Characterization and Methanogenic Potential Evaluation of Faecal Sludge: Case of the Kossodo Biogas Plant in Ouagadougou

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

1.     Line 462 C/N ratio: Did you try to achieve a higher C/N ratio with composite samples by mixing the sludge or increasing the settling time for each sludge?

2.     Line 498. the reason that was given to explain the high VS in the wet season seems not accurate. If the pit latrine is not waterproof and the rain tends to go inside this latrine, its mean that the water volume increases, and the dilution factor has to reduce the VS in wet seasons. Please improve the explanation

3.     did the temperature play any role between wet and dry seasons?

4.     Line 509 please explain why the TN increases in the wet season.

5.     Line 594 the settling increases the TS and the ratio C/N. please give the TC in table 4. Why this work did not focus on optimising the C/N ratio which is the main factor for valorisation of the faecal sludge?

6.     Also did the treatment plant plan to help empty frequently the septic tank. Based on the literature and on your work the organic matter gets mineralised after 2 years without emptying.

Author Response

Responses to questions, comments, and suggestions from Reviewer 1

We, the authors, extend our sincere gratitude to the editor and the reviewers for dedicating their time to review our manuscript. Your comments, questions, and concerns have been immensely valuable. We have meticulously reviewed and incorporated the suggested changes into this revised version. In response to your queries and feedback, we have provided a detailed point-by-point response below. Additionally, we have included supplementary background information and references to new sources that have been seamlessly integrated into this revision of the manuscript. We have also made additional adjustments that we believe would enhance the clarity and coherence of the manuscript. Comments from the reviewers are highlighted in blue, while the corresponding author's responses are in black. We are confident that this resubmission, along with the incorporated changes, effectively addresses the concerns raised in the initial review.

Reviewer #1 comments:

1. Line 462 C/N ratio: Did you try to achieve a higher C/N ratio with composite samples by mixing the sludge or increasing the settling time for each sludge?

Author’ response: Thank you for your question. The samples used to characterize the faecal sludge were composite samples constituted from faecal sludge taken from the emptying trucks during discharge operations at the treatment plant. The aim was to characterize the sludge without seeking to try to achieve a higher C/N ratio. In this revision, this information was provided in Table 1 and also in the manuscript, L150-153.

2. Line 498. The reason that was given to explain the high VS in the wet season seems not accurate. If the pit latrine is not waterproof and the rain tends to go inside this latrine, its mean that the water volume increases, and the dilution factor has to reduce the VS in wet seasons. Please improve the explanation

 

Author’ response: Thank you for your comment. We agree with your point, as our previous approach in terms of percentage could indeed lead to confusion. We have revised the section to focus on concentration for better understanding (Line 488 to 491).

 

 

3. Did the temperature play any role between wet and dry seasons?

Author’ response: Thank you for your question. Based on our measurements and observations, the temperature did not play any significant role between the two seasons. We provided the temperature values for both seasons in Table 3 in this revision, for clarity. It can be observed that the average values for both seasons are almost similar.

4. Line 509 please explain why the TN increases in the wet season

Author’ response: Thank you for your question. The high total nitrogen (TN) content observed during the wet season can be explained by several factors. First, during the wet season, the sludge is ‘fresher’, as indicated by the values of Volatile Solids (organic content) obtained in this study. Fresh sludge, in this context, refers to sludge that contains a higher proportion of organic matter. When sludge is fresher, it therefore contains more nitrogen than partially mineralized sludge, as evidenced by the high nitrogen levels observed in public toilets, which typically contain fresher sludge. In contrast, sludge in the dry season has more time to mineralize, which reduces the organic nitrogen content.

Second, most plausible explanation is that water infiltration into these pits can introduce an external source of nitrogen. Some soils naturally contain organic nitrogen, and runoff water can transport it into the pits. It is also important to note that the nitrogen content of the pits may depend on household habits, including dietary choices and pit management practices (such as the mixture of feces and urine). These are additional factors that could account for the elevated nitrogen content during the wet season. These explanations are not covered in the manuscript as they go beyond the scope of the paper.

5. Line 594 the settling increases the TS and the ratio C/N. please give the TC in Table 4. Why this work did not focus on optimizing the C/N ratio which is the main factor for valorisation of the faecal sludge?

 

Author’ response: Thank you for the question and the valuable suggestion. We have duly incorporated the total carbon (TC) data into the Table 4 in this revision. We completely agree with your point of view on the approach to our work. We could indeed have put a different emphasis, perhaps by focusing only on the C/N ratio. However, our main objective was to comprehensively evaluate the characteristics of the Kossodo plant faecal sludge and assess their suitability for anaerobic digestion. This is why we haven't tried and we didn’t focus our work on optimizing the C/N ratio.

 

6. Also did the treatment plant plan to help empty frequently the septic tank? Based on the literature and on your work the organic matter gets mineralised after 2 years without emptying.

 

Author’ response: Thank you for your questions. We acknowledge your concern regarding the timing of septic tank emptying. The treatment plant does not plan to help empty frequently the septic tank. It is important to note that this process is often contingent on household needs and financial considerations, with the associated costs being the responsibility of the household. While it is true that over a two-year period, these sludges undergo partial mineralization, our study has revealed that, particularly during the wet season, sludge concentration demonstrates a significant potential for methane production. With this in mind, we may suggest a heightened emphasis on sludge settling during the wet season to enhance the valorization of faecal sludge.

 

Additional response: Similarly, with regard to Figure 7, we have uploaded a new and improved version, in which the data relating to septic tanks were not considered in the first draft following a malfunction in the Tanagra software. We have solved the problem and presented in this revision the appropriate min/max and mean values for the C/N ratio in the text, see L558-569.

All the parts in yellow in the manuscript are the contributions requested and the parts where we found gaps or needed precision for a better understanding.

 

Reviewer 2 Report

Comments and Suggestions for Authors

To Sustainability

Title: Characterization and methanogenic potential evaluation of faecal sludge: Case of the Kossodo biogas plant in Ouagadougou

Ref.: sustainability-2670651

 

Dear Editor，

 

Ouedraogo et al. report a research paper in which they insist that knowledge of the composition of faecal sludge used in anaerobic digestion is essential. In this context, the author examined the Kossodo biogas plant in Ouagadougou, West Africa, where they received sludge from septic tanks and pit latrines. The author studied physicochemical properties (total suspended solids, volatile solids, chemical oxygen demand, biological oxygen demand, total carbon, nitrogen and phosphorus etc. of the sludge (130 trucks sample), heavy metals, microbiological parameters were studied and the biogas yield was evaluated. They found, that settled faecal sludge has higher E. coli and heavy metals compared to raw. I see that the author has managed to characterize and study the behaviour in detail. What they have in common is that the transition is smooth and well worked out. However, the authors need to add a paragraph in the introductory section that illustrates the novelty of the current experiments and provides a clearer objective that the reader can easily pursue. I believe that the work described is sufficient for this journal. Therefore, this article is assumed to be accepted with minor comments.

 

The detailed Minor comments are listed as follows:

 

1. Introduction- Author's claim manuscript in two parts (1) Characterization of raw and settled faecal sludge from the Kossodo plant

(2) Assessment of the methanogenic potential of the sludge. The author should write a paragraph and provide additional information for Part 2. In addition, the author should compare the advantages of characterization compared to other plants.

 

2. Is it possible for the author to divide the methods of the manuscript into two parts: the initial sampling and analysis and the assessment of the methanogenic potential of the sludge in Section 2?

 

3. Table 2 is very informative, but the author did not describe it in detail. It is recommended to present the numbers obtained, as well as their meaning and variations, in more detail.

 

4. The author has added the rainy season and dry season to Table 3. Is it possible to divide them by months or four seasons? – If not, the seasonal word is not appropriate, change it to dry or humid/wet atmospheric condition is more suitable.

 

5. In Table 4 all abbreviations used, i.e. TS and VS, for the convenience of readers, should be given with the full name and in a footnote.

 

6. Subsection you can put as 3.4.1…..3.4.2….. In a stand of just putting a bullet.

 7. Since the draft manuscript is divided into two parts, the conclusion must also be described in two parts to ensure a very consistent structure.

 

 

Author Response

Responses to questions, comments, and suggestions from Reviewer 2

We, the authors, extend our sincere gratitude to the editor and the reviewers for dedicating their time to review our manuscript. Your comments, questions, and concerns have been immensely valuable. We have meticulously reviewed and incorporated the suggested changes into this revised version. In response to your queries and feedback, we have provided a detailed point-by-point response below. Additionally, we have included supplementary background information and references to new sources that have been seamlessly integrated into this revision of the manuscript. We have also made additional adjustments that we believe would enhance the clarity and coherence of the manuscript. Comments from the reviewers are highlighted in blue, while the corresponding author's responses are in black. We are confident that this resubmission, along with the incorporated changes, effectively addresses the concerns raised in the initial review.

 

Reviewer #2 comments:

1. Introduction- Author's claim manuscript in two parts (1) Characterization of raw and settled faecal sludge from the Kossodo plant (2) Assessment of the methanogenic potential of the sludge. The author should write a paragraph and provide additional information for Part 2. In addition, the author should compare the advantages of characterization compared to other plants.

 

Author’ response:   Thank you for your comment. We have included a new paragraph (see L75-80) in the manuscript highlighting the benefits of sludge characterization, which demonstrate the usefulness of this work. Your contribution is much appreciated and contributes significantly to improving the quality of the work.

 

Additionally, the revised document has been reorganized to include a paragraph devoted to the assessment of methanogenic potential (see L81-102). Your input has been instrumental in enhancing the overall quality of the work

 

2. Is it possible for the author to divide the methods of the manuscript into two parts: the initial sampling and analysis and the assessment of the methanogenic potential of the sludge in Section 2?

 

Author’ response: Thank you for this suggestion. This recommendation has been covered in the revised document (see sections 2.1 and 2.2).

 

3. Table 2 is very informative, but the author did not describe it in detail. It is recommended to present the numbers obtained, as well as their meaning and variations, in more detail.

 

We would like to express our gratitude for your pertinent recommendations and suggestions.  We have tried to add some descriptions of some key parameters in the revised manuscript (see L316-342) to give general information about the parameters not covered Which will serve as databases for future studies. In addition, standardized thresholds for discharge rates have been added to Table 2 to provide more details on the urgency of adequate sludge treatment. There is no doubt that Table 2 contains a wealth of crucial information on the characteristics of faecal sludge, each of which could be subjected to in-depth analysis. However, in view of the main objective of our study, which focuses on the use of faecal sludge in an anaerobic digestion context, we have chosen to highlight the parameters that are straight related to anaerobic digestion. This focus enabled us to concentrate our attention on the factors that are crucial to the anaerobic digestion process.  For these parameters, the mean and standard deviations as well as the P values were considered in the discussion. 

 

The author has added the rainy season and dry season to Table 3. Is it possible to divide them by months or four seasons? – If not, the seasonal word is not appropriate, change it to dry or humid/wet atmospheric condition is more suitable.

Author’ response: Thanks for the suggestions. We appreciate the contribution. We would just like to point out that in the West African Sahel context, we only have a monomodal rainfall regime with two contrasting seasons, as mentioned in Section 2.1.1.  A long dry season, which runs from November to May, and a short-wet season, which runs from June to October. In response to your concern, we have corrected rainy by wet it in the revised manuscript as requested.

4. In Table 4 all abbreviations used, i.e. TS and VS, for the convenience of readers, should be given with the full name and in a footnote. 

Author’ response: Thank you for your comment. We have incorporated the suggestions into the revised manuscript. (line 541-542)

 

5. Subsection you can put as 3.4.1…..3.4.2….. In a stand of just putting a bullet.

Author’ response: Thank you for this suggestion. This recommendation has been considered in this revision.

6. Since the draft manuscript is divided into two parts, the conclusion must also be described in two parts to ensure a very consistent structure.

 

Author’s response: Thank you for your insightful comment. We have considered your concern about the separation of the two conclusions. We have improved this in the revised document. Below the second part of the conclusion Thanks you for your input.

Besides the reviewer’s comments, we have provided some more changes to the documents. We deeply proofread the manuscript for grammars and spelling mistakes, for inconsistencies, which were all fixed.

 

Additional response: Similarly, with regard to Figure 7, we have uploaded a new and improved version, in which the data relating to septic tanks were not considered in the first draft following a malfunction in the Tanagra software. We have solved the problem and presented in this revision the appropriate min/max and mean values for the C/N ratio in the text, see L558-569.

All the parts in yellow in the manuscript are the contributions requested and the parts where we found gaps or needed precision for a better understanding.