'Top-down' planning for scalable sustainable sanitation in high-density low-income urban areas: is it more appropriate than 'bottom-up' planning?

We argue that, if the sanitation target of the Sustainable Development Goals (universal access to 'safely-managed' sanitation by 2030) is to have any chance of success, then a community-sensitive top-down planning approach has to be adopted for sanitation provision in high-density low-income urban areas in developing countries, as 'bottom-up' planning is much more time-consuming and not yet successfully proven at scale. In high-density low-income urban areas, there is only a limited choice for safely-managed sanitation: (i) simplified/condominial sewerage (which becomes cheaper than on-site sanitation systems at the relatively low population densities of 160-200 people per ha), (ii) low-cost combined sewerage (if it is cheaper than separate simplified sewerage and stormwater drainage), (iii) community-managed sanitation blocks, and (iv) container-based sanitation (the last two of which are suitable, especially in slums, when neither simplified sewerage nor low-cost combined sewerage is affordable or technically feasible). These four sustainable sanitation options are as scalable in developing countries as conventional sewerage has been in industrialized countries.