What do we know about urban sanitation costs? (a review of Daudey, 2017)

A review paper (open access) on the costs of urban sanitation came out last year. Authored by Loïc Daudey (now of AFD but then a consultant for WSUP) it surveys the literature on lifecycle costs of full chain chain systems in Africa and Asia. I found it very useful for my purposes so thought I’d write a quick review.

The paper focuses on cost *ratios* between different sanitation systems analysed within the same study.  It’s a smart approach which avoids the pitfalls of comparing absolute costs across diverse contexts, which rarely sheds much light on things as there are so many determinants of costs. That’s the useful thing about one paper it reviews, Dodane et al. (2012) – also open access and the best study in this field – which compares a sewerage system to an FSM system in Dakar, Senegal. Crucially, the comparison is an area of the city where both are operating, thereby minimising contextual effects. More on that paper another time.

Daudey’s lit. review finds that conventional sewer systems are the most expensive solution, followed by a tie between ‘septic tank & FSM’ solutions and simplified sewerage, and finally various ‘pit & FSM’ solutions. He concludes that ST & FSM comes out more expensive than simplified sewerage, but that doesn’t seem to be supported by the results. See below the key figure with some annotations of my own, including red boxes to emphasise where the median is (the black dashes), and some analysis. It’s a neat way to present the results – each stack of datapoints is the ratio between the first and second technology type in the respective X-axis label. My beef with the conclusion above is that since the median for ‘ST & FSM’ versus ‘simplified sewer’ is more or less 1, that means there’s little between them. Sure, the mean would be higher due to the outlier where the ratio is 4, but arguably the median is a better measure of central tendency for this kind of data.

Another key point stands out of the figure – there is a huge range of cost ratios for conventional sewerage vs ST & FSM – seven datapoints ranging from 1:1 up to almost 5:1. That rams the point home that context matters – sewerage is often but not always more expensive. Daudey has a nice table on cost determinants – my impression from working in a few cities and talking to engineers is that population density and topography are likely to be the most important, but I’m not aware of research that has gone into depth on this (please msg me if you know of any!).

I think the policy Q here is a three-way debate between conventional sewerage V simplified sewerage V ST & FSM. Yes pit latrines are important in many places and will continue to be important (especially in places with limited water for flushing), but few cities will be prioritising them for expansion in master plans. So, as I argued in this other blog , while conventional and simplified sewerage need to be a big part of the picture, the population numbers mean that FSM-based solutions will be with us for some time. And what Daudey’s review shows is that we shouldn’t necessarily be under the impression that FSM-based solutions are always cheaper than sewerage. Context is key.

Finally, then, a bit of the critique of the paper (other than the point above that one key conclusion is weakly supported by the findings).

1.He could have applied a more structured approach to study quality ratings. This is  common in systematic reviews, see e..g. Appendix S5 of this key WASH/diarrhoea review (Wolf et al 2014). The rating process is implicit rather than explicit – maybe it would have been better to score studies and only including the very strongest in a sub-set of ratio analysis, or maybe colour-code the strongest studies in the figure above.

2. Related to that, the review process could have been made more transparent through using something like a PRISMA diagram. It’s fine in many circumstances not to actually do a systematic review, but it’s not hard to be transparent about what was actually done (which still may be very systematic). Stick it in “supplementary material” if you don’t have the space.

3. There could have been more detailed examples relating to the key findings, (e.g. the life-cycle cost ratios) and relegated to “supplementary material” some of the stuff that was inconclusive e.g. on OpEx.

4. He could have contacted some of the authors of studies when things weren’t clear. There is some valid criticism in the paper of a study I was involved in in Dhaka, Bangladesh, but that was a wide-ranging 120pp report and we only had space for 6pp on the costing part (with some huge caveats on data quality) . There is loads of underlying material and we could have answered some of his Qs if he’d emailed us. The same is probably true for other studies where it’s said that things aren’t clear.

5. Minor point, but there could been more on the effects side. Sure, that was outside the scope of the paper to address it in detail. But considering costs on their own isn’t necessarily that illuminating for a decision-maker. Some of these service levels are associated with different disease effects, and different non-health benefits to households and different types of public goods. There could have been a bit more emphasis on how the effects side should be a key part of any decision.

Notwithstanding all these points, I found it a very useful paper that I’ll surely be dipping in and out of in the next few years as I try to move forward some work on urban sanitation costs myself.

Overall, then, what do we know about urban sanitation costs? My answer would be “not enough”. Luckily, there are plenty of people now working on this. Leeds are doing their CACTUS project, and WSUP are about to contract some consultants to do some work on costing and willingness to pay. Aguatuya are also reportedly working on some kind of tool. So fingers crossed that in a couple of years time we’ll know a lot more! I’ll aim to write another blog in a few weeks about how we can better capture cost data that organisations are generating anyway, without much additional effort.