**1. Introduction**

The world is experiencing unprecedented urban population growth rates, and most of this growth is projected to take place in cities of low‐ and middle‐income countries in Africa and Asia. The United Nation's latest global population estimates, published in early 2015, project

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that the global population in 2100 will be 11.2 billion. It is estimated that Africa will have 39% of the world's population, almost as much as is estimated for Asia [1].

The rapidly urbanising societies of Africa, Asia and Latin America are constricted by a quadruple challenge: urban environmental degradation, global climate change with accentu‐ ating water stress, infrastructure deficits and fast expanding peri‐urban areas and informal settlements. As pointed out by [2], even when urban sanitation management infrastructure is available, it often serves only a small percentage of the urban population. Small‐ and medium‐ sized towns (<500,000 inhabitants) will carry the brunt of future urbanisation in low‐ and middle‐income countries and will have a pronounced backlog in urban sanitation infrastruc‐ ture. A recent infrastructure study of the World Bank highlighted the low access to improved sanitation in urban Africa with 51% of the population relying on traditional (unimproved) latrines, 14% on improved latrines and only 25% connected to sewers or a septic tank [3].

Urban sanitation is at an inflection point. The international community and national govern‐ ments are increasingly acknowledging that conventional sewer‐based sanitation cannot be the solution for all the different urban areas. Apart from the lack of capital, there are other good objective reasons why conventional urban water management does not offer the only solution for the rapidly growing cities in Asia and Africa: the lack of stable energy supplies, of spare parts and of human resources for reliable operation are factors that limit the expansion of centralised systems. In an increasing number of cities, water scarcity is also becoming an important bottleneck. As a special case, the improvement of sanitation conditions in informal settlements in low‐ and middle‐income countries has proven difficult due to disabling institutional environments, as well as the lack of secure tenure and of the rule of law, often preventing the development of innovative management schemes. Today, a majority of urban citizens rely on on‐site systems, such as septic tanks, pit latrines or cesspits. With sewer‐based systems out of reach for a large part of the global population, there is an urgent need to develop more cost‐effective and resource‐efficient systems that can deliver the desired water services necessary for public health, protection against flooding, and the preservation of natural resources. In this chapter, we present the main reasons for environmental sanitation deficits and lay out arguments for a holistic sectoral approach that is inclusive and that incorporates innovative management arrangements for growing urban areas.
