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Open AccessReview
Peer-Review Record

A Review of the Chemistry of Anaerobic Digestion: Methods of Accelerating and Optimizing Process Efficiency

Processes 2019, 7(8), 504; https://doi.org/10.3390/pr7080504
Reviewer 1: Anonymous
Reviewer 2: Frank Uhlenhut
Reviewer 3: Anonymous
Processes 2019, 7(8), 504; https://doi.org/10.3390/pr7080504
Received: 24 May 2019 / Revised: 24 July 2019 / Accepted: 30 July 2019 / Published: 2 August 2019
(This article belongs to the Special Issue Processes for Bioenergy and Resources Recovery from Biowaste)

Round 1

Reviewer 1 Report

The review content does not correspond to the title. The authors according to the title, are critically analysing the barriers of wide adoption of AD in developing countries. with the exception of one paragraph , that simply refers to technical problems and lack of technical knowledge as the main problem there is absolutely no analysis based on the problems the countries face. Instead the focus is on the actual problems that any AD process regardless of size in any part of the world can face. A part of the paper deals with the AD chemistry , which is well known information and does not feed directly to the rest of the paper. There is also no definition of size , what actually the authors consider small scale? To the reviewers knowledge both in central Europe and the North West part of the continent AD is mainly applied in a localised setting but also as part of WWT. There are numerous conflicting sentences in the paper as well for example  

lines 52-55 p:3, if AD is so problematic what is the incentive behind it? lines: 67-68 p:3, not sure what the authors mean, lines 209-212 p:7 is contradictory to line 290-292 p:9 and others. The authors have to decide what type of review they would like to write a more general one dealing with an overview of AD problems , or a specific one to developing countries per  continent , following the developing country definition as supplied by the World Bank or another internationally accepted institution. 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

General comment:

In principle the objective of the study is interesting and relevant (review of specific problems and possibilities for usage of small-scale AD systems in developing countries). The authors give an overview of the advantages of small-scale AD systems and describe the four different stages of the biological process. Subsequently, they summarize the current status of small-scale AD in developing countries and describe the specific problems and lack of knowledge in this field. In conclusion, they suggest some potential solutions and outline future directions and opportunities.

 

The following issues should be added/discussed:

1)  The authors mainly focus on problems caused by high sulfur content in the used substrates/feedstock. They should give some examples for such substrates and evaluate their relevance. In practice this problem is of minor importance, because biowaste/biosolids (which are the main substrates for small-scale AD) consist of carbohydrates, lipids and proteins. Carbohydrates and lipids don’t contain sulfur (only C, H and O). Only proteins can be a source for sulfur if they contain one or both amino acids cysteine and/or methionine.

In practice other problems like inhibition caused by high nitrogen content or accumulation of VFAs (especially acetic acid) caused by inappropriate feeding (too high volumetric load) leading to pH drop are of much greater importance.

2)  Page 9, lines 292 – 294: The authors state that the effects of sulfur and bacterial interaction within the AD process have not been adequately explained. There are many publications about the bacterial interaction and the role of sulfur in the AD process and possible inhibitions by sulfur (especially in connection with modelling and simulation of the biological processes and extensions/modifications of the used models like the well-established ADM 1). A short selection of publications is listed below:

Kalyuzhnyi, S. and Fedorovich, V. (1998) Mathematical modelling of competition between sulphate reduction and methanogenesis in anaerobic reactors. Biores. Tech. 65, 227-242.

Fedorovich, V., Lens, P. & Kalyuzhnyi, S. (2003). Extension of Anaerobic Digestion Model No. 1 with processes of sulfate reduction. Applied Biochemistry and Biotechnology, 109(1-3), 33-45.

Xavier Flores-Alsina, Kimberly Solon, Christian Kazadi Mbamba, Stephan Tait, Krist V. Gernaey, Ulf Jeppsson, Damien J. Batstone, Modelling phosphorus (P), sulfur (S) and iron (Fe) interactions for dynamic simulations of anaerobic digestion processes. Water Research, Volume 95, 15 May 2016, Pages 370-382.

E.L. Barrera, H. Spanjers, K. Solon, Y. Amerlinck, I. Nopens, J. Dewulf, Modeling the anaerobic digestion of cane-molasses vinasse: Extension of the Anaerobic Digestion Model No. 1 (ADM1) with sulfate reduction for a very high strength and sulfate rich wastewater. Water Research, 71 (2015), pp. 42-54

Ernesto L. Barrera, Henri Spanjers, Jo Dewulf, Osvaldo Romero and Elena Rosa. The sulfur chain in biogas production from sulfate-rich liquid substrates: a review on dynamic modeling with vinasse as model substrate. J Chem Technol Biotechnol, 2013, 88, 1405-1420

H.-H. Chou, J.-S. Huang, W.-G. Chen, R. Ohara, Competitive reaction kinetics of sulfate-reducing bacteria and methanogenic bacteria in anaerobic filters. Bioresource Technology, 99 (17) (2008), pp. 8061-8067

T. Hao, P. Xiang, H.R. Mackey, K. Chi, H. Lu, H. Chui, M.C.M. van Loosdrecht, G.-H. Chen. A review of biological sulfate conversions in wastewater treatment. Water Research, 65 (2014), pp. 1-21

J.B.M. Klok, M. de Graaff, P.L.F. van den Bosch, N.C. Boelee, K.J. Keesman, A.J.H. Janssen. A physiologically based kinetic model for bacterial sulfide oxidation. Water Research, 47 (2) (2013), pp. 483-492

Hélène Hauduc, Bruce Johnson, Charles Bott, Matthew Ward, Imre Takács. Incorporating Sulfur and Relevant Reactions into a General Plantwide and Sewer Model. IFAC PapersOnLine 50-1 (2017) 3935–3940

3)  Page 13, lines 322 and 323: The authors conclude that the disappearance of hydrogen and the accumulation of VFAs are linked to inhibitory effects of sulfur. These interactions should be described in more detail.

4)  Page 13 and Figure 1 on page 14: First the authors describe the effects of sulfur and H2S, but the following explanations and the reaction scheme in Figure 1 are dealing with the inhibition effects caused by halogenated aliphatic hydrocarbons (HAHs)! It has to be explained (and verified by literature) if this reaction scheme can really be transferred to the inhibition effects caused by sulfur.

5)  Page 16, chapter 7.1: The authors suggest to use analytical methods (like NDIR) to determine the exact amount of sulfur and the usage of techniques for sulfur removal. These suggestions don’t seem to be reasonable, because sophisticated analytical methods like NDIR are normally not used in small-scale biogas plants (especially not in developing countries). A small-scale biogas plant for developing countries has to be cheap, easy to use and with the lowest technical expenditure. Techniques for sulfur removal are normally not used in practice (not even in highly industrialised countries), because this would be too expensive. In addition, other opportunities are used to prevent problems caused be too high sulfur content. The optimum dosage of substrates (and optimum mixture of different substrates) can be calculated by simulation using an appropriate model (e.g. extended ADM 1 resp. ADM 1xp with incorporated processes for sulfur removal and inhibition).

6)  Page 16, chapter 7.2: The authors suggest the combination of small-scale AD with a microbial electrochemical system. How could this be put into practice (in which way could MES be implemented into a digester)?

7)  Page 17, line 497: The authors mention the use of magnetic fly ash for pretreatment of mainly livestock manure. This has to be described in more detail. What is the effect of magnetic fly ash (supply with trace elements? precipitation of S2-?...)?

 

So in the final analysis, the authors mainly focus on possible problems by high sulfur contents in the used substrates (which does not correspond to practical experience) and propose some solutions which are inappropriate for developing countries.

Thus, from my point of view the current version of the paper is not suitable for publication and a major revision of the entire manuscript is needed.

The specific comments are summarized in the attached pdf file “Specific comments_processes-523686”.

Comments for author File: Comments.pdf

Author Response

Please see attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

This critical review article reports on the technical barriers and opportunities for small-scale anaerobic digestion. The focus of this review is from the chemistry perspective and overall the authors have done decent work on summarising the concerned topic. There are some rooms for improvement and the followings are my comments:

Structurally, the section concentrating on the technical barrier and opportunity is not as much as the very general aspects of AD, for examples, section 2 has taken a big portion of the article while its presence is a drift from the core topic.

Even section 2 is to be included, it needs to be shortened substantially and some errors to be corrected. For example, section 2.1.4 should be methanogenesis not actogenesis.

More careful proof reading should be done as there are some obvious typos and mistakes throughout the article. For instance (line 257), the the misuse of m3 and m3(superscript).

The core sections of the article should be section 6 and 7, however, these sections are very briefly discussed and proper researches and case studies should be cited and discussed to support the example. Also, the barriers are not listed in section 6 while the corresponding solution are listed, although in a wrong way. For instance, the minitile for section 7.1 and 7.2 are the same. These improper assignment of section and inconsistent writing style lowered the readability of the article significantly.

Author Response

Please see attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The m/s is not readable at its present form. It is very difficult at its current state of so much deleted text to be evaluated to its full lenght. However , at tits present form , it does not enhance the field of AD, a large percentage of the information are already known. The edge of developing countries and small scale AD  was really interesting and needed in the field . Considering Processes is an OA journal that would give a significant advantage to the authors , having their novelty exposed and recited multiple times.The UK has funded numerous small AD projects globally and so has done the EU, it would be really interesting to see the impact of this funding.  Kindly rewrite completely the manuscript taking your time and putting effort to an initial really good idea.

Author Response

Point 1: The m/s is not readable at its present form. It is very difficult at its current state of so much deleted text to be evaluated to its full length. However, at its present form, it does not enhance the field of AD, a large percentage of the information are already known. The edge of developing countries and small scale AD was really interesting and needed in the field. Considering Processes is an OA journal that would give a significant advantage to the authors, having their novelty exposed and recited multiple times. The UK has funded numerous small AD projects globally and so has done the EU, it would be really interesting to see the impact of this funding. Kindly rewrite completely the manuscript taking your time and putting effort to an initial really good idea.

Response 1: We respectfully disagree with some of the reviewer’s comments as we feel that significant changes were made to our manuscript to improve it according his/her previous comments hence numerous texts were deleted from previous information. The initial version of the manuscript attempted to discuss the chemical aspect of AD and its importance to process optimization, with reference to developing countries of the world using the technology at a small-scale level. This was presented as an in-depth analysis of existing key technical barriers and opportunities for broader application and commercialization of the technology in rural communities of developing countries. The intention was to discuss key technical challenges of small-scale AD from a chemistry perspective and help identify the need for further research to better deal with the recurrent technical issues in the operation these systems. The motivation for the review was triggered by the conflicting literature reports about the chemical aspect of AD and its influence on process efficiency. However, we agree that the initial presentation of the manuscript was flawed and marred by multiple regrettable errors, omissions and conflicting sentences that were pointed out by the reviewer after his/her careful evaluation of the manuscript. We failed in our attempt to adequately and clearly describe the challenges facing small-scale AD from a chemistry standpoint. On the basis of this and since we were offered a second chance, we decided that our manuscript was worthy of painstaking perusal and should undergo extensive revision in order to address all reviewer’s comments and suggestions. As a matter of fact, the entire manuscript was re-organised and re-written in such a way that is unambiguous and understandable, with its title changed to fit its new content. We feel that the manuscript now has a focus that was ambiguously described in its previous form.

With regards to the reviewer’s comment that the present form of our manuscript does not enhance the field of AD, it should, once more, be pointed out that there are contradictory literature reports on how interspecies hydrogen transfer, hydrogen partial pressure and the use of non-biological conductive materials can improve methanogenic reactions for faster and more efficient methane yield. Furthermore, published reports on how a microbial electrochemical system can be successfully integrated with AD for improved methanogenic reactions as a consequence of increased syntrophic interactions of different microorganisms in the digestion process are equally incongruous, including the concept of volatile fatty acid accumulation and how it can be curbed. These were identified methods of accelerating and optimizing AD process efficiency that has been extensively and unambiguously discussed in the current version of our manuscript, with clearly identified areas of research that can facilitate better understanding. As a proof of the inconsistencies on how the above methods can accelerate and optimize AD, a number of articles (both research and review) are, in no particular order, listed below for the reviewer’s meticulous perusal.   

(1)  Elia Judith Martínez, Ana Sotres, Cristián B. Arenas, Daniel Blanco, Olegario Martínez and Xiomar Gómez. Improving anaerobic digestion of sewage sludge by hydrogen addition: Analysis of microbial populations and process performance. Energies 2019, 12(7): 1 – 15.

(2)  Fanghua Liu, Amelia E. Rotaru, Pravin M. Shrestha, Nikhil S. Malvankar, Kelly P. Nevin and Derek R. Lovley. Promoting direct interspecies electron transfer with activated carbon. Energy & Environmental Science 2012, 10: 1 – 9.  

(3)  Xiaofen Li, Janis E. Swan, Giridhar R. Nair, Alan G. Langdon. Preparation of volatile fatty acid (VFA) calcium salts by anaerobic digestion of glucose. Biotechnology & Applied Biochemistry 2014, 62(4): 476 – 482.

(4)  Sajib Barua, Bipro Ranjan Dhar. Advances towards understanding and engineering direct interspecies electron transfer in anaerobic digestion. Bioresource Technology 2017, 244: 698 – 707.

(5)  Anahita Rabii, Saad Aldin, Yaser Dahman and Elsayed Elbeshbishy. A review on anaerobic co-digestion with a focus on the microbial populations and the effect of multi-stage digester configuration. Energies 2019, 12: 2 – 25.

(6)  Rotaru, A.E., Shrestha, P.M., Liu, F., Markovaite, B., Chen, S., Nevin, K.P., Lovley, D.R. Direct interspecies electron transfer between Geobacter metallireducens and methanosarcina barkeri. Applied Environmental Microbiology 2014a, 80(15): 4599 – 4605.

(7)  Rotaru, A.E., Shrestha, P.M., Liu, F., Shrestha, M., Shrestha, D., Embree, M., Zengler, K., Wardman, C., Nevin, K.P., Lovley, D.R. A new model for electron flow during anaerobic digestion: direct interspecies electron transfer to methanosaeta for the reduction of carbon dioxide to methane. Energy & Environmental Science 2014b, 7(1), 408 – 415.

(8)  Zhiqiang Zhao, Yaobin Zhang, Yang Li, Yan Dang, Tingting Zhu, Xie Quan. Potentially shifting from interspecies hydrogen transfer to direct interspecies electron transfer for syntrophic metabolism to resist acidic impact with conductive carbon cloth. Chemical Engineering Journal 2017, 313: 10 – 18.

(9)  Lun Shi-yi and Chen Jian. The contribution of interspecies hydrogen transfer to the substrate removal in methanogenesis. Process Biochemistry 1992, 27(5): 285 – 289. 

It should be noted that the list of articles to prove our point of contradictions about the aforementioned methods of improving AD is quite exhaustive hence we were conservative in providing those listed above. In our view, the current form of our manuscript has presented critical analysis of already published information in which we carefully considered the progress of current research toward clarifying stated problems in the field of AD. Nonetheless, no matter the extent of research undertaken in the field of AD and no matter the information known to every researcher in this field, there will always be studies aimed at advancing the technology. The copious literature information on the AD technology doesn’t mean research in this interesting field must be halted.

With reference to the comment on the edge of developing countries and small scale AD, which the reviewer stated was really interesting and needed in the field, we agree with the reviewer on this and must aver that the journal (Processes) has given us this last chance to revise our manuscript according to all reviewers’ comments, which we have done to the best of our abilities according to the reviewers’ comments. Attempts to revert to the initial idea may further jeopardise the chances of acceptance, considering the fact that other reviewers may find the information incoherent/uninteresting and may reject the manuscript without a second thought. 

Finally, we agree that the UK has funded numerous small AD projects globally and so has done the EU. We also agree that it would be interesting to see the impact of this funding. As a reiteration, our manuscript has been written with specific focus on the chemical aspect of AD and its influence on methanogenic reactions and process efficiency. We would like to keep the focus intact. However, your point has been included as a consideration for future analysis (section 10. Conclusions, page 16, lines 518 – 521) and we thank you for your suggestion.   


Author Response File: Author Response.pdf

Reviewer 2 Report

General comment:

In principle the objective of the study is interesting and relevant (review of specific problems and possibilities for usage of AD technology and preview of the future developments). The authors give an overview of the advantages of AD systems and describe the four different stages of the biological process. Subsequently, they summarize the current status of the use of AD technology and describe the specific problems and lack of knowledge in this field. In conclusion, they suggest some potential solutions and outline future directions and opportunities.

 

The following issues should be added/discussed:

1)  Page 3, line 40: “…known as biogas (CH4)…” This is an inaccurate simplification, because CH4 is only the main component of biogas (but not the only one).

2)  Page 4, line 93: “…,leading to the formation of digestate (decomposed feedstock) and CH4 gas…” This is an inaccurate simplification, because CH4 is only the main component of the produced biogas (but not the only one).

3)  Page 15, lines 367 and 375: The authors describe the effects of non-biological conductive materials and GAC on toxic organic compounds and suggest that these substances can be absorbed. Is it really an absorption or is it more likely an adsorption?

4)  Page 4 (2.1.1. Hydrolysis) line 102 and “Responses from authors”: The authors claim that the hydrolysis reaction is directly associated with the production of elemental hydrogen. The chemical equation (1) is definitely incorrect, because the mass balance is not fulfilled (on the left side of the equation there is a total of 7 oxygen atoms, on the right side of the equation there is a total of 6 oxygen atoms). For the (in my opinion correct) equation (C6H10O5)n + n H2O --> n C6H12O6                                                                  the mass balance would be fulfilled. In addition, the three hydrolysis processes (for carbohydrates, proteins and lipids) of the well-established ADM 1 model are not coupled with the production of hydrogen (Batstone, D. J., Keller, J., Angelidaki, I., Kalyuzhyni, S. V., Pavlovstathis, S. G., Rozzi, A., Sanders, W. T. M., Siegrist, H., Vavilin, V. A. (2002) Anarobic Digestion Model No. 1, IWA Task Group on Mathematical Modelling of Anaerobic Digestion Processes, IWA Scientific and Technical Report No. 13; Table 3.1). Hydrogen and acetate are the products of the following two phases of AD (Acidogenesis and Acetogenesis), like e.g. illustrated in Figure 12.2 of the cited source

https://www.e-education.psu.edu/egee439/node/727.

So this point has to be discussed and clarified.

 

In the final analysis, the current version of the paper is suitable for publication after minor revision of the entire manuscript and clarification of the outstanding issues.

The specific comments are summarized in the attached pdf file “Specific comments_processes-523686_v1”.



Comments for author File: Comments.pdf

Author Response

General comment:

In principle the objective of the study is interesting and relevant (review of specific problems and possibilities for usage of AD technology and preview of the future developments). The authors give an overview of the advantages of AD systems and describe the four different stages of the biological process. Subsequently, they summarize the current status of the use of AD technology and describe the specific problems and lack of knowledge in this field. In conclusion, they suggest some potential solutions and outline future directions and opportunities.

The following issues should be added/discussed:

Point 1: Page 3, line 40: “…known as biogas (CH4)…” This is an inaccurate simplification, because CH4 is only the main component of biogas (but not the only one).


Response 1: This was noted and corrected on page 3, line 47.

 

Point 2: Page 4, line 93: “…,leading to the formation of digestate (decomposed feedstock) and CH4 gas…” This is an inaccurate simplification, because CH4 is only the main component of the produced biogas (but not the only one).

 

Response 2: This has been noted and corrected in lines 99/100 of page 4.

 

Point 3: Page 15, lines 367 and 375: The authors describe the effects of non-biological conductive materials and GAC on toxic organic compounds and suggest that these substances can be absorbed. Is it really an absorption or is it more likely an adsorption?

 

Response 3: This has been noted and correct in lines 371/372 on page 13. The authors actually meant ‘’adsorb’’.

 

Point 4: Page 4 (2.1.1. Hydrolysis) line 102 and “Responses from authors”: The authors claim that the hydrolysis reaction is directly associated with the production of elemental hydrogen. The chemical equation (1) is definitely incorrect, because the mass balance is not fulfilled (on the left side of the equation there is a total of 7 oxygen atoms, on the right side of the equation there is a total of 6 oxygen atoms). For the (in my opinion correct) equation (C6H10O5)n + n H2O --> n C6H12O6 the mass balance would be fulfilled. In addition, the three hydrolysis processes (for carbohydrates, proteins and lipids) of the well-established ADM 1 model are not coupled with the production of hydrogen (Batstone, D. J., Keller, J., Angelidaki, I., Kalyuzhyni, S. V., Pavlovstathis, S. G., Rozzi, A., Sanders, W. T. M., Siegrist, H., Vavilin, V. A. (2002) Anarobic Digestion Model No. 1, IWA Task Group on Mathematical Modelling of Anaerobic Digestion Processes, IWA Scientific and Technical Report No. 13; Table 3.1). Hydrogen and acetate are the products of the following two phases of AD (Acidogenesis and Acetogenesis), like e.g. illustrated in Figure 12.2 of the cited source

https://www.e-education.psu.edu/egee439/node/727.

So this point has to be discussed and clarified.


Response 4: We agree with the reviewer that the mass balance of the chemical equation (1) is incorrect and we are grateful to him/her for calling our attention to this. The equation has been corrected to reflect the proper mass balance. However, the reviewer must be respectfully reminded that, beyond anaerobic digestion, hydrolysis means the addition of water (H2O) to a substance. This addition causes both the substance and H2O molecules to split into two parts such that one fragment of the parent molecule gains a hydrogen ion. Now, relating this analogy to the process of AD, which occurs in four different stages with hydrolysis as its first reaction stage, H2O is added to the biomass (large organic polymers) in this stage to initiate a reaction that will produce short chain organic compounds like acetate and hydrogen, both of which can be used by fermentative microorganisms in the second stage (Acidogenesis) of the AD process where greater chain organic compounds like VFAs are formed. The VFAs are then further broken down in the next stage of the AD process (Acetogenesis) by obligate hydrogen-producing acetogenic microorganisms to produce other organic compounds like acetic acid, carbon dioxide and, yet again, hydrogen. This is because some amount of H2O from the previous stages is still available and acts as an electron source to facilitate the conversion of the VFAs. This point has been discussed and clarified in the manuscript (section 2.1.1. Hydrolysis, lines 134 – 136 and in section 2.1.3. Acetogenesis, lines 178 – 181, pages 5 and 6 respectively). The equations presented in the following articles are exactly correct as they clearly explain the reactions of the AD process.

(a) https://www.e-education.psu.edu/egee439/node/727.

(b) http://www.fao.org/3/w7241e/w7241e0f.htm#4.1.2%20acetogenesis%20and%20dehydrogenation.


Specific comments from Reviewer 2 found in his/her attached file


Specific comments:


Furthermore, there are a number of minor errors that should be corrected. The text contains a number of mistakes and missing blank spaces (e. g. between value and unit):


Point 1: Page 1 (Introduction) line 30: Superfluous full stop “…[1].” instead of “…[1]. .”. Page 1 (Introduction) line 34: “…the process also has the tendency...” instead of “…the process also have the tendency...”.


Response 1: The statement on page 1 has been corrected in line 34.


Point 2: Page 3 (Introduction) line 45: “…organic matter that consists primarily of nondegraded cells...” instead of “…organic matter that is mostly non-degraded cells...”. Page 3 (Introduction) line 49: “…hydrogen partial pressure...” instead of “…hydrogen partial pressures...”.


Response 2: The errors have been corrected accordingly in lines 53 and 57 of the revised manuscript.


Point 3: Page 3 (Introduction) line 52: “…volatile fatty acids (VFAs)...” instead of “…volatile fatty (VFAs) acids...”.


Response 3: This has been noted and corrected in line 60 of page 3.


Point 4: Page 3 (Introduction) line 55: “…,butyric acid and propionate;...” instead of “…,butyric acids and propionates;...”.


Response 4: Correction effected in line 63.


Point 5: Page 3 (Introduction) line 70: “…, there seems to be...” instead of “…,there seem to be...”.


Response 5: Noted and corrected on page 3, line 78.


Point 6: Page 4 (2.1. AD process chemistry) line 103: C6H10O5 cannot be used as a molecular formula for carbohydrates, it should be (C6H10O5)n.


Response 6: This was corrected in a previous revision.


Point 7: Page 4 (2.1.1. Hydrolysis) lines 117/118: “…in order to render...” instead of “…in order to render...”.


Response 7: This has been corrected on page 4 in line 125.


Point 8: Page 5 (2.1.2. Acidogenesis) line 126: “…, and it is the most important organic acid used as a substrate...” instead of “…, and it is the principal organic acid used as a material...”.


Response 8: This has been corrected accordingly in section 2.1.2 (Acidogenesis), lines 133 and 134, respectively.


Point 9:  Page 5 (2.1.2. Acidogenesis) line 132: Missing blank space between “2” and “H2“ (“2 H2“ instead of “2H2“).


Response 9: Missing blank space added in equation 3.


Point 10: Page 5 (2.1.3. Acetogenesis) line 135: Misleading wording (“…hydrogen-scavenging bacteria that can be converted to CH4 [32].” The whole bacteria will not be converted to CH4!


Response 10:  This has been rephrased to indicate that ‘’some’’ of the bacteria would be converted to methane by the hydrogen-scavenging bacteria. This correction is in section 2.1.3 (Acetogenesis) on page 5, lines 153/154.


Point 11: Page 5 (2.1.3. Acetogenesis) lines 153, 154, 157 and 159: Hydrogen is a diatomic molecule, “H2” instead of “H”.


Response 11: This mistake has been corrected in lines 160, 161, 164, 166 and 170 respectively.


Point 12:  Page 6 (2.1.3. Acetogenesis) lines 161/162: “…reduction of acetate, which is the key intermediary product of…” instead of “…reduction of acetates, which are key intermediary products of…”.


Response 12: These have been corrected in lines 168 and 169 in section 2.1.3.


Point 13: Page 6 (2.1.3. Acetogenesis) line 162: Missing blank space between value and unit (“25 %” instead of “25%”).


Response 13: Missing blank space added in line 169 as “25 %”.


Point 14: Page 6 (2.1.3. Acetogenesis) line 163: Hydrogen is a diatomic molecule, “H2” instead of “H”.


Response 14: This has been noted and corrected accordingly on page 5, line 170.


Point 15: Page 6 (2.1.4. Methanogenesis) line 166: Hydrogen is a diatomic molecule, “H2” instead of “H”.


Response 15: This has been noted and corrected line 173, section 2.1.4 (Methanogenesis).


Point 16: Page 8 (2.2. The percentage composition of CH4 from AD) lines 229/230: Missing blank space between value and unit (e.g. “45 °C” instead of “45°C”).


Response 16: Missing blank space added in line 246 on page 8 (2.2. The percentage composition of CH4 from AD).


Point 17: Page 8 (2.2. The percentage composition of CH4 from AD) lines 229/230: The expression “…to bacteria high temperature range…” is not comprehensible and should be reworded.


Response 17: This has been reworded for better understanding and can be found in lines 247 and 248 on page 8.


Point 18: Page 10 (Table 4): Missing blank space between value and unit (“35 °C” instead of “35°C”).


Response 18: Missing blank space added on page 9 (Table 4) next to ref 64 (“35 °C” added instead of “35°C”).


Point 19: Page 14 (3. Overview of previous research on AD) line 286: “…; not much has been done…” instead of “…; not much have been done…”.


Response 19: This has been corrected and can be found of page 8, line 296.


Point 20: Page 14 (3. Overview of previous research on AD) line 294: “interaction” instead of “intaeraction”.


Response 20: This has been noted and corrected in line 304 on page 12.


Point 21: Page 14 (3. Overview of previous research on AD) lines 293/294: The expression “…are not adept at their physiological interaction,…” is not comprehensible and should be reworded.


Response 21: For better understanding, a more appropriate word has been used to replace ‘’adept’’ in the sentence, which can be found on page 12 (3. Overview of previous research on AD), line 303.


Point 22: Page 14 (3. Overview of previous research on AD) line 294: “accumulation” instead of “acumulation”.


Response 22: This has been noted and corrected on page 12 (3. Overview of previous research on AD), line 305.


Point 23: Page 14 (4. Brief synopsis of issues hitherto addressed in AD research) line 300: “operational aspects” instead of “operation aspects”.


Response 23: This has been corrected in line 310, page 12.


Point 24: Page 14 (5. The concepts of IHT and HPP) line 329: “…butyric and propionic acid…” instead of “…butyric and propionic acids…”.


Response 24: This was noted and corrected in line 340 on page 13.


Point 25: Page 15 (5. The concepts of IHT and HPP) line 331: The expression “…must be dexterous at their…” is a colloquial phrase and should be reworded.


Response 25: The expression has been reworded and can be found on page 13, line 341.


Point 26: Page 15 (5. The concepts of IHT and HPP) line 333: The expression “Bringing about this…” is not comprehensible and should be reworded.


Response 26: The expression has been reworded and clearly highlighted on page 13 in lines 343 and 344, respectively.


Point 27: Page 15 (5. The concepts of IHT and HPP) line 340: Missing blank space, “…effect in the syntrophic interaction…” instead of “…effect inthe syntrophic interaction…”. Page 15 (6. Methods of accelerating and optimizing AD) line 359: Superfluous full stop “…[95].” instead of “…[95]. .”.


Response 27: Missing blank space added in the expression on page 13, line 351. Superfluous full stop also corrected in line 368 of the revised version of the manuscript.


Point 28: Page 15 (6. Methods of accelerating and optimizing AD) line 361: Missing blank space and superfluous “e”, “…could stimulate syntrophic interactions…” instead of “…could stimulateesyntrophic interactions…”.


Response 28: This has been corrected in line 370.


Point 29: Page 15 (6. Methods of accelerating and optimizing AD) line 359: “…that need to be...” instead of “…that needs to be...”.


Response 29: This has been corrected and highlighted in line 373 on page 13.


Point 30: Page 16 (6. Methods of accelerating and optimizing AD) line 385: “…butyric and propionic acid,…” instead of “…butyric and propionic acids,…”.


Response 30: This was noted and corrected in line 394, page 14.


Point 31: Page 16 (6. Methods of accelerating and optimizing AD) line 385: The expression “…must be adept at their physiological interaction.” is not comprehensible and should be reworded.


Response 31: This expression has been reworded as suggested. It is highlighted in line 394 and 395 on page 14 of the manuscript.


Point 32: Page 16 (6. Methods of accelerating and optimizing AD) line 399: The expression “…for microorganisms to thrive.” is not comprehensible and should be reworded. Page 16 (6. Methods of accelerating and optimizing AD) line 409: Superfluous blank space, “economical” instead of “economic al”.


Response 32: this has been reworded in lines 408/409. The superfluous blank space “economical” instead of “economic al” has also been corrected in line 420, page 14.


Point 33: Page 17 (7. Recent advances in the AD technology) line 419: “…materials like biochar…” instead of “…materials such biochar…”.


Response 33: The authors’ choice of word has been replaced with the reviewer’s and the word is highlighted in line 430, page 15 (7. Recent advances in the AD technology).


Point 34: Page 17 (7. Recent advances in the AD technology) line 419: “…, Guatemala, Nicaragua and Peru…” instead of “…, Guatemala Nicaragua and Peru…”.


Response 34: This was noted, corrected and highlighted in line 442 on page 15 of the revised version of the manuscript.


Point 35: Page 18 (9. Discussion) line 484: The expression “…tacitly described…” is inappropriate and should be reworded.


Response 35: The expression has been reworded. The word ‘’tacitly’’ was replaced with ‘’unambiguously’’ in line 495 on page 16 (9. Discussion).


Point 36: Page 18 (9. Discussion) line 489: Missing blank space, “…must include critical evaluation…” instead of “…must include critical evaluation…”.


Response 36: This was noted and corrected in line 500 of page 16 (9. Discussion).


Point 37: Page 18 (10. Conclusions) line 489: “…,there still exist knowledge gaps…” instead of “…,there still exists knowledge gaps…”.


Response 37:  This has been noted and corrected in line 513, page 16 (10. Conclusions).


Point 38: Page 18 (10. Conclusions) line 502: The expression “…from have a thorough understanding...” is not comprehensible and should be reworded.


Response 38: The expression has been reworded for better understanding and it is highlighted on page 16, lines 413/414. 
















Reviewer 3 Report

Significant changes has been made to the manuscript and it is acceptable for publication in present form.

Author Response

Point 1: Significant changes has been made to the manuscript and it is acceptable for publication in present form.


Response 1: We are grateful to the reviewer for acknowledging that our manuscript is acceptable for publication in its present form. The manuscript was extensively revised according to the reviewer’s insightful and invaluable comments and suggestions. It is believed that the comments, corrections and suggestions that ensued from the careful review of our manuscript helped improve it tremendously. A courteous and objective tone that dealt clearly and thoroughly with every detail and issues raised was adopted to ensure our manuscript was re-written and re-organised in a form that would render it acceptable for publication. Every sentence provided in the manuscript is now clear, accurate and precise. Once more, we thank you for reviewing our manuscript and providing constructive comments and feedback.    


Round 3

Reviewer 1 Report

The authors have effectively repositioned both the content and the title of the paper, with a fair approach to the subject. The reviewer is finding the paper at its present form suitable for publication. 

Reviewer 2 Report

From my point of view the authors have answered most of the questions sufficiently and have also taken the comments of the reviewers into consideration in most cases. Overall, this led to an improvement of the article. So in the final analysis, the reviewed article seems to be suitable for publication.


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