Pathways for the Valorization of Animal and Human Waste to Biofuels, Sustainable Materials, and Value-Added Chemicals

Round 1

Reviewer 1 Report

environments-2188938-peer-review-v1

Pathways for the valorization of animal and human waste to biofuels, sustainable materials and value-added chemicals.

Review

General comments

The article presents a review of technologies used for the valorization of animal and human waste. In the opinion of this reviewer, the work done here is interesting and it is clear that some effort has been done to write it.

In my opinion the article should be more clear on what is the focus of the contribution for the paper. In other words, first it should be evaluated what was missing in literature in this regard and then you could add some justification of why this article represent a contribution to the science.

The technologies presented are well known and some of them are even missing. For example hydrothermal carbonization. The point is not to focus on a general description of the processes but focus on the novelty of this article.

For instance, if the literature is missing in data related to the recovery of chemicals, then focus on this aspect for each technology. If the goal is the maximum energy recovery, then focus all the processes described on that particular area.

More detailed comments

1.      Already in the abstract session and in the Introduction, write clearly what is the added value of the present article in regard to the most recent literature. What is the added value of this paper and what is its contribution.

2.      Figure 1 doesn’t need to have the actual animal figures on that drawing. The reader can understand the meaning also by using text only.

3.      Gasification is done in a very large range of conditions. Specify more details on this technology. Different configurations can be found in the book chapter: (2019). Notions of Biomass Gasification. In: Fundamentals of Biofuels Engineering and Technology. Green Energy and Technology. Springer, Cham. https://doi.org/10.1007/978-3-030-11599-9_21

4.      Supercritical water gasification is not easy to operate. Please explain the main mechanisms and also the problems encountered with this technology. You could look at some more details related to the mechanisms of gasification at this conditions. Some reference could be: https://doi.org/10.1016/j.ijheatmasstransfer.2021.122200

5.      In this article there is a description of diverse process conditions and different technologies. For each temperature and pressure range, there are different products and different reactions are taking place. For example, in lines 287-294 there is a figure describing the mechanisms of reaction there and this can be done also for other types of thermochemical processes.

6.      Fig. 6 for example is not very detailed. There are much better descriptions in literature about the gasification reactions.

7.      The HTL description, should be really improved. Describe the differences between HTL, SCWG and HTC and what are the different mechanisms of reaction there.

8.      Hydrothermal Carbonization is not even mentioned in this study. It is important to underline the mechanisms of these 3 hydrothermal technologies and why different conditions are required for different reactions to take place. How in practice, catalysis effects the products for these technologies? Why this catalysts work in that way?

9.      You can find some more information in the following article: https://doi.org/10.1016/j.enconman.2022.116215

10.   The sentence “The journals with the highest number of citations in each cluster include scientific reports, Plos one, applied and environmental microbiology, nutrients, science of the total environment and xenobiotica.” Is not clear. Are scientific reports taken from different sources? Do you mean that scientific reports in diverse journals had the most of citations?

Author Response

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Reviewer 2 Report

Good work but I recommend indicating with the individual quantities (C, S, O, H) whether it is a dry or wet state.

Author Response

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Reviewer 3 Report

This study presents a review of methods for utilizing animal and human waste and summarizes the results of a large number of studies in this field. Additionally this manuscript also analyses bibliographic trends in the literature.

I have seen many reviews on biomass valorization and this has a larger scope than previous reviews I have read on waste valorization. So I think it is interesting.

However, I have a number of comments

1) A number of figures in the manuscript have been copied and pasted from other articles (simply referencing the articles is not enough, copyright permission must be obtained). So I believe this is a plagiarism issue although I believe the source materials have been referenced. If the authors wish to re-use these figures from the literature they must obtain copyright permission from all the relevant publishers. For example figures 4, 6 and 7. If other figures have also been copied in this way then permission must also be obtained for those.

Alternatively, these could be removed.

2) In figure 1 the authors define “animal waste” as consisting of Animal feces, urine and manure. However, in the text in section 2 the author refer to animal waste as simply “manure” for example saying “Animal manure is a combination of waste feeds, feces, urine, and bedding material”. Please correct this to be more consistent with your definitions.

3) Do you have a reference for the information provided in figure 3?

4) It would be good to briefly mention the different types of gasification reactor: fixed bed (downdraft or updraft), fluidized bed etc.

5) References 63 and 64 are identical referring to the same article.

6) why do you recommend on-site recovery of resources in your conclusions? Is this supported by the results of your review? Can Offsite or district waste processing could also be viable?

Author Response

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Reviewer 4 Report

The paper by J. A. Okolie deals with an analysis of contemporary literature relevant to thermochemical and biological pathways of conversion of human and animal wastes into value-added intermediates and products including biofuels. The advantages and weak points of the specific methods are also discussed. This review is valuable and interesting as a guide for searching effective routes to valorization of wastes, which were previously regarded as environment pollutants.

Bibliometric analysis concerning input of worldwide research centers is also presented.

This review may be published in a present form. The referee suggests only that presentation of the possibility of specific reuse/recycling techniques depending on specific country may be interesting for integration of efforts for searching the renewable feedstocks.

Author Response

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Round 2

Reviewer 1 Report

Authors have addressed correctly the changes suggested. The article can be published in my opinion

Author Response

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