Low-Temperature Pretreatment of Biomass for Enhancing Biogas Production: A Review

Round 1

Reviewer 1 Report (Previous Reviewer 2)

Authors have improved the manuscript, as requested. New references were added and the manuscript is better organized. Now I recommend its publication.

Author Response

Thank you very much, please see the document for our response!

Author Response File: Author Response.pdf

Reviewer 2 Report (New Reviewer)

Comments to the manuscript fermentation-1969135: Low-temperature pretreatment of biomass for enhancing biogas production: a review

The present manuscript deals with the low-temperature pretreatment of different types of waste to improve biogas yields during anaerobic digestion. The manuscript impressively reviews different kinds of waste and – even more relevant the energy balances of this pretreatments. It is really good work and only some minor aspects are of relevance. The main weakness of the manuscript is, that it strongly focus on thermal treatment and ignores other techniques also available. Even though some references are addressed later, the presence of these other techniques should be addressed at the beginning of the paper. The content fits to Journal. Lingual presentation is fine. Hence, the manuscript should be accepted after some minor changes listed in detail as followed:

·         Line 39-40: It should be pointed out under which conditions reduction of GHG is possible, because anaerobic digestion strongly enforces production of CO₂, CH₄ and N₂O as GHGs. In case of aerobic digestion only CO₂ is produced, which is of lower relevance due to the differences in GHG factors.

·         Line 44: It should be pointed out that there is not only a high investment in the biogas techniques itself (large digestion tank…), but also in waste air treatment techniques both to avoid emissions of these compounds and to pre-treat the biogas itself to get higher quality before use. An example of a reference dealing with these problems is:

o   https://doi.org/10.1016/j.jclepro.2019.04.258 (including Supplementaries)

·         Line 49: Explanation why thermal treatment is preferred is quite confusing as the exergy is the part of the energy which can be used or recovered by heat exchangers. If its is lower than for other techniques energy losses are higher.

·         Line 49: Furthermore, it is not satisfactory to compact all potential pre-treatment techniques to the groups mechanical and chemical processes without further descriptions. Even though scope of this paper are thermal processes 1-2 additional sentences should be added dealing with these other techniques. The following references might be helpful:

o   General aspects of biogas optimization:

§  https://doi.org/10.1016/j.bej.2008.06.020

§  https://doi.org/10.1016/j.ibiod.2015.03.025

o   Biogas out of algae:

§  https://doi.org/10.1016/j.rser.2015.12.303

o   Alternatives to thermal processes:

§  https://doi.org/10.1016/j.biortech.2011.05.047 (ultrasound)

§  https://doi.org/10.1016/j.fuel.2019.05.057 (AOP)

§  https://doi.org/10.1016/j.biombioe.2019.03.018 (alcaline cracking)

·         Line 55: There are false friends as “weaking viscosity”. Please change.

·         Line 94-96: This sentence is hard to understand.

·         Line 96: …of many types of biomass….

·         Line 114: ….also proved that pretreatment at….

·         Table 1: Abbreviation ODS should be explained (most probably organic dry sludge)

·         Table 1: In case of yields for kitchen waste of reference no. 47 the amount of material is not presented, biogas yield ist mL per kg of waste?

·         Line 456: Analysis with FTIR is not clear in this contect. How was the sample pre-treated to get a good test result? How can FTIR help to differentiate between crystalline and amorphous cellulose?

Author Response

Thank you very much, please see the document for our response!

Author Response File: Author Response.pdf

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

1. There are many vague sentences in the introduction and other parts of the paper that need significant improvement. For instance, the first sentence of the introduction doesn’t make sense to me. Also, please check page 2, lines 50-53, and line 61.

2. Page 2, line 62, the application of HTPT is not just for the dissolution of lignocellulosic components. It can be used for other components as well.

3. Many papers demonstrate that LTPT is considered below 100 ˚C. In this review, the authors consider below 140 and 100 ˚C as LTPT, which doesn’t make sense. In many papers on sludge solubilization, 140 ˚C is considered an HTPT.

4. Page 2, line 70, please specify the feedstock type.

5. Page 2, line 93, needs to be re-written.

6. Page 3, line 108, ammonia can have a buffering capacity in the AD system, and since it’s a weak base, it can’t reduce the pH.

7. The authors need to provide a table for each type of feedstock they are discussing and compare them. It is better to provide experimental conditions such as temperature, exposure time, AD operating condition (mesophilic/thermophilic), and operating mode (batch/continuous) in one table and then compare them based on the obtained results in each study. Maybe we can’t compare some studies with each other. I feel the provided discussion is superficial, and it seems the authors are just providing results from different papers.

8. There are tons of inconsistencies in the paper, which makes the reader confused. For instance, in line 146, the authors provide some results for 150 ˚C, which is already classified as HTPT, whereas the topic of discussion is LTPT.

9. The authors are using many old references, whereas there has been a lot of new research in recent years on the same topic. For instance, in part 3.2, the reviewed references (61-65) are from 1997-2016. So, papers published in recent years are not reviewed and provided.

10. Chemical-assisted pretreatment is totally different than LTPT, and section 3.3 should be removed from the paper.

Reviewer 2 Report

The manuscript brings interesting information about pretreatment of raw material for biogas production using low temperatures. However, a strong English revision is necessary. Abstract is confusing and needs improvement. There are a lot of abbreviations in the text, making necessary the addition of an abbreviation list in the manuscript. But above all, the article is weak as it is, a lot of information is presented in a confusing and catalog-like way, without figures, tables or critical discussion. A good review article should present the information in a clear and attractive way. In addition, the treatment of the data collected or a topic of challenges and perspectives is important to direct the reader about the future of the topic. Due to all the points raised, the article cannot be considered for publication.