Current Status and Prospects of Valorizing Organic Waste via Arrested Anaerobic Digestion: Production and Separation of Volatile Fatty Acids

Round 1

Reviewer 1 Report

The manuscript titled covers the recent status of volatile fatty acids production by anaerobic digestion. I would recommend that the manuscript be accepted for publication in Fermentation after minor revision. Here are some detailed comments and suggestions;

Line 32: The percentage of VFA production from fossil fuels needs to be included.

Line 40: Biobased VFA production has adverse effects on the environment, too (so, not without, but less). Please rephrase your statement accordingly.

Line 46: Please support your indication with solid results (g VFA/g substrate).

Line 88: Reference and detailed information for the MixAlco process should be provided.

Line 105 - 106: There are extra spaces in the sentence, please remove them.

Line 149: I assume the authors refer to Figure 1 in this sentence? If so, please correct it.

Line 173: The title "Parameters affecting methanogenesis" should be updated with arrested methanogenesis since the text mostly focuses on VFA production rather than methane production.

Line 177 - 178: pH is used two times in the sentence, please remove one of them.

Line 177: Inoculum source is also a very important parameter for AD. Please include it.

Table 2: Journal template for the references should be used for Murali et al. 2017a.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Very well rounded review paper for the production of VFAs.

Minor review for spell check and grammar needed.

Several Comments.

1) I see Figure 1 is a reprint with permission, however, is there a more shorthand way of denoting amount of Iron (abundance of red circle). Also I am slightly confused on the representation of each metal "circle" in Figure 1. Please add this information in the caption.

2) Table 1 should include the type of VFA produced as done in Table 3.

3) Table 2, last column header has g/L but items listed in parenthesis are in g/g, slightly confusing, maybe another column is needed for the (g/g

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

This paper is a very interesting review on valorisation of organic waste by Arrested Anaerobic Digestion. The review is well written, well structured, and easy to follow. I think the review is very relevant and certainly will be useful for many researchers, including myself. I think it could be further polished to improve its quality. I’d like to recommend publication, after minor revisions. Hope my comments and suggestions, given below, will be useful.

Lines 54 – 56: The sentence mentions that soil-amendment products have a low value. This depends on location. E.g., in the EU, due to recent increases in prices of gas, all sources of nutrients became valuable. I suggest either removing the remark about the price or stating the geographic location, where such statement is relevant (maybe it’s like that in the US?).

Lines 61 – 63: “Waste like garden waste as well as lignocellulosic biomass materials often need a pretreatment step to break the barrier made of bonds between lignin and carbohydrateprevents bioconversion of the materials [15].” – The reference cited (Malik et al. 2020) not only mentions breaking the structure and delignification in order to make carbohydrates more accessible to enzymes. It is clear from the results in this paper that not only lignin, but also cellulose and hemicellulose are broken down into simple sugars. Please refer e.g., to figure 3 in the cited paper and notice that in comparison to the blank the contents of lignin, cellulose, and hemicellulose are decreasing with increased ozonation time and ozone dose, and that decrease is connected with increased content of total reducing sugar. I suggest rephrasing Your sentence, to obtain more comprehensive picture of the need for pre-treatment.

Line 68 – 69: “To become directly useful in the current energy supply chain, AD also requires additional removal of CO2 to convert biogas to RNG (Renewable Natural Gas).” – I suggest removing this sentence as strictly speaking it’s not true. There are more than 17 000 biogas plants in the EU, which generate energy (electricity and heat) without conversion of biogas to biomethane.

Lines 115 – 124: It’s a very nice summary. For convenience of the readers I suggest making 3 bullet points to name each pathway separately.

Lines 144 – 146: Among inhibitors of methanogenesis, some lack references (ammonia, sulfides, heavy metals). “Organic compounds” seems to be too broad of a category to be used here. Also, in the work of Koniuszewska et al. [ref 42 in the manuscript] four different antibiotics are studied. I think it’d be beneficial for the readers to include all 4 examples, i.e., amoxicillin, oxytetracycline, sulfamethoxazole, and metronidazole as examples of antibiotics, also showing that many different antibiotics have been investigated.

Lines 194 – 196: “Studies have shown that pH 4 together with a retention time (HRT) of 5 days resulted in production of high concentration of VFA while at pH 10 the process needed long HRTs to produce high VFA concentration.” - please provide reference.

Line 366: In table 3 adsorption and membrane separation are lumped together. I think it’d be better to have two separate tables for each. This would provide space for some additional information that could be useful. E, g. for membranes I suggest providing additional information, such as cut-off size and membrane material. Also, for activated carbon it’d be useful to know the feedstock and activation procedure.

Author Response

Please see the attachment

Author Response File: Author Response.pdf