**3. Sewered or non‐sewered: various sanitation systems**

The previous section highlights urban diversity and the related heterogeneity in terms of sanitation status. Different contexts mean different sanitation systems, therefore it is important to adopt a sanitation system approach, as illustrated by [6]. Sanitation cannot be reduced to latrines or sewers but needs to be considered as a whole value chain, consisting of a user interface (the toilet), collection/storage (e.g. pits and septic tanks), conveyance (e.g. sewers or vacuum trucks), treatment and use and/or disposal. In particular, systems can be categorised as sewered or non‐sewered, as well as dry or wet (without or with water). They can also be differentiated through their scale/domain of application: on‐site (for households or buildings), small‐scale (for cluster of houses or neighbourhoods) or large‐scale (centralised at city level). Urban centres and western cities are usually served by a single‐sewer network that conveys the wastewater to a treatment plant; we will refer to this type of system in this chapter as the 'conventional system'. However, most cities around the world still rely mainly on on‐site systems, where domestic wastewater accumulates in pits and tanks in a form called 'faecal sludge' or 'septage'. Now and then, this faecal sludge needs to be pumped and transported to a faecal sludge treatment plant.

It is clear that it will not be possible to connect all urban areas to a conventional system in the foreseeable future. Nor is it desirable. The main bottleneck is of course financial: the amount of money needed to provide this is immense, and most governments do not have enough funds to build such infrastructure; besides, when such an infrastructure exists, the risk of failure is high when there is no financial means and capacities to operate and maintain the system properly. The further one is from the city centre, the more expensive it also gets. The economies of scale that can be achieved at the treatment plant level are outweighed by the dis‐economies of scale of the sewer network [7]. Low‐density peri‐urban areas especially have a very high cost per capita. Sometimes, the geography and population density of the city make a conven‐ tional system simply unrealistic, such as in Durban, South Africa, which is characterised by low density and a hilly topography. Other cities such as Dakar, Senegal, fully assume faecal sludge management (FSM) as part of its sanitation system in its own right, cohabiting with sewers. By accepting this and by investing in FSM, the utility ends up treating a lesser volume of wastewater per capita, while avoiding the massive investments required to provide a sewer connection to all. It is the role of a good master planning exercise to determine which areas are best connected to sewers and which can be best served by FSM.

Besides the crucial system choice, there is also the choice of scale. This results from the comparison of both the costs involved, on a life‐cycle basis, and the associated management schemes. For both sewered and unsewered systems, there are viable alternatives at different scales, from city to neighbourhood and the individual building level. Small‐scale sanitation systems or 'decentralised systems' are seen as a promising alternative for some selected urban areas. They offer the opportunity to implement new management schemes and go beyond some of the current sanitation bottlenecks.

Total sewerage coverage may also not be desirable in cities that are water scarce. In such circumstances, it does not make sense to use large amounts of drinking water simply to flush excreta and to keep the sewer system running. Besides the current threat of water scarcity to our societies, there is also the forecasted lack of nutrients for agriculture, especially for phosphorus. While phosphorus and nitrogen are now considered treatment priorities for Western governments as they may lead to eutrophication problems in natural water bodies, they are also essential for food safety. Since these nutrients are located in urine and excreta, the sound management of resources advocates for source separation to avoid diluting them in large amounts of water, which renders their recovery or treatment both cumbersome and expensive. Source separation or even on‐site treatment could offer major advantages for future sustainable urban water‐management systems [8].

The following sections cover the current bottlenecks for the fast increase in sanitation coverage and how innovative sanitation planning and management can contribute to the development of more sustainable sanitation systems that are contextual, integrative and inclusive.
