poultry. It might be also fine refer to livestock wastes.
Line 54. Please indicate the chemical methods.
Lines 59-62. The sentence is confusing, please rephrase it. Why did you report the corn stover as “additive”?
Materials and methods:
The description of the technology to obtain the oil sludge sand can be improved. Moreover, I suggest to report exactly the location where all materials were collected.
Concerning the experimental design, please add the duration of the experiments and explain better why a different amount of water was added (see Table 2).
Results and discussion:
This section must to be deeply improved with more bibliography, since the results obtained in the study are not enough to support the conclusions. It might be useful for instance to add a complete chemical characterization of the initial and final mixtures, in order to evaluate the anaerobic process also in terms of organic matter variation. Concerning this aspect, I suggest also to improve the discussion of the organic matter conversion. Moreover, the authors report the biogas productions in mL, while it would be more useful to show the results as Nm3/t SV.
Line 137. It is not clear how comparing the biogas production of pure oil sludge with the biogas production obtained from the co-digestion in Figure 2.
Line 142. It is not completely correct to refer to “organic components” looking in the Table 1.
Lines 142-145. Authors mentioned that the microbial activity might be inhibited, but it was not explained why and how the microbial activity is affected by the chemical characteristics of the tested material.
Lines 154-157. Please explain how the addition of corn stover can affect positively the biogas production.
Line 161. It is not enough to describe the general behavior, but it might be important also to explain how the different ratios enhance the biogas production.
Line 173. It is not clear why the authors refer to corn stover qualities.
Lines 177-179. Please add the importance to study the evolution of soluble organic C during the anaerobic digestion.
Line 191. What do you mean with “microbial activity is not sufficient”?
Lines 194-197. Please explain why the increased amount of inoculum affects positively the microbial activity and then biogas production.
Conclusions:
I suggest to focus the conclusions on the optimization of preparing the initial mixture with the oil sludge sand, considering the possible effect of hazardous compounds.
Figures and Tables:
Table 1. Please report in the caption the meaning of abbreviations. In addition, I suggest to remove “water”.
Table 2. Please add in the second column that the ratios refer to corn stover : oil sludge, and delete the next two columns.
Figures 3 and 5. See general comments about the unit of measure.
Figure 4. It would be interesting to change the Figure in Table, by adding the correlation between the mass conversion and the cumulative biogas production or moving this information in the text.
Author Response
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Author Response File: Author Response.docx
Reviewer 3 Report
The purpose of the article is unclear. Test results are inconsistent, given in terms of various units, which makes it difficult to compare them. The explanation of the results is not clear and even contradicts itself. There is no discussion of the research results obtained. Applications also need improvement.
Detailed comments in the attached file.
Comments for author File: 
Comments.pdf
Author Response
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Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
Author Response
We have check the English language and style in the manuscript.
Reviewer 2 Report
The article (sustainability-692589) entitled "Study on gas production performance of anaerobic digestion of oil sludge sand" was improved, but the scientific soundness is still low. The lack of statistical analysis does not permit to evaluate if the results are significant or not, and then the conclusion can't be supported by the results obtained.
Moreover, I suggested improving the bibliography to explain better the results, but the authors gave the explanation in the response file and they did not improve the text (Line 142-145, 154-161, 177-179, 194-197).
Line 231 in the new manuscript : I suggest to mention only organic matter content.
Please, check the manuscript for fine/minor errors spelling in both Figures and text.
Author Response
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Author Response File: Author Response.docx
Reviewer 3 Report
The article was corrected in accordance with the comments in the review. In my opinion, it can be published in its current form.
Author Response
Thank you.
Round 3
Reviewer 2 Report
The authors did not modify the text as requested, especially they did not add new references for supporting the results.
Please, add and explain the analysis of variance in the section of materials and methods.
Line 148 in the new manuscript: I suggest to mention only organic matter content.
Author Response
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Author Response File: Author Response.docx
