Conventional Sewer Systems Are Too Time-Consuming, Costly and Inflexible to Meet the Challenges of the 21st Century

Round 1

Reviewer 1 Report

The title needs to be modified. Please make it more academic rather than a blog title.

The research contribution is not clear.

The research methodology needs to be elaborated by giving more details in data collection and data analysis.

In section 2.1 line 59: informal settlements can not be counted as a feature since they are not included in urban planning. Plus they have hundreds of more problems besides sewer networks.

In section 2.2, lines 73-75: what is the difference between the statements?

In section 2.2, lines 82-87: explain more about the scenarios as the whole paper is based on these scenarios

In section 2.3.2, line 134: The construction cost in Canada (with GDP per Capita of $46,510) cannot be compared to construction costs in Nigeria with lower labor and material cost. The assumption needs to be revised. 

In sections 4 and 5, the authors after an exploratory data analysis found that the current sewer systems are not efficient. It is expected to provide the alternatives with the ground back up data.

Author Response

Comment R1.1: The title needs to be modified. Please make it more academic rather than a blog title.

Response: Thank you for your thoughts on our title. We agree that the current title departs from the predominant academic convention and style. However, we respectfully decline the opportunity to revise. We believe that it is important to use plain language and be clear with our core message, especially for topics relating to the WASH sector, following the principles of the Nakuru Accord  https://wash.leeds.ac.uk/failing-better-in-the-wash-sector/.

Comment R1.2: The research contribution is not clear.

Response: On line 455-460 in the Conclusions, we have added new text describing that this is the first analysis to use data on sewer network build-out times and costs—as opposed to anecdotal information—to show that conventional sewer systems are an unrealistic solution for expanding sanitation coverage in rapidly growing cities with informal settlements.

Comment R1.3: The research methodology needs to be elaborated by giving more details in data collection and data analysis.

Response: We have revised and expanded the text in the method section to better clarify the procedures used for data collection and analysis. More precisely: section 2.2, the introduction to 2.3 as well as section 2.3.2.

Comment R1.4: In section 2.1 line 59: informal settlements can not be counted as a feature since they are not included in urban planning. Plus they have hundreds of more problems besides sewer networks.

Response: We agree that there are methodological and practical challenges to estimating the number of individuals living in informal settlements. This is why we use data from the World Bank, which is the most authoritative source of this information and is routinely used by researchers and policymakers. We have now clarified this and made the World Bank definition more explicit in the 1^st paragraph of the methods section.  

Line 344-352, we have added new text that addresses your concern that there are many problems associated with informal settlements other than a lack of sanitation

Comment R1.5: In section 2.2, lines 73-75: what is the difference between the statements?

Response: We have revised the text in these three statements to make the difference between them clearer (now lines 77-81).

Comment R1.6: In section 2.2, lines 82-87: explain more about the scenarios as the whole paper is based on these scenarios

Response: Lines 84-112: We have added new text to each scenario description to provide more detail. We have also added two new equations (line 96-97 and line 107-108) that provide a mathematical representation of the role of population growth in serviced and underserviced areas in each scenario.

Comment R1.7: In section 2.3.2, line 134: The construction cost in Canada (with GDP per Capita of $46,510) cannot be compared to construction costs in Nigeria with lower labor and material cost. The assumption needs to be revised.

Response: We agree that there are a number of limitations with using construction costs in Canada to use as a comparison with potential construction costs in Nigeria. In response to your comment, we have reorganized and revised this section of the paper (Subsection 2.3.2) to emphasize that this theoretical estimate is to complement the three other empirically based approaches we use to estimate build-out costs for Lagos, none of which rely on data from Canada. In addition, we have added new text that reinforces the limitations of all four of our estimation approaches and note how each city is unique in how costing would work.

Comment R1.8: In sections 4 and 5, the authors after an exploratory data analysis found that the current sewer systems are not efficient. It is expected to provide the alternatives with the ground back up data.

Response: We agree that it would be really valuable to have research showing the effectiveness of alternative solutions to centralized sewer systems. However, given that implementations of these alternative solutions are still in their infancy, and many of the applications are in non-urban settings, we are not aware of any datasets that allow for an analysis of build-out rates or costs. As demonstrated in this paper, data is still lacking for a reliable estimate for centralized sewer systems, even though they are much more common than any of the alternatives. As such, an analysis of build-out rates and costs for distributed/local/alternative sanitation solutions is beyond the scope of our paper. In response to your comment, we have added new text in Lines 363-368 reinforcing how our analysis highlights the need for the kind of research you suggest.

Reviewer 2 Report

The paper has been very well written, and clear explanation has been provided for the presented results. The analysis of the results is excellent.

Some of the references are not available. Please check reference 27 and 30 and provide correct link.

Author Response

Response to Reviewer #2

Comment R2.1: Some of the references are not available. Please check reference 27 and 30 and provide correct link.

Response: We have corrected these references. Thank you for pointing this out. (now references 29 and 31)

Reviewer 3 Report

The manuscript entitled "Conventional sewer systems are too time-consuming, costly and inflexible to meet the challenges of the  21st century" is very interesting as it is deal with an emerging issue with regards to the sanitation and hygiene of cities. instead of the many assumptiones, in my humble opinion it is suitable for publication in this journal after considering the following comments:

1- First of all, the author mentioned that there is a SI but they are missing!

2-Figure 1: "%" should be written in the X-axis

3- Figure 1: correct the following in the caption, populations between 300,000 and 5 million (Circle not squares);

4- Line 399: developed world instead of developing world

 

Author Response

Response to Reviewers for Manuscript #sustainability-823703: “Conventional Sewer Systems Are Too Time-Consuming, Costly and Inflexible to Meet the Challenges of the 21st Century”

We thank all four reviewers for their comments and suggestions. We have addressed all of them and believe that the changes have improved the paper.

 

Response to Reviewer #3

Comment R3.1: First of all, the author mentioned that there is a SI but they are missing!

Response: We apologize for this omission. The SI has been uploaded.

Comment R3.2: Figure 1: "%" should be written in the X-axis

Response: We have added the “%” sign to the x-axis of this figure. Thank you for pointing this out.

Comment R3.3: Figure 1: correct the following in the caption, populations between 300,000 and 5 million (Circle not squares);

Response: Thank you for letting us know about this error. It has been corrected.

Comment R3.4: Line 399: developed world instead of developing world

Response: Thank you for letting us know about this error. It has been corrected (now line 403)

Reviewer 4 Report

The paper presents a key issue, the real capacity of cities of developing countries to update their sanitation system with conventional sewers, in order to supply their total population. It stresses that for most of the cities, it is rather an impossible target, considering the current and forthcoming situations. The demonstration is clear, and the paper includes a huge set of references. Meanwhile, this paper suffers from oversimplifications/approximations in the methodology, the discussion and in the conclusion. It must be improved.

You’ll find hereafter my detailed comments:

• Comment 1: Lines 132-135: It seems not relevant to consider that the cost would be similar in Canada as in Lagos. This not the case, as it strongly depends of the mean salary of the population that is totally different in these two countries.
• Comment 2: Lines 135--137: Here again, the hypothesis seems not pertinent as the cost per capita tend to decrease with increase of the project size.
• Comment 3: Lines 138-139: It would not meet the targeted regulation in developed countries. Moreover, a complete removing of Suspended Solids (SS) is not a target (even in developed countries), as most regulations include a threshold level of SS in effluent, not the total removal of SS.
• Comment 4: Lines 139-142: Here again, your hypothesis seems wrong. The Canadian cities have generally a lot of space, meaning a low-population/habitat density, then a high ratio of pipe length per capita. Moreover, this pipe length per capita is also dependent of the urban street’s density. You must revise your data with African/Asian/South American data.
• Comment 5: line 149: The cost is not strictly proportional to population size, your hypothesis meaning a stable cost/capita whereas the cost/capita is not stable.
• Comment 6: Lines 150-151: This hypothesis is not fully demonstrated by comparison of similar projects in both countries. Could you provide some data?
• Comment 7: Lines 204-214: What is curious in your approach is that practically this is totally dependent of the number of projects per city, whereas you seem to consider only one project/city.
• Comment 8: Lines 230-231: Practically, it mainly demonstrates that the current infrastructure’s building is not sufficient. It could be the results of other priorities at the scale of the cities. I am still not convinced that 1800 households/month is a maximum for a city that is 10million people. It would mean 1% of the population per year. What occurs in China demonstrate it is not always the case.
• Comment 9: Global comment: The issue is much more complex than what you consider, and you must include the institutional, regulations, urban planning, economic, and sociological issues, and the technical capacity of the city to organize and operate these extended networks.
• Comment 10: Global comment: One of the huge limitations of your paper is that you forgot to link the sanitation extend to the drinking water network extend. Practically, it is a non-sense to have a sewer network where people have no access to a potable water source. In such a case, they have to use local sanitation systems. This must be included in your approach. I am sure it would strongly impact your results, by limiting the needs. And conversely, if you add both drinking and sanitation centric networks, the situation would be much more hazardous to build and finance.
• Comment 11: Lines 317-324: China has a very strong increase of urban population, and at the same time is able to build its networks.
• Comment 12: Lines 327-330: That’s effectively the case.
• Comment 13: Global comment: You must add a paragraph addressing the limitations of your approach and the corresponding uncertainty when applying to specific cities.
• Comment 14: Global comment: Solutions exist. You have to look at local management of water in sites where you have no centric systems. There are projects on that.
• Comment 15: Lines 375-394: Practically it is effectively the more common situation. But you are wrong when you consider that research, innovation and development are focused on centric systems. I have personally been working for 10 years on alternative way of managing water in the future sustainable city. Researches exist on that, solutions are also being developed.

Author Response

Response to Reviewers for Manuscript #sustainability-823703: “Conventional Sewer Systems Are Too Time-Consuming, Costly and Inflexible to Meet the Challenges of the 21st Century”

We thank all four reviewers for their comments and suggestions. We have addressed all of them and believe that the changes have improved the paper.

Response to Reviewer #4

Comment R4.1: Lines 132-135: It seems not relevant to consider that the cost would be similar in Canada as in Lagos. This not the case, as it strongly depends of the mean salary of the population that is totally different in these two countries.

Response: Thank you for this insight on differences in labor costs across countries. See response to Reviewer #1, who made a similar comment.

Comment R4.2: Lines 135--137: Here again, the hypothesis seems not pertinent as the cost per capita tend to decrease with increase of the project size.

Response: We agree that many factors create nonlinearities that lead to differences in per capita building cost at different project sizes. However, most available data sources provide cost information in terms of a constant per capita rate. The Canadian costing templates do take into account decreasing costs. We point this out and have added new text in Lines 201-202 noting that some economies of scale might be realized that leads to changes in per capita building costs.

Comments R4.3 and R4.4: Lines 138-139: It would not meet the targeted regulation in developed countries. Moreover, a complete removing of Suspended Solids (SS) is not a target (even in developed countries), as most regulations include a threshold level of SS in effluent, not the total removal of SS.

Lines 139-142: Here again, your hypothesis seems wrong. The Canadian cities have generally a lot of space, meaning a low-population/habitat density, then a high ratio of pipe length per capita. Moreover, this pipe length per capita is also dependent of the urban street’s density. You must revise your data with African/Asian/South American data.

Response: We agree that these are both additional limitations of the comparison between Lagos and Canada. As we describe in our response to your earlier comment (Comment R4.1), we have modified the text in Subsection 2.3.2 to raise the profile of our three other approaches to estimate build-out costs and be clearer about the limitations of the Canada comparison and localized regulations.

Comment R4.5: line 149: The cost is not strictly proportional to population size, your hypothesis meaning a stable cost/capita whereas the cost/capita is not stable.

Response: This is related to your previous comment (Comment R4.2). We agree that per capita construction cost may be nonlinear in project size, but we are not able to find data that allows us to construct a marginal cost curve for Lagos. We have added new text in Lines 201-202 noting that some economies of scale might be realized that leads to changes in per capita building costs. On line 299-305, we note other factors that are likely to impact the costs.

Comment R4.6: Lines 150-151: This hypothesis is not fully demonstrated by comparison of similar projects in both countries. Could you provide some data?

Response: We have rewritten this sentence to clarify the role of our assumption (that there are five persons per household) on the comparison between the two countries.

Comment R4.7: Lines 204-214: What is curious in your approach is that practically this is totally dependent of the number of projects per city, whereas you seem to consider only one project/city.

Response: You make a good point that our projections are based on a single World Bank sanitation project within each city. In theory, multiplying the number of projects in each city could increase the rate of new people connected to sewers. In response to your comment, we have added text on 269-276 noting that the majority of cities would need to increase the rate ten to 50 times in order to fulfill their target. However, we also note that cities have a finite capacity for the number of sanitation projects at any given time, limited by the number of ways a city can be divided up for projects , and there would be significant cost implications to ramping up the number of projects per city.

Comment R4.8: Lines 230-231: Practically, it mainly demonstrates that the current infrastructure’s building is not sufficient. It could be the results of other priorities at the scale of the cities. I am still not convinced that 1800 households/month is a maximum for a city that is 10million people. It would mean 1% of the population per year. What occurs in China demonstrate it is not always the case.

Response: China is often hailed for its effectiveness, but even China is struggling to close the sanitation gap. Notably, several of the most rapidly growing cities included in the present study are located in China and we have not managed to locate any studies that suggest that their completion rate is higher than indicated in our study. We have added some text to that end (lines 277-279 and 387-390), supported by a recent publication (32).

Comment R4.9: Global comment: The issue is much more complex than what you consider, and you must include the institutional, regulations, urban planning, economic, and sociological issues, and the technical capacity of the city to organize and operate these extended networks.

Response: We are in complete agreement that there are many more dimensions to the wicked problem of providing sanitation coverage in rapidly growing cities with informal settlements. Our analysis shows that, even absent these additional complications, build-out costs and times alone make centralized sewer systems an unrealistic solution. In response to your comment, we have added new text in Lines 249-256 indicating that there are many other complex challenges that need to be overcome before any sanitation solution can fully cover populations in these cities.

Comment R4.10: Global comment: One of the huge limitations of your paper is that you forgot to link the sanitation extend to the drinking water network extend. Practically, it is a non-sense to have a sewer network where people have no access to a potable water source. In such a case, they have to use local sanitation systems. This must be included in your approach. I am sure it would strongly impact your results, by limiting the needs. And conversely, if you add both drinking and sanitation centric networks, the situation would be much more hazardous to build and finance.

Response: We agree that the extent of drinking water systems can be linked to the extent of sanitation systems in certain contexts. We have added new text in at the end of the 1^st paragraph of the discussion section noting this potential linkage in the context of the many infrastructure priorities that need to be addressed in informal settlements. We believe that our analysis is informative without explicitly modeling linkages to drinking water systems because most drinking water networks are physically separate from sewage networks and drinking water connections are more common than sewage connections in many contexts (see example graphic below with World Bank data that shows that although drinking water and sanitation are indeed correlated, drinking water access is generally higher). Moreover, many informal settlements that do not have piped drinking water still have access to drinking water through groundwater wells or trucked water, so sanitation solutions are required in these contexts as well. We agree that distributed solutions are more feasible here, and our analysis demonstrates the limitations of using a centralized sewer system in these contexts.

Comment R4.11: Lines 317-324: China has a very strong increase of urban population, and at the same time is able to build its networks.

Response: See response to R.4.8.

Comment R4.12: Lines 327-330: That’s effectively the case.

Response: We are glad that you agree with our conclusion here.

Comment R4.13: Global comment: You must add a paragraph addressing the limitations of your approach and the corresponding uncertainty when applying to specific cities.

Response: In revising the methods section, we are now more clearly highlighting the limitations of the study, particularly with respect to the cost-estimates in section 2.3.

Comment R4.14: Global comment: Solutions exist. You have to look at local management of water in sites where you have no centric systems. There are projects on that.

Response: As we already note in the last paragraph of the Discussion (Section 4), distributed/local sanitation systems are an important class of alternatives to centralized sewers. We have added new text in to the 3^rd paragraph of the conclusion reiterating the potential important role for these distributed/local systems.

Comment R4.15: Lines 375-394: Practically it is effectively the more common situation. But you are wrong when you consider that research, innovation and development are focused on centric systems. I have personally been working for 10 years on alternative way of managing water in the future sustainable city. Researches exist on that, solutions are also being developed.

Response: We agree that great work is being done on alternative sanitation solutions. We have expanded the text in the Discussion (Section 4) and Conclusion (Section 5) to make it clearer that these alternative solutions are being explored and should be prioritized in academic and research programs given the evidence we present of the limitations of centralized solutions.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Nice presentation but it is expected to explain your contribution clearly.

Author Response

In the abstract, we write

Our study provides clear evidence that the conventional model for sanitation is unworkable for rapidly growing urban areas.

And we end our paper with the following statement

The work presented here is the first analysis to use data on sewer network build-out times and costs—as opposed to anecdotal information—to show that conventional sewer systems are an unrealistic solution for expanding sanitation coverage in rapidly growing cities with informal settlements.

Reviewer 4 Report

Conventional sewer systems are too time-consuming, costly and inflexible to meet the challenges of the 21st century Treated – V2b Global comments on the paper:

Thanks for this revised paper that has been improved. Meanwhile there are still some issues. You’ll find hereafter my detailed comments (I refer to my first comment number for easiness):

• Comment 1: Be so kind to forward me the answer you have provided to the other reviewers.

• Comment 2: Practically, most of the provided costs for water projects don’t include full lifecycle costs and cannot include any future modification of the regulations. If you want to fully compare costs, these have not to be included. My comment on savings at economies of scale is then still valid. So, your approach is effectively not conservative.

• Comments 3 and 4: Thanks for the revised text. Meanwhile, I suggest you address more the limitations of your approach in the discussion step. Here is one additional comment on your new revised text:

• Lines 196-198: So, you only look at primary treatment (not at complete one), that is only a part of the treatment in most of developed countries such as in Canada. This is not common practice.

• Comment 8: I want to react to your sentence lines 387-390. The reality is that Chinese growing urban population was very high for 20 years due to mass migration of population from rural areas to cities, to provide employments to growing industries (as requested/sustained by Chinese authorities). It differs from what occurs in African countries where the migration is not politically sustained and leads to strong issues in the capital cities. The strong increasing middle-class population in China that has access to a true sanitation demonstrates that sanitation has strongly improved in few years (in this country). I personally worked on Chinese cities, and what I have seen is not in agreement with your maximum 1800 connected people/month/city. So even if they have some difficulties to provide a true sanitation to all of their urban population, they have improved the situation, even with hundreds of millions of people moving from rural areas to cities. It would have not been possible strictly considering your 1800 connected people/month/city.

• Comment 9: Practically if I am correct, your adding is lines 162-164, not lines 249-256. But that’s OK. I have found it.

• Comment 10: May be, I was not totally clear. Where you have no direct access to a drinking water from a centric network, the water consumption per capita is generally much lower and it can have an impact on the potential sanitation systems and costs. Moreover, in informal settlements, sanitation is a key issue, but my own experience is that there are very few centric sanitation systems in informal settlements for various reasons. Sanitation is then specifically based on local simple systems or on nothing. So, my feeling is that you over estimate the real sanitation cost by considering the same approach to all of the population of a city. You should at least address this issue.

 

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 3

Reviewer 4 Report

Thanks for this revised version of your paper.

I still think it would have been useful to more address the limitations of the approach in the discussion part (for instance the impact of no drinking water network in informal settlements).

Meanwhile, the paper is sufficiently improved to be published.