Portable Biogas Digester: A Review
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsDespite the promising title of the reviewed article, its content is somewhat disappointing. The article is largely a review of the literature on biogas production with the term "portable biogas digester" inserted from time to time. I believe it needs improvement before being published on Gases
1. The introduction is too long and too detailed. It seems to me that the description of the 4 phases in which biogas is produced is unnecessary. However, too little attention was paid to the need to produce portable digesters. I also lack information on how many such devices exist on the market. What is the demand.
2. Chapter 2 - please insert some sample drawings or, better yet, photos of the described digesters. Moreover, in this chapter the authors describe quite specific examples of digesters, adapted to work in warm climates. The article is a review of the literature, and its title does not limit the subject of research only to this type of digesters. However, the reader should know that the article only covers such specific solutions.
3. Chapter 4 - it describes the impact of various parameters on biogas production, but there is little connection with the topic of the article.
4. Lines 56-57 – reference needed
5. Please use subscripts in chemical compounds, e.g. line 89.
6. Lines 110-111 - wording: Uddin & Wright et al. [12] is unclear
7. Lines 207-208 - the word "recently" repeated
8. Lines 222-224 - why do the authors not mention anything about "psychrophilic digestion", which takes place at lower temperatures?
9. Table 3 – the issue of total solids takes up a lot of space here. Why don't the authors mention this before? I mean wet and dry fermentation. What type of fermentation usually occurs in a portable biogas digester?
10. Lines 395-408 – please explain where these assumptions came from?
11. Table 5 - item 51 in the list of references is given as the source. Meanwhile, this item does not contain the information shown in the table. Also, this table should be number 4.
12. Line 490 – article by Issahaku et al. it is number 33 while in the literature list it is 34
13. Table 4 (Line 527) – does the presented data refer to biogas yield or methane yield?
Author Response
Despite the promising title of the reviewed article, its content is somewhat disappointing. The article is largely a review of the literature on biogas production, with the term "portable biogas digester" inserted from time to time. I believe it needs improvement before being published on Gases.
Comment 1: The introduction is too long and too detailed. It seems to me that the description of the 4 phases in which biogas is produced is unnecessary. However, too little attention was paid to the need to produce portable digesters. I also lack information on how many such devices exist on the market. What is the demand.
Response: The authors appreciate the reviewer’s comment. Based on the comment, the introduction has been reduced, leaving those of much interest to the reader and focusing on the topic. The 4 phases or stages of anaerobic have been removed as recommended by the reviewer, and the authors understand that this aspect is not necessary for the study. The need for a portable biogas digester was addressed in section 4. It is worth knowing that the system's existence is not as common on a large scale; however, recommendations and further studies, such as the present one, encourage more of the system's design for household use.
Comment 2: Chapter 2 - please insert some sample drawings or, better yet, photos of the described digesters. Moreover, in this chapter the authors describe quite specific examples of digesters, adapted to work in warm climates. The article is a review of the literature, and its title does not limit the subject of research only to this type of digester. However, the reader should know that the article only covers such specific solutions.
Response: The authors appreciate the reviewer’s comment. As suggested by the reviewers, examples of the specified biogas digester design via sample drawing are presented and shown in Figures 1, 2, and 3 and Figure 4 for the portable digester. Studies have shown that portable biogas digester designs result from modifying fixed dome, floating, and balloon digesters.
Comment 3: Chapter 4 - it describes the impact of various parameters on biogas production, but there is little connection with the topic of the article.
Response: The authors appreciate the reviewer’s comment on this aspect/section (section 3, currently). However, the review is on portable biogas digesters. Hence, the authors sought it right that this section is necessary since it focuses on the factors affecting the performance and operation of the system to arrive at the desired and optimized biogas yield. Having established that, the studies and references in the sections are recent and not outdated, which is important.
Comment 4: Lines 56-57 – reference needed.
Response: The authors appreciate the reviewer’s comment. Reference added as Kabeyi and Olarewaju [3], line 51.
Comment 5: Please use subscripts in chemical compounds, e.g., line 89.
Response: The authors appreciate the reviewer’s comment. Kindly know that this statement from which the comment was made was deleted and removed based on the first comment by reviewer 1.
Comment 6: Lines 110-111 - wording: Uddin & Wright et al. [12] is unclear.
Response: The authors appreciate the reviewer’s comment. The authors would like to say this was addressed by rephrasing and revising the wording in Uddin and Wright [Now 5]. This can be seen in lines 70 – 71.
Comment 7: Lines 207-208 - the word "recently" repeated
Response: The authors appreciate the reviewer’s comment. This has been corrected, as seen in lines 356-357.
Comment 8: Lines 222-224 - why do the authors not mention anything about "psychrophilic digestion," which takes place at lower temperatures?
Response: The authors appreciate the reviewer’s comment and thank you for pointing this out. The authors would like to apologize for the mistake, and we want to state that the information on psychrophilic digestion is necessary. However, this has been mentioned in the study, as seen in lines 205 to 209.
Comment 9: Table 3 – the issue of total solids takes up a lot of space here. Why don't the authors mention this before? I mean wet and dry fermentation. What type of fermentation usually occurs in a portable biogas digester?
Response: The authors appreciate the reviewer’s comment. Due to the advantages and benefits of total solids towards biogas production, most designs from the literature deal with organic waste (substrate/feedstock) as a major factor. However, dry fermentation is preferred and employed for biogas digesters, including portable ones. See lines 424 – 427 for the author’s response as highlighted.
Comment 10: Lines 395-408 – please explain where these assumptions came from?
Response: The authors appreciate the reviewer’s comment. As a case study, the assumption came from the study conducted by Alkahalidi et al. [2019] on portable biogas digesters for domestic use in Jordanian villages. As a reference, this can be found in line 429 – 430.
Comment 11: Table 5 - item 51 in the list of references is given as the source. Meanwhile, this item does not contain the information shown in the table. Also, this table should be number 4.
Response: The authors appreciate the reviewer’s comment. The authors have corrected and used the right reference for Table 5, which is now Table 4 in the revised manuscript. The right reference is [20] and has been corrected and can be seen in line 450 (Table 4)
Comment 12: Line 490 – article by Issahaku et al. it is number 33; in the literature list, it is 34.
Response: The authors appreciate the reviewer’s comment. The above comment has been corrected. Issahaku et al. have been given the right number in the text and reference section. The same is applicable to other references and sources used in the study.
Comment 13: Table 4 (Line 527) – does the presented data refer to biogas yield or methane yield?
Response: The authors appreciate the reviewer’s comment. The authors meant biogas yield, which has been corrected as seen in Table 5 and previously in Table 4.
To our knowledge, we believe we have responded well to the comments raised by reviewer 1.
Reviewer 2 Report
Comments and Suggestions for AuthorsThe manuscript deals with a subject that deserves review. However, extensive corrections are needed before it can be considered suitable for publication. The authors should be more critical with the process; many biogas digester present failure and low productivity, a subject scarcely commented on the manuscript.
1. An extensive English edition is needed. Some phrases just lack any sense or are incorrectly expressed.
2. L37-39: Landfills are still used today, although organic recovery of material is needed. There are also requirements for running landfills as reaction systems, thus the statement is not fully correct
3. L44: Why is acid rain prevented?
4. L45-46: Only fuels (methane, hydrogen) made of biogas, a flammable gas. Please be more specific
5. L26-75: Please separate into independent paragraphs. The text is too long to be in a single paragraph.
6. L53-55: incorrectly expressed and the text is repetitive with the previous statement
7. L65-70: This text is not linked with previous lines. Information is misplaced here. A Language edition is needed. L67 has similar content to the previous lines
8. L77-119: Please schematized in a diagram. This process is well known; better use this information to create a figure explaining different stages
9. L110: Correct reference insertion
10. L110-127: Please introduce here information regarding unsuccessful experiences reported in the scientific literature dealing with reactor failure after being constructed and installed in different low-income countries. Explain the causes of failure.
11. L130-131: Please introduce the subject. Currently, the authors are not linking in an acceptable manner the different sections of the document
12. Section 2: A figure is needed where a schematic representation of the different digesters is graphically observed.
13. L191-192: compared with what other process?
14. L202: What other development?
15. L204: high-density populated areas benefit from centralized treatment units rather than having a small decentralized treatment system. Please explain what the authors mean by conditions areas.
16. L208-209: What is the implication of the type of feedstock and the digester being portable?
17. Section 4 refers to general factors affecting anaerobic digestion, not exclusively portable digesters. Reduce this section and place this general information in a table where the digestion description was made.
18. L356-392: Modify this information; it is too general. The description in this section should contain more technical details regarding construction, restrictions, and organic loading constraints.
19. L376: LPG needs to be defined
20. section 7: biogas production should be indicated in terms of gas volume per unit mass of volatile or total solids.
21. L447: stirring engine?
22. L522-526: expression erroneous. Biogas yield should be expressed per unit of mass of the feed, which has not been reported. Therefore, the information in Table 4 is also erroneous. Authors should indicate the OLR and water demand of the system to attain the desired solid content in the feed. Many failure experiences deal with a lack of water in desertic climatic conditions.
23. L537-538: indicate money conversion unit to dollars or euros
24. L547: Definition is missing.
25. L548: Rp?
26. L551: Description should've been placed earlier
27. L554: R6?, L555: R63?
28. L550-557: Better description is needed. No explanation was given regarding the study performed in two different schools, may I guess?
29. L562: Mixing with human excreta has a risk of spreading diseases due to the low temperature of the process. Fecal contamination and the presence of viruses may create a health risk.
30. The conclusion section should be reduced.
Comments on the Quality of English Language
The manuscript requires extensive changes and complete English eddition
Author Response
The manuscript deals with a subject that deserves review. However, extensive corrections are needed before it can be considered suitable for publication. The authors should be more critical with the process; many biogas digester present failure and low productivity, a subject scarcely commented on the manuscript.
Comment 1: An extensive English edition is needed. Some phrases just lack any sense or are incorrectly expressed.
Response: The authors are grateful for the reviewer’s comment on improving the manuscript. The authors subjected the whole manuscript to a professional English editor. Hence, the certificate confirming editing and proofreading is available at the request of the corresponding author.
Comment 2: L37-39: Landfills are still used today, although organic recovery of material is needed. There are also requirements for running landfills as reaction systems, thus the statement is not fully correct
Response: The authors are grateful for the reviewer’s comment on improving the manuscript. The authors deleted and removed the statement from the manuscript.
Comments 3: L44: Why is acid rain prevented?
Response: The authors are grateful for the reviewer’s comment on improving the manuscript. The authors' responses can be found in lines 41 – 44, as highlighted in the manuscript.
Comment 4: L45-46: Only fuels (methane, hydrogen) made of biogas, a flammable gas. Please be more specific.
Response: The authors are grateful for the reviewer’s comment on improving the manuscript. As highlighted in the manuscript, the authors revised and rephrased the statement, as seen in lines 45 – 46.
Comment 5: L26-75: Please separate into independent paragraphs. The text is too long to be in a single paragraph.
Response: The authors are grateful for the reviewer’s comment on improving the manuscript. However, this has been adhered to, thereby separating the affected parts into independent paragraphs.
Comment 6: L53-55: incorrectly expressed and the text is repetitive with the previous statement
Response: The authors are grateful for the reviewer’s comment on improving the manuscript. The authors considered removing or deleting the previous statement to address the comment.
Comment 7: L65-70: This text is not linked with previous lines. Information is misplaced here. A Language edition is needed. L67 has similar content to the previous lines
Response: The authors are grateful for the reviewer’s comment on improving the manuscript. The entire manuscript has now been subjected to professional editing, and a certificate was issued based on the comment from the reviewer.
Comment 8: L77-119: Please schematized in a diagram. This process is well known; better use this information to create a figure explaining different stages.
Response: The authors are grateful for the reviewer’s comment on improving the manuscript. Reviewer 1 recommended and suggested removing the affected areas where the above comment is coming from. Hence, as explained earlier, the stages of anaerobic digestion were removed.
Comment 9: L110: Correct reference insertion
Response: The authors are grateful for the reviewer’s comment on improving the manuscript. The correct reference has been inserted as Uddin and Wright [5], as seen in lines 70 – 71.
Comment 10: L110-127: Please introduce here information regarding unsuccessful experiences reported in the scientific literature dealing with reactor failure after being constructed and installed in different low-income countries. Explain the causes of failure.
Response: The authors are grateful for the reviewer’s comment on improving the manuscript. The authors responded to the comment as seen in lines 75 – 94 as highlighted in the manuscript.
Comment 11: L130-131: Please introduce the subject. Currently, the authors are not linking in an acceptable manner the different sections of the document.
Response: The authors are grateful for the reviewer’s comment on improving the manuscript. The first paragraph deals with the need for renewable energy through biogas in handling and contributing to the solution of the global energy demand. The second paragraph focuses on biogas composition and the waste usually employed for anaerobic digestion. Thereby, it briefly introduces anaerobic digestion towards the end. Paragraph 3 of the introduction section defines the technology responsible for portable biogas digestion for biogas production and the factors responsible for the failure of the system technology, mostly in low-income African countries. Paragraphs introduce the topic due to the challenges and limitations of the biogas digester. The authors believe the current revised manuscript has links properly and more than the previous document.
Comment 12: Section 2: A figure is needed where a schematic representation of the different digesters is graphically observed.
Response: The authors are grateful for the reviewer’s comment on improving the manuscript. The authors have presented Figures 1, 2, and 3 to show the example of different biogas digesters.
Comment 13: L191-192: compared with what other process?
Response: The authors are grateful for the reviewer’s comment on improving the manuscript. The authors could not ascertain and understand what the reviewer means by “compared with other process”. Clarification is needed. The affected statement is now in lines 331 – 332.
Comment 14: L202: What other development?
Response: The authors are grateful for the reviewer’s comment on improving the manuscript. The authors removed the “in another development” since it does not make sense. However, the paragraph has been revised and rephrased. (See line 346 – 351)
Comment 15: L204: high-density populated areas benefit from centralized treatment units rather than having a small decentralized treatment system. Please explain what the authors mean by conditions areas.
Response: The authors are grateful for the review comment in improving the manuscript. The authors rephrased this as densely populated areas and under difficult conditions against the previously used conditions areas. This can be found in line 353.
Comment 16. L208-209: What is the implication of the type of feedstock and the digester being portable?
Response: The authors are grateful for the reviewer's comment on improving the manuscript. Based on the reviewer's comment during the revision, the authors decided to remove and delete the type of feedstock mentioned earlier in the statement, as this does not go well and makes no sense. Hence, the statement was rephrased and revised, as seen in lines 356 – 361. However, the same type of feedstock used in large-scale digesters also applies to portable biogas digesters.
Comment 17. Section 4 refers to general factors affecting anaerobic digestion, not exclusively portable digesters. Reduce this section and place this general information in a table where the digestion description was made.
Response: The authors are grateful for the review comment on improving the manuscript. The authors adhere to the comment by moving the affected area from section 4 to now section 3 immediately after the digester description.
Comment 18: L356-392: Modify this information; it is too general. The description in this section should contain more technical details regarding construction, restrictions, and organic loading constraints.
Response: The authors are grateful for the reviewer's comment on improving the manuscript. With reference to the comment, the authors modified the general information and now focused on specific areas as suggested by the reviewers. These include design and construction, restriction, and organic loading constraints. As seen in lines 382 – 415, these have been implemented immediately after Table 2.
Comment 19: L376: LPG needs to be defined
Response: The authors are grateful for the reviewer's comment on improving the manuscript. With reference to the previous reviewer's comment regarding modifying the generalized information, this present comment falls within the areas that the authors removed. Therefore, the information in LPG has been removed from the manuscript.
Comment 20: section 7: biogas production should be indicated in terms of gas volume per unit mass of volatile or total solids.
Response: The authors are grateful for the reviewer's comment on improving the manuscript.
Comment 21: L447: stirring engine?
Response: The authors are grateful for the reviewer's comment on improving the manuscript. The wording has been corrected as “stirrer,” as highlighted in line 485.
Comment 22: L522-526: expression erroneous. Biogas yield should be expressed per unit of mass of the feed, which has not been reported. Therefore, the information in Table 4 is also erroneous. Authors should indicate the OLR and water demand of the system to attain the desired solid content in the feed. Many failure experiences deal with a lack of water in desertic climatic conditions.
Response: The authors are grateful for the reviewer's comment on improving the manuscript. We agreed with the reviewer's comment; this is correct and hence authors have addressed this by indicating m3/kg TS as the correct expression. This can be found now in Table 5.
- L537-538: indicate money conversion unit to dollars or euros
Response: The authors are grateful for the reviewer's comment on improving the manuscript. The money conversion unit to euro has been addressed where appropriate. See lines 587 – 588 of the revised manuscript.
- L547: Definition is missing.
Response: The authors are grateful for the reviewer's comment on improving the manuscript. The definitions of the various economic indicators have been defined where appropriate and accordingly, as seen in lines 594 – 595.
Comment 25: L548: Rp?
Response: The authors are grateful for the reviewer's comment on improving the manuscript. The rupiah (Rp) is a currency used in Indonesia. However, the value is now expressed in euros because of the conversion to euros, as seen in lines 595 – 596.
Comment 26: L551: Description should've been placed earlier
Comment 27: L554: R6?, L555: R63?
Comment 28: L550-557: Better description is needed. No explanation was given regarding the study performed in two different schools, may I guess?
Comment 29: L562: Mixing with human excreta has a risk of spreading diseases due to the low temperature of the process. Fecal contamination and the presence of viruses may create a health risk.
Response: The authors are grateful for the reviewer's comment on improving the manuscript. During the revision and after a second look, the authors had to remove the paragraph where comments No 26 to 29 were raised. The removal had to deal with the biogas digester not being portable. The study conducted by Mkabela et al. [2021] is a large-scale biogas digester, not a portable one; hence, it was removed.
Comment 30: The conclusion section should be reduced.
Response: The authors are satisfied and grateful for the reviewer's comment on improving the manuscript. Based on the comment, the authors had to reduce the conclusion section, as pointed out by the reviewer.
To our knowledge, we believe we have responded well to the comments raised by reviewer 2.
Reviewer 3 Report
Comments and Suggestions for AuthorsThe text contains a number of mistakes and missing blank spaces (e. g. between numerical value and unit). There are also some sentences and phrases that are not comprehensible.
In the final analysis, the current version of the paper would be suitable for publication after revision of the entire manuscript.
The specific comments are summarized in the attached pdf file “Specific comments_gases-3062242”.
Comments for author File: 
Comments.pdf
(See above)
Author Response
The text contains a number of mistakes and missing blank spaces (e. g. between numerical value and unit). There are also some sentences and phrases that are not comprehensible. In the final analysis, the current version of the paper would be suitable for publication after revision of the entire manuscript. The specific comments are summarized in the attached pdf file “Specific comments_gases-3062242”.
peer-review-37701192.v1.pdf
Response: The authors would like to appreciate the comments from the reviewers as they tend to improve the manuscript for possible consideration for publication. However, as seen in the attached, the comments raised have been responded to accordingly and appropriately, as highlighted in the revised manuscript. As the reviewers pointed out, these comments focus more on blank spaces of numerical values and units, as well as a few instances that deal with rewording and incomprehensible. All these have been addressed.
To our knowledge, we believe we have responded well to the comments raised by reviewer 3.
Reviewer 4 Report
Comments and Suggestions for Authors The reviewed paper entitled "Portable biogas digester: review" requires a number of additions and corrections. First of all, in my opinion, the wording "portable" should be reconsidered. The authors describe digesters buried in the ground as portable solutions. Is the buried reactor still portable? Would not the term micro-biogas plant be more appropriate? I think the authors should focus on pointing out possible differences between micro plants and industrial biogas plants. I agree that in many cases, for many applications, it is only advisable to build a micro-biogas plant. However, a lot of very basic information about the methane fermentation process has been provided, such as the influence of temperature and pH on the process. In my opinion, a scientific publication does not have to repeat ordinary textbook data. A person reading this type of publication already has the basic knowledge. The work lacks comparisons and figures on operating costs and construction costs. The authors provide information that is not very precise. Table 1, for example, shows the results for different types of portable biogas plants. They indicate the substrates used, but there is no information on the loads used. The use of the term kitchen waste itself is very imprecise. Information on the use of plastic tanks has no practical significance. For the fermentation process itself, it does not matter whether the housing is made of plastic or concrete. Table 2 shows a comparison of portable biogas plants with other solutions. The figures are not very precise. This means: efficiency from 0.2 m3 to 3.6 m3? The volume of biogas should be given in relation to the volume of the fermentation chamber or in relation to the organic mass of the substrate introduced. What is the advantage of portability? Is a reactor with a volume of 7.5 m3 (the authors give such an example) buried in the ground easy to transport? When the authors point out that the solution with portable biogas plants is cheaper, it is worth making calculations (figures, percentages) in relation to e.g. 1m3 of biogas produced. If the biogas plant is to be used only for the production of biogas to be burned in a kitchen burner, the costs are of course lower, but if it is also to serve as a source of electricity, the same elements are needed. There is no reliable numerical data, but there are many general terms in the work.Author Response
Comment: The reviewed paper entitled “Portable biogas digester: review” requires a number of additions and corrections. First of all, in my opinion, the wording “portable” should be reconsidered. The authors describe digester buried underground as portable solutions. Is the buried reactor still portable? Would not the term micro biogas plant be appropriate? I think authors should highlight possible differences between micro plants and industrial biogas plants. However, I agree that in many cases, for many applications, it is only advisable to build a micro biogas plant.
Response: The authors appreciate the comment from the reviewer. However, the sentence or statement describing the underground biogas digester as portable has been removed and seen as a mistake. And this has been corrected. We thank the reviewer for pointing out this. Secondly, we agreed on the use of a micro biogas digester, and it is still correct. Interestingly, both portable and micro biogas digesters refer to small units designed for individual/family household and institutional purposes. More specifically, the word “Portable biogas digester” is always used in literature. Therefore, we maintain and go with the topic of the manuscript as appropriate.
Comment: However, a lot of basic information about the methane fermentation process has been provided, such as the influence of the temperature and pH on the process. In my opinion, a scientific publication does not have to repeat ordinary textbook data. A person reading this type of publication already has the basic knowledge.
Response: The authors appreciate the comment from the reviewer. We agreed with the reviewer that information on the methane fermentation process has been provided and published, especially the factors. However, with due respect, we disagreed with the reviewer on the issue of repeating original textbook data. Our review sources are mainly journals and not textbooks. Hence, section 3 refers to “Recent studies on factors affecting the performance of portable biogas digester.” We implore the reviewers and editor to check the references in the section, these are recent, which implies that studies are still conducted in that regard. The information is not necessary for academics and researchers in the field but for readers who are interested in the technology, especially as the world is experiencing an energy crisis.
Comment: Table 1, for example, shows the results for the different types of portable biogas plants. They indicate the substrate used, but no information exists on the loads.
Response: We appreciate the reviewer's comment and thank the reviewer for pointing this out. However, the comment has been addressed, as seen in Table 1.
Comment: The use of the term kitchen waste itself is very imprecise. Information on the use of plastic tanks has no practical significance.
Response: We appreciate the reviewer's comment. Hence, due to the imprecise of the kitchen waste used in Table 1, as pointed out by the reviewer, the authors decided to remove or delete the column on the type of waste since it makes no difference with other designs of biogas digesters in terms of feedstock type.
Comment: For the fermentation process itself, it does not matter whether the housing is made of plastic or concrete. Table 2 shows a comparison of portable biogas plants with other solutions. The figures are not very precise. This means efficiency from 0.2 m3 to 3.6 m3. The volume of biogas should be given in relation to the volume of the fermentation chamber or in relation to the organic mass of the substrate introduced. What is the advantage of portability? Is a reactor with a volume of 7.5 m3 (the authors give such an example) buried in the ground easy to transport? When the authors point out that the solution with portable biogas plants is cheaper, it is worth making calculations (figures, percentages) in relation to ,e.g., 1m3 of biogas produced. If the biogas plant is to be used only for the production of biogas to be burned in a kitchen burner, the costs are of course lower, but if it is also to serve as a source of electricity, the same elements are needed. There is no reliable numerical data, but there are many general terms in the work
Response: The authors appreciate the response and concerns from the reviewer in an effort to improve the manuscript for publication. First, the capacity of 0.2 m3 to 3.6 m3, as reported in Table 2, signifies the volume of the biogas digester and not the biogas volume or yield. The issue of underground biogas digester being referred to as portable was removed and deleted in the revised manuscript. Hence, a portable biogas digester has the advantage of convenient transport and easy assembly even up to a volume of 7.5 m3, provided it is not underground. Secondly, the authors refer to costs in terms of the cost of material for fabrication and construction for being cheaper than the large scale and not the biogas energy value application.
To our knowledge, we believe we have responded well to the comments raised by reviewer 4.
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThe authors improved the article according to my suggestions. Therefore, I believe it can be published in Gases.
Author Response
Comment: The authors improved the article according to my suggestions. Therefore, I believe it can be published in Gases.
Response: We appreciate the reviewer's positive response regarding the possible publication of our manuscript. Many thanks
Reviewer 2 Report
Comments and Suggestions for AuthorsAuthors addressed main corrections, although some minor changes are still needed.
Minor corrections:
1. L41-43: NOx emissions are associated with any fuel burning, no matter if being fossil fuel, carbon or renewable material. Reducing NOx in burnt gases deals with burning conditions, that is, attaining premixed and lean burning conditions. Use excess air to attain low flame temperature and avoid diffusion flames in combustion chamber. My recommendation is to either remove the lines containing the erroneous concept, or correct in proper form to indicate modes of reducing NOx emissions when using biogas.
2. Figure 1 and Figure 3: improve quality
3. L205-206: Meaning not clear
4. L206: What does author intent of express with "this type of temperature condition is unstable"?. Please clarify. Reducing temperature affects reaction rates, therefore it has an intrinsic effect in the time needed to complete the degradation process and it does affect OLR and reactor volume. Please clarify this concepts in this section.
5. L210-213: Contains similar information to that added in previous lines (highlighted in yellow: L197-200). Please integrate to avoid repetition.
6. L219-221: Indicate type of temperature control systems. Controlling temperature makes the design more complex, please clarify.
7. section 3.3. Please link temperature effect and HRT
8. section 3.4: Add comments regarding the effect of increasing C/N ratio to optimum level and hence increasing VS removal (therefor greater biogas production). Add references where this feature is evaluated.
Comments on the Quality of English Language
Minor edition, check the use of some units
Author Response
Comment 1: L41-43: NOx emissions are associated with any fuel burning, no matter if being fossil fuel, carbon or renewable material. Reducing NOx in burnt gases deals with burning conditions, that is, attaining premixed and lean burning conditions. Use excess air to attain low flame temperature and avoid diffusion flames in combustion chamber. My recommendation is to either remove the lines containing the erroneous concept, or correct in proper form to indicate modes of reducing NOx emissions when using biogas.
Response: The authors appropriate the response from the comment from the reviewer. Based on the comment, the authors rephrased and corrected the concept in a proper form, as seen in lines 41 – 46 of the manuscript.
Comment 2: Figure 1 and Figure 3: improve quality
Response: The authors appreciate the response from the reviewer. Figures 1 and 3 have been improved for clarity.
Comment 3: L205-206: Meaning not clear and L206: What does author intent of express with "this type of temperature condition is unstable"?. Please clarify. Reducing temperature affects reaction rates, therefore it has an intrinsic effect in the time needed to complete the degradation process and it does affect OLR and reactor volume. Please clarify this concepts in this section.
Response: The authors appreciate the response from the reviewers. The affected lines and phrases have been rephrased and revised as highlighted in lines 200 – 201 and 208 – 213.
Comment 5. L210-213: Contains similar information to that added in previous lines (highlighted in yellow: L197-200). Please integrate to avoid repetition.
Response: The authors appreciate the comments from the reviewer. Based on the comment, the authors removed the repeated part and integrated it into the previous line, as seen in lines 200 – 201.
Comment 6. L219-221: Indicate type of temperature control systems. Controlling temperature makes the design more complex, please clarify.
Response: The authors are grateful for the reviewer's comment on improving the manuscript. However, the comment has been addressed, as seen in lines 221 – 228 of the manuscript.
Comment 7. section 3.3. Please link temperature effect and HRT
Response: The authors are grateful for the reviewer's comment on improving the manuscript. Lines 227 – 287 link the temperature effect and HRT
Comment 8. section 3.4: Add comments regarding the effect of increasing C/N ratio to optimum level and hence increasing VS removal (therefor greater biogas production). Add references where this feature is evaluated.
Response: The authors are grateful for the reviewer's comment on improving the manuscript. The authors responded to the comment as seen in lines 308 – 323 as well as lines 335 – 344.
Reviewer 4 Report
Comments and Suggestions for Authors The comment regarding the inclusion of basic information about the fermentation process in the publication does not refer to the quality of the publications to which the authors refer. The point is not that the authors refer to poor publications. The point is that they are presenting completely basic data. In my opinion, a review publication should present the views of different researchers on a particular topic. It should present the latest results and views on a particular topic. I don't think it is necessary to explain the basics of the process to the readers of a scientific journal. Scientific journals are usually read by experts. In my opinion, chapter 3 should focus on the specific characteristics of portable fermenters and emphasize them more. The title of the chapter is good, it is worth working on the content. Information on the differences between portable fermenters and transfer fermenters would be an important innovation.Author Response
Comment: The comment regarding the inclusion of basic information about the fermentation process in the publication does not refer to the quality of the publications to which the authors refer. The point is not that the authors refer to poor publications. The point is that they are presenting completely basic data. In my opinion, a review publication should present the views of different researchers on a particular topic. It should present the latest results and views on a particular topic. I don't think it is necessary to explain the basics of the process to the readers of a scientific journal. Scientific journals are usually read by experts. In my opinion, chapter 3 should focus on the specific characteristics of portable fermenters and emphasize them more. The title of the chapter is good, it is worth working on the content. Information on the differences between portable fermenters and transfer fermenters would be an important innovation.
Response: We appreciate the comment from the reviewer. However, at this point, we disagree with the comment raised by the reviewer. First, the comment raised by the reviewer 4 is not necessary. Our manuscript has been subjected to 4 reviewers during the first round, and 3 of the reviewers did not see anything wrong with our inclusion of the content in Section 3 of the manuscript. Despite our initial response after careful consultation in the first round of the reviewing process, the same comment is coming up again. Please note that our response still stands in this regard. Secondly, we, the authors, assume that reviewer 4 is in a chemical engineering area of study and is using a term that is not appropriate for renewable energy, which our manuscript focuses on, specifically energy storage. The issue of portable fermenters could be understood as portable digesters, but that of transfer fermenters seems incorrect and confusing. Hence, it does not have any connection or link with our study. The last sentence of the reviewer comment needs further clarification. Therefore, we ask the academic editor to check the comments and the manuscript as well our response. If possible, subject our manuscript to another reviewer for his decision or verdict.
