Possibilities for Anaerobic Digestion of Slaughter Waste and Flotates for Biomethane Production

Round 1

Reviewer 1 Report

The following report is based on my review of the manuscript entitled “Possibilities for anaerobic digestion of slaughter waste and flotates for biomethane production”. The manuscript water-2358223 is within the scope of “Water” and is also interesting. However, the following major comments have been pointed out and need to be addressed properly for further improvement of the manuscript. They are as follows:

1.     The present work is appreciable. The authors should include brief novelty statement in the abstract.

2.     The introduction is written very briefly. The authors should elaborate the previous works. The authors may refer the following studies to improve impact and visibility of the study.

·       https://doi.org/10.1016/j.matpr.2022.06.493

3.     The problem statement should be clearly stated.

4.     The novelty of the work is also not clearly mentioned. Authors need to state the novelty of the study.

5.     The authors should improve the discussion of the results to support the study.

6.     The authors should include relative comparison data of the treatment efficiency of studied materials in the conclusion of the study.

7.     Authors should avoid citations of non-index articles. Kindly, update the reference list with valid indexed citations. Authors should cite only recent 5 years published articles.

The manuscript is well written. However, minor English editing will improve the manuscript.

Author Response

Dear reviewer, 

thank you very much for your very helpful comments. We have tried to incorporate them into the revised manuscript to the best of our ability. The following are our responses to the individual comments.

1.     The present work is appreciable. The authors should include brief novelty statement in the abstract.

Thanks for the comment, a novelty statement has been added to the abstract.

2.     The introduction is written very briefly. The authors should elaborate the previous works. The authors may refer the following studies to improve impact and visibility of the study.

·       https://doi.org/10.1016/j.matpr.2022.06.493

 The introduction was complemented with a novelty statement and reference was made to recent publications in the field.

3.     The problem statement should be clearly stated.

Thank you for highlighting this, a problem statement has been added.

4.     The novelty of the work is also not clearly mentioned. Authors need to state the novelty of the study.

Thanks for the suggestion a novelty statement has been added to the abstract and the introduction.

5.     The authors should improve the discussion of the results to support the study.

Thank you for pointing out that the discussion of the results has been revised.

6.     The authors should include relative comparison data of the treatment efficiency of studied materials in the conclusion of the study.

Thank you for your suggestion, we will make sure to include relative comparison data of the treatment efficiency of the studied materials in the conclusion of our study.

7.     Authors should avoid citations of non-index articles. Kindly, update the reference list with valid indexed citations. Authors should cite only recent 5 years published articles.

Thank you for this advice, the collection of literature sources has been revised.

Comments on the Quality of English Language

The manuscript is well written. However, minor English editing will improve the manuscript.

Thank you for this assessment, the manuscript was revised by a colleague who is a native English speaker.

Reviewer 2 Report

This paper, entitled Possibilities for anaerobic digestion of slaughter waste and flotates for biomethane production, is a scholarly work and can increase knowledge on this domain. The authors provide an interestig and original study, the content is relevant to Water.

I have some general and specific comments:

- The manuscript is quite well written and well related to existing literature.

- The abstract and keywords are meaningful.

- The Introduction section is quite short and could be improved, especially with the description of the context more in depth and the presentation of the objectives. The study should be more introduced in details and better justified according to the context and the state-of-art.

- Please start the Materials and Methods with the substrate and inoculum used for this study, then after setup continuous experiment and the the BMP measurements and finally the analytical methods. This section should be rearranged from my point of view.

- Please provide more details about the substrate and the inoculum used in this study.

- About BMP measurements, the authors give the ratio of total TS inoculum to substrate, it could be interesting to provide this ratio expressed with VS.

- About Setup continuous experiment, biocarriers were added. What are these biocarriers? Wha is the origin? Please provide also operational parameters of the pre-acidification step.

- Please provide accuracy or standard deviation of data in Table 1. Some parameters were not determined or the results are under the limit of detection? If not determined, I propose to write ND instead of (-) in the Table. Why providing Table 1 and Figure 2 for the characterization of substrate? One of these should be sufficient.

- Is there any measurement of total VFA and alkalinity of substrate and inoculum, or on the mix during experiments? Is there any pH measurement also carried out?

- Please provide error bars in Figure 2. Same comment for following figures (Fig. 4, Fig. 5, Fig. 7, Fig. 9, Fig. 10).

- Please provide curves of methane production in Figure 2 expressed in NmL Ch4 per g of VS instead of TS.

- What about biodegradability of substrate? Authors carried out some COD measurements, then theroretical production of methane could be calculated and correlated to COD (1 g of COD provide 350 mL of methane).

- About the OLR, please discuss about the choice of such strategy. Why not choosing an increase of OLR step by step as usual? Why not waiting 3 HRT for stabilization as usual in AD reactor for OLR experiments and methane production?

- Please discuss about pH evolution (Fig. 10) during the experiments. Is there any link to total VFA or specific VFA?

As it, this paper is not fully acceptable for publication and requires some amendments and additional information. I recommend the following decision: RECONSIDER AFTER MAJOR REVISION.

Author Response

Dear reviewer,

Thank you for your helpful comments and suggestions, and for helping us to improve the quality of the article even further.

Here are our responses to your comments:

- The manuscript is quite well written and well related to existing literature.

We appreciate your positive feedback on the manuscript's writing quality and its relation to existing literature.

- The abstract and keywords are meaningful.

We are glad to hear that you found the abstract and keywords meaningful.

- The Introduction section is quite short and could be improved, especially with the description of the context more in depth and the presentation of the objectives. The study should be more introduced in details and better justified according to the context and the state-of-art.

Thank you for your suggestion to improve the Introduction section. We have added a novelty statement and referenced recent publications in the field to provide a more in-depth context and better justification for the study.

- Please start the Materials and Methods with the substrate and inoculum used for this study, then after setup continuous experiment and the the BMP measurements and finally the analytical methods. This section should be rearranged from my point of view.

We appreciate your recommendation to reorganize the Materials and Methods section by starting with the substrate and inoculum, followed by the continuous experiment setup and BMP measurements, and ending with the analytical methods. We have made the necessary adjustments accordingly.

- Please provide more details about the substrate and the inoculum used in this study.

Thank you for your comment on the substrate and inoculum used in this study. We have included additional information about them.

- About BMP measurements, the authors give the ratio of total TS inoculum to substrate, it could be interesting to provide this ratio expressed with VS.

We have added the ratio of total VS inoculum to substrate, as you suggested.

- About Setup continuous experiment, biocarriers were added. What are these biocarriers? Wha is the origin? Please provide also operational parameters of the pre-acidification step.

We have provided more details about the biocarriers used in the continuous experiment setup and the operational parameters of the pre-acidification step, as per your request.

- Please provide accuracy or standard deviation of data in Table 1. Some parameters were not determined or the results are under the limit of detection? If not determined, I propose to write ND instead of (-) in the Table. Why providing Table 1 and Figure 2 for the characterization of substrate? One of these should be sufficient.

Thank you for your suggestion to provide accuracy or standard deviation of data in Table 3. We have included them where appropriate and replaced (-) with ND for parameters not determined. We have decided to keep both Table 3 and Figure 2, as they serve different purposes: Table 3 provides a broader overview of measured parameters, while Figure 2 highlights the key parameters' large variance.

- Is there any measurement of total VFA and alkalinity of substrate and inoculum, or on the mix during experiments? Is there any pH measurement also carried out?

The parameters VFA and were unfortunately not included. The work was limited to the parameters FOG and organic acids, as our measurement technology could not allow more at that time. However, we will definitely include these parameters in the next experiment. pH values of the substrates were added in Table 3. 

- Please provide error bars in Figure 2. Same comment for following figures (Fig. 4, Fig. 5, Fig. 7, Fig. 9, Fig. 10).

Thank you for your suggestion to provide error bars in the figures. We have added error bars for the point plots in Figures 4, 5, 6, 7, 9, and 10, where appropriate. We understand that the boxplots in Figure 2 already contain error bars.

- Please provide curves of methane production in Figure 2 expressed in NmL Ch4 per g of VS instead of TS.

we assume that you are referring to "Figure 3. Trend of the methane yield of the biochemical methane potential measurement". We have expressed the methane production curves in Figure 3 in NmL Ch4 per g of VS, as you recommended.

- What about biodegradability of substrate? Authors carried out some COD measurements, then theroretical production of methane could be calculated and correlated to COD (1 g of COD provide 350 mL of methane).

This is an interesting point, BOD measurements were also carried out, which demonstrate a good biodegradability, which is also illustrated by the high TS degradation rate. Since this is an anaerobic process and the COD is not a balance parameter, the focus here was placed on the TS/TC and the TS/TC degradation rate.

- About the OLR, please discuss about the choice of such strategy. Why not choosing an increase of OLR step by step as usual? Why not waiting 3 HRT for stabilization as usual in AD reactor for OLR experiments and methane production?

Thank you for your comment on our experimental design. We agree that a stepwise increase in OLR is a common approach. This was also applied in the first experiment. Thereby, the maximum stable ORL was also determined. In our specific experimental design, we subsequently chose to start with a high OLR after an unrecorded adaptation phase to investigate the maximum capacity of the reactor. We also found that due to time constraints and substrate availability, it was not possible to wait for 3 HRT for stabilisation. While we acknowledge that waiting for 3 HRT for stabilization is a common approach, we found that a shorter stabilization period was sufficient to achieve steady-state conditions in our experiment.

- Please discuss about pH evolution (Fig. 10) during the experiments. Is there any link to total VFA or specific VFA?

That's an interesting hint, we did a corelation test of the pH and the concentration of the organic acids with the following result:

t = 0.52846, df = 29, p-value = 0.6012
alternative hypothesis: true correlation is not equal to 0
95 percent confidence interval:
 -0.2658777  0.4368847
sample estimates:
      cor 
0.0976634 

Thus, the correlation is less than 10 %, which rules out a connection.

Best regards,

Round 2

Reviewer 1 Report

The authors have revised the manuscript accordingly. The manuscript can be accepted in its present form.

Reviewer 2 Report

The authors provide a revised version of their manuscript taking into account all the comments and requests of amendments listed in the previous review. The authors provide detailed and justified answers to all the comments and requests of amendments, I agree with all these answers . As it, the manuscript is now fully acceptable for publication in Water and I recommend the following decision: ACCEPT IN PRESENT FORM.