Sustainable Production of Medium-Chain Fatty Acids from Fresh Leachates in the District of Abidjan: Study of the Feasibility of the Process and Environmental Benefits

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1. To correlate the difference in leachates with the level of poverty seems to me to be more of a social science correlation. From an environmental engineering point of view, it seems to be more directly related to the composition of waste and rainfall in the area or solid waste transfer stations.
2. This manuscript has a good research idea, but the experimental design lacks QA/QC concepts. There is a lack of information, such as: date of sampling, number of times, sampling method, location (exact location), how to take samples (uniform method), how many samples to take, how to collect the samples for backup, whether to measure the water temperature or quality at the site, how to handle the data, and whether to repeat the experiment.
3. The data in Table 2 appear to have been processed, but should be labeled to indicate how much of the data were processed.
4. 4. 2.5. Setup of bioreactor and operation of chain elongation: Batch reactors were set up in duplicate, each containing 1 L of leachate from the three SWTS, operating under anaerobic condition and uncontrolled pH system as described [8]. Were strains added? Source of strain? HRT? 1 L of leachate per batch for 55 weeks? How often is the leachate changed and is the nature of the leachate the same each time?
5. Figure 2.: Was gas production and composition monitored? Was there hydrogen and methane?
6. P.6: The results revealed a substantial organic load, with COD levels exceeding 50 g/L in all samples [12,14]. LBG exhibited the highest COD concentration (114,5 ± 14,2 gO2/l), nearly double that of LYO (76,7 ± 2,3 gO2/l), while LAD recorded 94,1 ± 5,2 gO2/l. Why compare with references [12,14]? Did 50 g/L have any special significance?
7. Was the unit of COD in Table 2 was mg.l-1 incorrect? The meanings of TOA, TFA, VFA, MCFA should be listed below the table.
8. P.6: However, LBG appears to be the most suitable substrate likely due to its origin and the composition of the waste from which it is derived. The actual the composition of the waste should be presented in literature or research.
9. Figure 1: What was the name of the horizontal coordinate? Was D35 day 35? The experiment did not last 55 weeks, D35 was only the first 5 weeks, what about the data for the remaining 50 weeks? Why did COD increase? Why did the TOA go up and down? The names of the upper and lower graphs were garbled.
10. The difference between Fig. 4 and Fig. 5 comes from different leachate and its contained flora, was it possible to monitor the change of flora during or after the experiment? In order to know the difference of advantageous flora after taming and whether it is good for methanization or not.

Author Response

Dear reviewer, 

Please see the attachment

Best regards

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Publication for African countries is important. Interesting results. 

My remarks: "Figure 2 Bioreactor set up for chain elongation processing". At what temperature were the experiments conducted?

Table 2 Page 6. No ammonia value given it can be important.

Page 8 as well a H2 and CO2 ....Have you determined these substances?

Page 8 Figure 1 ? No Y -axis description

Author Response

Dear reviewer, 

Please see the attachment

Best regards

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Dear Editor,

The manuscript fermentation-3576165 deals with the fermentation of fresh leachates in the context of Coté d’Ivoire for medium chain fatty acids production. The paper is of scientific interest and adheres to the scope of the journal. The reviewer may have some questions or suggestions to improve the paper:

Major comments:

1. Lines 63-66, the effect of adding ethanol on the medium-chain fatty acids production should be further explained in introduction and discussed in a more fundamental level while discussing the results. The quantity of ethanol addition should be justified as well.
2. Section 2.5, the operating temperature of the bioreactor should be reported and its choice should be justified.
3. Same section, the effect of pH on the fermentation process and the fatty acids profile should discussed, which justifies the choice of the starting pH range for bioreactor.
4. The section 3.2 Microbial quality of the fresh leachates is dissociated from the rest of the study and should be correlated with the characteristics of fresh leachates and fermented media.
5. What was the gas production of the process? Was the composition of gas measured?
6. Figure 1 should be absolutely improved; it is a screenshot with no axis titles and units.
7. Lines 266-271, it is quite surprising that the COD increased. Normally we shall have a COD balance in a bioreactor, both batch and continuous. The creation of COD is against this theory. Further explanation should be given to justify the observation. The first COD measurement is the mixture including ethanol ?
8. Figures 4 and 5, the 100% VFA in the acid profile at D7 of LYO (c) should be explained.
9. The reviewer does not understand very well the separation of 3.4 and 3.5 as well as the figures 4 and 5.
10. The interest of producing medium fatty acids is not very clear. What could the acids produced be used for?
11. A more general discussion associating all results obtained and the previous studies would be beneficial to enhance the paper impact.

Author Response

Dear reviewer, 

Please see the attachment

Best regards

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The unit of COD was still wrong, either g/L or mg/L. No further comments on the rest.

Author Response

Manuscript Number: fermentation-3576165

Dear Reviewer,

Thank you for giving us again the opportunity to resubmit a revised draft of our manuscript titled “Sustainable production of medium-chain fatty acids from fresh leachates in the District of Abidjan: study of the feasibility of the process and environmental benefits” to the Journal Fermentation. We appreciate the time and effort that you have dedicated to improve our paper. We are grateful to the reviewers for their comments.

Please see below for our point-by-point responses to the reviewer comment. All the changes in the manuscript are highlighted in red color. The lines named in the responses refer to the track changes document. If any responses are unclear, or you wish additional changes, please do not hesitate to let us know.

Comment : The unit of COD was still wrong, either g/L or mg/L. No further comments on the rest.

Response : Thank you for pointing this out. We corrected the mistake. Table 2 page 7

Reviewer 3 Report

Comments and Suggestions for Authors

Most of the questions raised by the reviewer has been addressed. However, the reviewer has to point out that the response to the point 7 is not satisfactory and against the theory of COD balance. In a batch system, the total organic matter, and thus total COD, should remain constant if: no additional substrate is added, no sampling/removal of reactor contents affects volume significantly, no gases or volatile organics leave the system with significant COD value and no analytical artifacts or contamination occurs.

Fermentation processes redistribute the organic matter (e.g., from complex molecules to volatile fatty acids, no matter the chain length), but do not increase the total amount of COD unless external COD is introduced or measurement errors occur.

The result should be further scientifically explored to explain the possible reason why COD increased.

Author Response

Manuscript Number: fermentation-3576165

Dear Reviewer,

Thank you for giving us again the opportunity to resubmit a revised draft of our manuscript titled “Sustainable production of medium-chain fatty acids from fresh leachates in the District of Abidjan: study of the feasibility of the process and environmental benefits” to the Journal Fermentation. We appreciate the time and effort that you have dedicated to improve our paper. We are grateful to the reviewers for their comments.

Please see below for our point-by-point responses to the reviewer comment. All the changes in the manuscript are highlighted in red color. The lines named in the responses refer to the track changes document. If any responses are unclear, or you wish additional changes, please do not hesitate to let us know.

Comment: Most of the questions raised by the reviewer has been addressed. However, the reviewer has to point out that the response to point 7 is not satisfactory and against the theory of COD balance. In a batch system, the total organic matter, and thus total COD, should remain constant if: no additional substrate is added, no sampling/removal of reactor contents affects volume significantly, no gases or volatile organics leave the system with significant COD value and no analytical artifacts or contamination occurs.

Fermentation processes redistribute the organic matter (e.g., from complex molecules to volatile fatty acids, no matter the chain length), but do not increase the total amount of COD unless external COD is introduced or measurement errors occur.

The result should be furthered scientifically explored to explain the possible reason why COD increased.

Response: Thank you for raising a valid theoretical concern about COD balance. We acknowledge that in a closed batch system; total COD should theoretically remain constant. However, several scientific explanations can account for the observed COD increase:

1. Solubilization of particulate organic matter: Fresh leachates contain significant amounts of particulate and colloidal organic matter that may not be fully measured in the initial COD determination due to incomplete extraction. During fermentation, hydrolytic enzymes produced by bacteria (particularly Bacillus spp., as shown in Table 3) can solubilize previously inaccessible organic compounds, making them detectable in subsequent COD measurements [Batstone et al., 2002].
2. Methodological considerations: The initial COD measurement was performed on filtered samples (0.45 μm), which may have underestimated the total organic content by excluding particulate matter. As fermentation proceeds, particulate organic matter is hydrolyzed into soluble forms that are fully captured in COD analysis.
3. Formation of intermediate metabolites: The chain elongation process involves complex metabolic pathways that can temporarily accumulate intermediate compounds with higher oxygen demand than the original substrates, particularly during the transition phase from acidogenesis to chain elongation (D7-D14 period observed in our study).
4. Biomass lysis and cell turnover: In mixed culture systems, some bacterial populations may lyse during pH stress or substrate competition, releasing intracellular organic compounds that contribute to measured COD [12]. 

We agree that this observation warrants further investigation. Future studies should incorporate detailed mass balance analyses, including separate quantification of soluble and particulate COD, as well as monitoring of intermediate metabolites to better elucidate the transformation pathways of organic matter during chain elongation.