Volatile Fatty Acid Production vs. Methane and Hydrogen in Anaerobic Digestion
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe title of the manuscript and the abstract suggest that the authors want to compare the efficiency and cost-effectiveness of the anaerobic digestion process when volatile fatty acids are treated as a final product compared to methane and hydrogen. This is an interesting approach. It is a pity that the authors practically omit this topic from the content of the manuscript. Detailed comments, the introduction is very general and the basic content is presented at a high level of generality. For example, when discussing the topic of OLR or HRT, which is crucial for VFA production, no numerical data are mentioned. The subsection titled: Methane vs. VFA formation, which is crucial for the whole manuscript, should be expanded and presented in more detail. What is the outcome of the proposed case study? Why is this reactor type presented? What is its uniqueness? There is a lack of comparisons and contrasts of VFA production versus methane production. This was only hinted at in the introduction. I suggest focusing on a specific topic and discussing it in more detail, omitting general data. For example, pretreatment methods are discussed in very general terms. If they are not the topic of the manuscript, they can be omitted. It is worth expanding the chapter on VFA removal and acquisition, as this is an interesting and relatively new topic.
Author Response
- For example, when discussing the topic of OLR or HRT, which is crucial for VFA production, no numerical data are mentioned.
Answer. We agree with the critics. The para considering OLR and HRT are extended in the revised version and more detailed discussion including numerical data is presented. However, the balance of the different texts in the article does not admit extremely detailed discussions anywhere.
- The subsection titled: Methane vs. VFA formation, which is crucial for the whole manuscript, should be expanded and presented in more detail.
Answer: In the revised version this section is extended with more detailed comments and comparison of the annual production of VFAs and their prices compared to the biogas prices and other applications. It is mentioned that the choice of the target product (VFAs or biogas) depends on different factors, described in the revised section.
- What is the outcome of the proposed case study? Why is this reactor type presented? What is its uniqueness?
Answer: In the revised version of section 2.3 introductory text is added to explain the aim of the case study with the baffled multi-step bioreactor. Its purpose is to separate the consecutive processes of hydrolysis, acidogenesis and methanogenesis in separate compartments and space. Thus we are able to avoid the negative effect of VFAs accumulation on methanogenesis and to extract VFAs from different compartments when necessary and suitable. Glycerol is typical substrate with very rapid VFs production with strong negative effect on methanogenesis.
- There is a lack of comparisons and contrasts of VFA production versus methane production. This was only hinted at in the introduction.
Answer: we tried to do it, cf. the answer of question 2.
- I suggest focusing on a specific topic and discussing it in more detail, omitting general data. For example, pretreatment methods are discussed in very general terms. If they are not the topic of the manuscript, they can be omitted.
Answer: we extended the section on substrate pre-treatment with discussion on the different methods and their effect on further digestion.
- It is worth expanding the chapter on VFA removal and acquisition, as this is an interesting and relatively new topic.
Answer: some information on membrane methods for product separation at fermentation is added with short discussion.
Author Response File: 
Author Response.docx
Reviewer 2 Report
Comments and Suggestions for AuthorsIntroduction
Dear Authors, thank you for this instructive review which elaborate and report the state of the art in VFAs production within an anaerobic digestion process.
Please find my comment below which are mainly oriented at bringing some more today’s innovation and consideration to further emphasize the compatibility of biogas production with the biorefinery concept that you strongly highlight at line 14-15 in your abstract.
Major comments:
[22]: reformulate the sentence “that is why their targeted production with desired production rate may shift aims of the anaerobic digestion”. If I understood correctly, your aim is to say that if we focus biogas production towards VFA production, the “gaseous” product might not be the primary target and therefore also its composition might change redefining the role of anaerobic digestion in the overall picture?
[24]: The sentence on combined VFAs production with biogas release can make the overall process self consistent with energy production sufficient to maintain the target processes. Could your reformulate the sentence trying to clarify what you mean with “energy production sufficient to maintain the target processes”? You mean from a bioethermodynamic or boundary limit point of view?
Figure 1: You defined the compounds considered for VFAs in the abstract. However, biogas production can also lead elongated fatty acids such as octanoic acid. An example of such company is Chaincraft in the Netherlands which is working in the frame of horizon framework GoodByO European project at optimizing chain elongation. Hence, the state of the art would consider adapting the overall figure 2 to consider also what others are doing in the field and that biogas, since you also focus on the paper about that specific topic, is not only about methane production.
In fact, in the actual frame biogas production can also be nicely integrated with low content CH4 by upgrading it with innovative means such as ex-situ biomethanation performed at high volumetric rates (ca. 1000 times higher than methane produce in biogas plants) and conversion to enable a direct upgrade to natural gas with the addition of hydrogen (or better using native hydrogen from VFA production).
Hence, as of today, this figure 1 could benefit from an update and is not reflecting enough the potential of biogas and its integration within the biorefinery concepts that you highlighted in the abstract.
Even though it is well understandable in the “technical jargon” what VFAs are, your review would benefit from a precise introduction and reference to where the molecular weight cut is placed between VFAs and non volatile ones e.g. obtained from further chain elongation performed by bacterial consortium for products with more added value e.g. octanoic acid. Especially if then you discuss product removal. Also, since volatile, maybe you can refere to “failed” project in which e.g. pervaporation was applied to remove them from offgas but e.g. was not enough to become economic and therefore “only liquid based extraction”§ are mainly considered? So i.e. since you say, first, at line 375, liquid-liquid then, at line 390, io-exchange one could refere to “third, why is extraction from the gas sincbe those are “VFAs” was not successful or not economic e.g. by pervaporation.
At conclusion point 2 and in view of the above highlighted aspects, does one need to see “negatively” that methane (at very low production rate) is not the favored product in the gas? Wouldn´t it actually be equally great to twist biogas production to a production of biogenic CO2 + H2 and VFAs or even elongated fatty acids chain e.g. octanoic acid? While the paper provide a very good overview of the know state of the art, the review would greatly benefit of highlights in the innovation to further sustain your abstract claim that biogas, twisted differently, (not only towards methane production) can provide new opportunities in a biorefinery frame.
Comments on the Quality of English Language
Minor comments:
14: The fermentation methods are in common à Exchange with are compatible with the biorefinery concept
19: of integrated processes for product removal and not of product.
42: you could provide a reference for the claim on biogas methane content
118: with demand of 105 kilotons by 2020 (this was reched and is in the past) adapt sentence.
133: Continuous process can lead to reasonable conversion rates (25-50% w/w) at food waste as substrate. You mean with food waste as substrate?
[345-349]: From these studies, it can be concluded that bioprocesses that 345 are favorable for the formation of the VFAs at high yields usually produce hydrogen as a 346 by-product, and vice versa. Indeed, it has been observed that the presence of VFAs 347 interferes with hydrogen production, either as a result of inhibition by undissociated acid 348 molecules [72, 73] or due to consumption of hydrogen by homoacetogens [74]. How shall “interferes with hydrogen production” be understood? Is this sentence the “anti thesis” of the previous one?
Author Response
Major comments:
- [22]: reformulate the sentence “that is why their targeted production with desired production rate may shift aims of the anaerobic digestion”. If I understood correctly, your aim is to say that if we focus biogas production towards VFA production, the “gaseous” product might not be the primary target and therefore also its composition might change redefining the role of anaerobic digestion in the overall picture?
Answer: Yes. The main aim of the anaerobic digestion might be focused on biogas or VFAs production, depending on economic reasons. The commented sentence in the abstract is re-written, as follows.
“That is why their targeted production with desired production rate may shift the aims of the anaerobic digestion toward volatile fatty acids instead of biogas release”.
- [24]: The sentence on combined VFAs production with biogas release can make the overall process self consistent with energy production sufficient to maintain the target processes. Could your reformulate the sentence trying to clarify what you mean with “energy production sufficient to maintain the target processes”? You mean from a bioethermodynamic or boundary limit point of view?
Answer: Yes. Now it is:
On the other hand, VFAs production combined with biogas release can make the overall process self-consistent with energy production sufficient to maintain the target processes using biogas for heating the digestor.
- Figure 1: You defined the compounds considered for VFAs in the abstract. However, biogas production can also lead elongated fatty acids such as octanoic acid. An example of such company is Chaincraft in the Netherlands which is working in the frame of horizon framework GoodByO European project at optimizing chain elongation. Hence, the state of the art would consider adapting the overall figure 2(1?) to consider also what others are doing in the field and that biogas, since you also focus on the paper about that specific topic, is not only about methane production.
Hence, as of today, this figure 1 could benefit from an update and is not reflecting enough the potential of biogas and its integration within the biorefinery concepts that you highlighted in the abstract.
Answer: Figure 1 is re-drawn including VFAs and medium chain FA extraction and recovery. The option for production of fatty acid with longer chain is mentioned elsewhere with references in the revised version, lines 91-93; 141-143;148-153.
- In fact, in the actual frame biogas production can also be nicely integrated with low content CH4 by upgrading it with innovative means such as ex-situ biomethanation performed at high volumetric rates (ca. 1000 times higher than methane produce in biogas plants) and conversion to enable a direct upgrade to natural gas with the addition of hydrogen (or better using native hydrogen from VFA production).
Answer: The possibility of the upgrading of low- grade biogas by methane or hydrogen is interesting, but there is no time to discuss it properly in this paper. It is mentioned briefly, lines 314-319.
Hydrogen production by fermentation is discussed in section 2.4. There hydrogen production is competitive to VFAs and methane production.
- Even though it is well understandable in the “technical jargon” what VFAs are, your review would benefit from a precise introduction and reference to where the molecular weight cut is placed between VFAs and non volatile ones e.g. obtained from further chain elongation performed by bacterial consortium for products with more added value e.g. octanoic acid.
Answer: it is answered to comments 3.
- Especially if then you discuss product removal. Also, since volatile, maybe you can refere to “failed” project in which e.g. pervaporation was applied to remove them from offgas but e.g. was not enough to become economic and therefore “only liquid based extraction”§ are mainly considered? So i.e. since you say, first, at line 375, liquid-liquid then, at line 390, io-exchange one could refere to “third, why is extraction from the gas sincbe those are “VFAs” was not successful or not economic e.g. by pervaporation.
Answer: the product (i.e. VFAs) removal from fermentation broth is extended in section 2.5 including membrane methods, inclusive pervaporation with its constraints.
- At conclusion point 2 and in view of the above highlighted aspects, does one need to see “negatively” that methane (at very low production rate) is not the favored product in the gas? Wouldn´t it actually be equally great to twist biogas production to a production of biogenic CO2 + H2 and VFAs or even elongated fatty acids chain e.g. octanoic acid? While the paper provide a very good overview of the know state of the art, the review would greatly benefit of highlights in the innovation to further sustain your abstract claim that biogas, twisted differently, (not only towards methane production) can provide new opportunities in a biorefinery frame.
Answer: Conclusion 3 is modified as follows.
Combined VFAs production with biogas release can make integrated and self-consistent process with biogas production sufficient to supply with energy the target processes. The biogas can be utilized also as a feedstock for value-added chemicals or fuels (light hydrocarbons) and hydrogen.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsAccept in present form
Author Response
Dear reviewer, thank you for the constructive remark. Please find attached the second revised version, including two more review paper with their contribution to the subject, noted in the introduction section. The novelty of the preset review are included at the end of the introduction. The newly added texts and papers are marked in red.
Sincerely yours, Venko Beschkov, corresponding author.
Author Response File: Author Response.docx
