Infiltrated Pits: Using Regional Groundwater Data to Estimate Methane Emissions from Pit Latrines
Round 1
Reviewer 1 Report (Previous Reviewer 2)
The reviewed manuscript has been greatly improved. However, there are still some points that should be considered:
- Line 63: "MFC" should be "MCF"
- Line 86: It is claimed that proposed model can be used to predict methane emissions from pit latrines with greater certainty. The manuscript entertained a bit of a qualitative discussion on this, but provided limited results to back this.
- Line 200: Do you mean "wastewater"
- Line 263: Indicate the reference year not just the month and check the rest of the text.
- Line 277: Addressing GHG emissions is an important issue, but in practice one could be skeptical if this alone should motivate emptying practices. Emptying could have other benefits/constraints that are worth including in the discussion.
- Line 280: sentence finishes off abruptly, seems like something is missing.
- Line 325: This passage may need to be reworked. The underlying premise used in this study is that inundated pits have higher emissions as there is presumably greater anaerobic degradation. Septic tank design varies... true, but it would not be a far fetched thought to assume that their design involves a containment chamber which will be submerged with flush water (with possible groudwater infiltration). That is, it is always submerged. This passage in the Discussion seems to indicate that septic tanks would behave differently regarding emissions depending on whether they are inundated or not. Not clear what the role of inundation is for septic tanks.
- Line 363: "regard" not "regards"
- Line 404: perhaps the second peak (Sept/Oct) is a bit clearer on the GRACE graphic, but not the first peak. Why not plot a line (using a different colour) indicating some form of central tendency measurement of the 2002-2022 period? Maybe even a correlation curve to quantify this mirroring using an R2 (although I acknowledge the GRACE graph is not groundwater depth)?
- Line 456: It seems more than a conceptual model. Is this the right descriptor?
Author Response
Thank you for taking the time to review our manuscript - please see the attached document for responses.
Author Response File: 
Author Response.docx
Reviewer 2 Report (New Reviewer)
The topic is important and relevant and the paper is well written. However, the proposed model is more at a level of a proposed idea or concept than at the level of a useful model or a journal article. In addition the model is neither built on actual data nor validated.
Since the premise of this paper is that septic tanks and pit latrines have different emissions of CH4 due to moisture content and interface with soils, the authors need to define exactly what they mean by septic tanks and pit latrines. There is a lot of literature describing that the terms are used very loosely, and differently by geographic location. For this study, it would be more useful to provide information on the actual construction (e.g. lined, unlined, fully lined, overflow, no overflow). This is particularly true for Dakar, Senegal, where the majority of “septic tanks” are in fact more analogous to cess pits with permeable linings.
In line 67 the authors state that it is time consuming and expensive to measure gas flux from pit latrines. Agreed, but this work does need to be done before reasonably accurate models can be built, and/or models based on best estimates need to be validated.
Assumptions need to be field validated. For example people claim that pit latrines require more emptying in rainy seasons, possibly due to infiltration from higher groundwater or possibly due to surface drainage, but studies evaluating whether this is true have not found statistical evidence to support it. Assumptions are purely hypothetical and not supported by science.
Rural and urban pit latrines have much different usage patterns, and total solids and moisture content. Since the authors postulate that moisture content is a key driver of CH4 generation, this fact cannot be ignored.
The authors state the value of the model is to be able to evaluate monthly variations in CH4, but then only apply it for one month.
Line 258, the authors propose that models like this could be used for recommendations on emptying practices, but do not consider what happens once the fecal sludge is emptied. Before recommendations like this could be made, a comparison of GHG emissions from emptying, transport, and treatment would need to be considered.
Line 271 incomplete sentence.
The model does not consider the amount of fecal sludge in pit latrines, or the level of stabilization of the sludge. I assume this will make a huge contribution to amount of GHG production.
Author Response
Thank you for taking the time to review our manuscript - please see the attached document for responses.
Author Response File: Author Response.docx
Reviewer 3 Report (New Reviewer)
Good day! See file
Comments for author File: 
Comments.pdf
Author Response
Thank you for taking the time to review our manuscript - please see the attached document for responses.
Author Response File: Author Response.docx
Round 2
Reviewer 1 Report (Previous Reviewer 2)
The manuscript has improved and minor corrections of last round have been addressed.
Author Response
Thank you for taking the time to give feedback.
Reviewer 2 Report (New Reviewer)
NA
Author Response
Thank you for taking the time to give feedback.
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
The manuscript " Infiltrated pits: using satellite and historical groundwater data 2 to estimate methane emissions from pit latrines"has a positive impact on the study of methane emissions in low - and middle-income countries. The manuscript is valuable for research, but the low accuracy of GRACE satellite makes it difficult for the GRACE satellite to complete the work. The structure of the manuscript is relatively complete, and there are no obvious problems with the use of English. The research is interesting, but there are several points that needed to be clearly explained, which are as follows:
1. Please explain the relationship between groundwater level and methane release in detail and add 45-48 lines.
2. Generally, rainfall is the cause of ponding in pit latrines. Why not consider rainfall?
3. The depth of pit toilets is generally 0.5m to 3m, and the underground water depth is generally greater than this depth. Are there groundwater impacts to consider?
4.Can the groundwater measurement accuracy of GRACE satellites meet the requirements?
5.Figure 2 has no legend, pointer, or scale.
6. The conclusion is too brief. Please improve the conclusion.
7.A model was developed to analyze methane emissions from pit latrines using GRACE satellites. However, whether GRACE satellite is suitable for this work and the difficulties and challenges of GRACE satellite application in this work are less described.
8.Does methane gas from human waste contribute to the greenhouse effect? Please provide details on lines 36 -40.
9.We have doubts about the feasibility of the GRACE study to analyze whether pit latrines are filled with water. We have questions about the validity of research into the possible greenhouse effect of human faecal fermentation of methane. The manuscript does not give a good account of its research significance.
Human respiration produces carbon dioxide, but human respiration is not the cause of the greenhouse effect. Methane is produced by the fermentation of human waste, but the methane produced should not be harmful to the environment.
Reviewer 2 Report
The topic of this manuscript is an interesting one that has been subject of recent studies and has highlighted what we know and what we don't know about onsite sanitation systems. The objective of this study was to develop a model to estimate degree of flooding in pit latrines and infer resulting methane emissions. The novelty here is the generation of information for data-scarce regions using GRACE satellite data. This sounds like a promising contribution, but there was no assessment of the approach to verify its performance. Overall, the title of the paper overpromised and the content underdelivered with a few major shortcomings, namely:
- There were notable literature omissions (e.g. Diaz et al. 2011, Ryals et al. 2019, Bourgault et al. 2019, McNicol et al. 2020, Shaw et al. 2021, Shaw & Dorea 2021) in the Introduction of the manuscript (and Discussion). Not only did this miss some key literature, but also some conceptual issues that are fundamental to the assumptions made for this model. Some of the missed literature is on other forms of onsite sanitation, but some of the missed references listed do address pit latrines (contrary to what the authors claim to be missing from literature).
- Authors have made an oversimplification in their model that degree of pit wetness equates to level of methane generation (presumably from anaerobic degradation). There was no consideration for other key factors like temperature which can significantly affect anaerobic degradation processes (i.e. groundwater can be relatively cold - < 20C - which can limit the type and kinetics of anaerobic degradation).
- One of the studies that the authors' have referenced (van Eekert et al. 2019) has been criticised (Bourgault et al., 2019) regarding their methodology that resulted in overestimated methane emission from onsite sanitation. Furthermore, Shaw & Dorea (2021) have shown that anaerobic degradation in onsite systems is not as complete as one assumes and therefore emission factors such as those from IPCC are likely overestimated. If true, the model presented in the current study could be fundamentally flawed. To me, current thinking in the sector is moving in that direction. Authors may or may not agree with these other perspectives, but to not acknowledge and position themselves when justifying their study and methodology is a critical blindspot of this manuscript.
- What was presented was limited to a description of the model, which is where the manuscript felt incomplete to be considered a full article. At best, this could be a Short Communication or some other abridged contribution format. However, given some of the points above, authors may want to consider reworking their model, which already has some conceptual fundamental issues.
- There was no clear assessment of the model. How did it perform? It was strange to see the absence of any attempt to validate the model or verify its performance. Uncertainties were qualitatively speculated on, but there was no quantitative verification.
- There was no attempt to show an application of the model. Instead there is only a fictional figure presented. Instead, the Results section was a further description of the model. A submission for publication felt very premature.
- The Discussion section did not present much depth. Would have been nice to see how the estimates from this model compare to other ones.
References
Diaz-Valbuena, L.R.; Leverenz, H.L.; Cappa, C.D.; Tchobanoglous, G.; Horwath, W.R.; Darby, J.L. Methane, Carbon Dioxide, and Nitrous Oxide Emissions from Septic Tank Systems. Environ. Sci. Technol. 2011, 45, 2741–2747.
Ryals, R.; McNicol, G.; Porder, S.; Kramer, S. Greenhouse gas fluxes from human waste management pathways in Haiti. J. Clean. Prod. 2019, 226, 106–113.
Bourgault C, Shaw K & Dorea CC (2019) Dominant decomposition pathways in pit latrines: A commentary. Water Science & Technology, 80(7): 1392-4.
McNicol, G.; Jeliazovski, J.; François, J.J.; Kramer, S.; Ryals, R. Climate change mitigation potential in sanitation via off-site composting of human waste. Nat. Clim. Chang. 2020, 10, 545–549.
Shaw K, Kennedy C & Dorea CC (2021) Non-sewered sanitation systems’ global greenhouse gas emissions: Balancing Sustainable Development Goal tradeoffs to end open defecation. Sustainability, 13(21): 11884.
Shaw, K.; Dorea, C.C. Biodegradation mechanisms and functional microbiology in conventional septic tanks: A systematic review and meta-analysis. Environ. Sci. Water Res. Technol. 2021, 7, 144–155.
