Review Reports

Round 1

Reviewer 1 Report

This paper evaluated the possibility of using overpressure in the digester to improve the efficiency of co-digestion of SS and OF-MSW using block-modular anaerobic bioreactors based on railway tanks. The data and literature summarized in the article are relatively rich, the analysis and discussion of the results are relatively sufficient, and the research results have certain significance and reference value. However, there are several problems: (1) The abstract is not perfect, and it is recommended to polish and revise it. (2) Too many keywords, it is recommended to simplify.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

This paper presents a study on the feasibility of improving the anaerobic co-digestion of sewage sludge (SS) and organic fraction of municipal solid wastes (OF-MSW), under conditions of moderate overpressure. In the study, three overpressure conditions and the effect of organic load in a pilot scale digester are evaluated. The manuscript is well organized and has sufficient scientific merit. However, before the manuscript is accepted for publication, the authors should pay attention to the following:

1. The title could be more representative if the co-substrates used in the study are indicated. Indicating that it is a case of the Russian Federation is not relevant.
2. The introduction adequately presents the motivation and scope of the work, however the studies where overpressure is implemented in anaerobic digestion and codigestion processes are described very lightly. Doing a deeper review could help the reader to understand the frontier of knowledge in this regard.
3. The ratio of inoculum-substrate or between co-substrates is very important to understand the overall performance of the process. It is recommended to include the specific criteria for the selection of the co-substrates-inoculum ratio. Likewise, it is necessary to include the statistical analyzes applied to verify the validity of the experimental procedures and results.
4. The authors could include some remarks or comments on the scaling of the results, either for a bioreactor with the same configuration as the pilot plant or for the block-modular bioreactor.

Author Response

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Author Response File: Author Response.docx

Reviewer 3 Report

The paper discusses the influence of pressure on the methane co-digestion process of sewage sludge and organic waste. It should be explained why the efficiency of the thermophilic anaerobic digestion was analysed instead of the mesophilic one. Is the thermophilic anaerobic digestion more widely used in the Russian Federation?

Questions for the paper:

1. Please explain why the sewage sludge in Table 1 is characterized by VS/TS = 65.34%. Is this a typical value?
2. The paper notes a significant decrease in pH for 0 and 100 kPa in series II and III. Please explain what caused this decrease. Should not the effects of: OLR, HRT, pH and pressure on the dynamics of biogas and methane production be evaluated together?

Author Response

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Author Response File: Author Response.docx

Reviewer 4 Report

The current review reports an interesting topic that points out the feasibility study of anaerobic co-digestion in moderately pressurized digesters in Russian Federation. It presents originality and novelty especially in the Russian Federation where no earlier works were performed in similar methodology and aims.  Overall, the manuscript is written in an average standard English and moderate adjustments are needed in this concern. The manuscript covers all the corresponding relevant topic. All used references by the authors are appropriate. The study is correctly designed and sounds technically.

The Abstract part is clear and well formulated and all keywords fit well. The Introduction part is a well structured and aiming one. All the study’s aims are very clear at the end of the Introduction part. The Materials and methods part is clear and contains essential information and explanations about the used materials and adopted methodologies. The Results part needs some adjustments in terms of findings analysis manner but shows a good statistical presentation in figures. Moving to the Discussion part, it is a clear and well formulated one; however, it needs some adjustments in the linking between current findings and earlier studies. Regarding the Conclusions part, it summarizes the findings and supports well the reported data in Results section (they are well justified). Also, potential reliable research was suggested.

Briefly, based on the above and below detailed explanation, the manuscript needs adjustments, but have a merit to be published in “Applied Sciences” journal once all suggestions and recommendations are fully addressed.

 

Abstract

1- Page 1, lines 17–33: The Abstract part is clear and well formulated.

2- Page 1, lines 34–35: All keywords fit well.

 

1. Introduction

1- Pages 1–3, lines 38–125: A very well structured and built introduction.

2- Page 2, line 49: Kindly adjust as follow: “are formed”.

3- Page 2, line 52: Kindly adjust as follow: “60 million tons”.

4- Page 2, lines 52–54: “Annually… waste”: These statements lack reliable sources (references); accordingly, kindly provide them.

5- Page 2, lines 61–64: “Anaerobic… ecosystem”: Kindly reformulate this sentence.

6- Page 2, line 69: Kindly adjust as follow: “According to Monnet [18]…”

7- Page 2, line 70: Kindly adjust as follow: “range from 4 to 16 inches”.

8- Page 2, lines 70–71: Kindly remove “(there are 27.7 inches of the water column in one psi))” and adjust the sentence as follow: “4 to 16 inches (101.6–406.4 mm of water column; 0.001–0.004 MPa)”.

9- Page 2, lines 82–86: “It should… archaea”: The sentence is long and heavy; accordingly, kindly reformulate it.

10- Page 2, line 86: Kindly replace “So” by “Therefore”.

11- Page 2, lines 91–92: “Typically… per day”: This statement lacks reliable sources (references); accordingly, kindly provide them.

12- Page 2, lines 92–93: Kindly adjust the sentence as follow: “At low OLR values”.

13- Page 3, lines 97–99: “In Ref… 1:40”: The sentence is badly written in standard English; accordingly, kindly reformulate it.

14- Page 3, lines 102–103: Kindly remove “(13.5–18 instead of 6 kg VS/(m³ day))”.

15- Page 3, line 104: Kindly adjust the sentence as follow: “However, there are few studies on the effect of…”

16- Page 3, lines 108–110: “The authors… COD”: Kindly reformulate this sentence avoiding the mention of studies’ authors in the beginning of the sentence.

17- Page 3, lines 120–125: The study’s aims are very clear.

 

2. Materials and methods

1- Pages 3–7, lines 130–207: The Materials and methods part is clear and contains essential information and explanations about the used materials and adopted methodologies.

2- 2.1. Inoculum and substrate: Page 4, lines 135–138: “Given… work”: Kindly avoid the first voice form of the sentence and adopt the impersonal form instead.

3- 2.1. Inoculum and substrate: Page 4, line 138: Kindly adjust as follow: “by-products”.

4- 2.2. Analytical methods: Page 4, line 162: Kindly add “respectively” after “200°C”.

5- 2.2. Analytical methods: Page 4, line 164: Kindly describe briefly the external standard method.

6- 2.3. Digesters configuration: Page 5, lines 173–174: “The co-substrate… litres”: The sentence is badly written in standard English; accordingly, kindly reformulate it.

7- 2.3. Digesters configuration: Page 5, line 177: Kindly adjust the sentence as follow: “… 250 litres and a flat bottom…”

8- 2.4. Operational conditions: Page 6, lines 194–196: “Once… digester”: The sentence is badly written in standard English; accordingly, kindly reformulate it.

9- 2.4. Operational conditions: Page 6, lines 202–203: “Time… Table 2”: Kindly remove this sentence.

 

3. Results

1- Pages 7–11, lines 209–261: The Results part needs some adjustments in terms of findings analysis manner but shows a good statistical presentation in figures.

2- Page 7, lines 209–218: “Throughout… 10 days”: These sentences are not appropriately formulated; accordingly, kindly reformulate them.

3- Page 10, line 245: Kindly adjust as follow: “in the range of 6.4–7.3”.

4- Page 10, line 246: Kindly adjust the sentence as follow: “except for control and with digesters having an overpressure of 100 KPa”.

5- Page 11, lines 258–261: “The highest… 7 days”: There is no significant difference between the findings with 150 KPa and 200 KPa. Accordingly, it is better to go ahead with the 150 KPa for both EPR and EY. Kindly mention that there was no significant difference between them and that they both the highest values.

 

4. Discussion

1- Pages 11–13, lines 263–316: This part is clear and well formulated but needs some adjustments in the linking between current findings and earlier studies.

2- Page 11, lines 266–267: Kindly adjust the sentence as follow: “in 3-liter digesters by Kim et al. [43]”.

3- Page 11, line 267: Kindly adjust as follow: “bars” instead of “bar”.

4- Page 11, line 272: Kindly adjust the sentence as follow: “in the current work compared to Kim et al. [43]”.

5- Page 13, lines 276–277: Kindly remove “described in Refs.” and adjust as follow: “288–333 L”, “1–3 bars”.

6- Page 13, lines 280–297: “The results… bar”: These findings originating from different sources are mentioned in a non-appropriate manner (it looks like a literature review). Accordingly, kindly reformulate this section pointing out each point and its agreement or contradiction with the current study’s findings.

7- Page 13, line 298: Kindly adjust the sentence as follow: “Thus, the currents findings correlate…”

8- Page 13, line 299: Kindly adjust as follow: “in the current study”.

9- Page 13, lines 301–306: “We think… [16]”: First, the sentence is too long and heavy; accordingly, kindly reformulate in order to make it more concise and aiming. Second, the statement is written in the first voice form; accordingly, kindly adopt the impersonal form instead.

10- Page 13, line 308: Kindly replace “Here” by “Herein”.

 

5. Conclusions

1- Pages 13–14, lines 318–344: This part summarizes the findings and supports well the reported data in Results section. Also, potential reliable research was suggested.

2- Pages 13–14, lines 323–335: In the Conclusions part, the findings should not be re-written. Accordingly, kindly remove this section.

3- Page 14, line 336: Kindly remove “Thus”.

Author Response

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Author Response File: Author Response.docx