precipitation patterns around the world, melt mountain glaciers, and worsen the extremes of droughts and floods. Drought-affected areas are projected to increase in extent, with higher frequency and intensity of drought. In particular, the subtropics and mid-latitudes, where much of the world’s poorest populations live, are expected to become substantially drier, resulting in heightened water stress (Meehl _et al_ ., 2007). Climate change will also affect water quality in water scarce regions, with reduced river flows losing assimilative capacity, and salinity increasing (Sadoff and Muller, 2009). As a result of these changes, the demand for irrigation water and in particular for wastewater as an extremely reliable water resource will rise, and it will need to be increasingly considered an integral component of local water resources.

_Growing urbanization_ . An ever larger share of the world population lives in cities. Particularly in developing countries, urbanization is growing very rapidly. From 2010 to 2030, the population living in urban areas in developing countries is expected to increase from about 2.6 billion to 4.0 billion in 2030; and in lower-income countries the urban population is expected to more than double from 254 million to 539 million (UNDESA, 2008).

_Growing urban wastewater generation_ . Associated with the large growth in the urban population in developing countries will be a growth in urban wastewater generation. As countries’ income levels rise, the levels of piped water supply and sewer networks also tend to rise, and lead to further increased wastewater flows.

In the recent past impressive achievements have been made in the provision of improved urban sanitation services, including sewerage. From 1990 to 2006 improved sanitation facilities were provided to some 779 million additional urban residents, a large share of whom are also connected to sewers (WHO/UNICEF, 2008). However, despite these efforts, the provision of these services has barely kept pace with urban population growth, and the urban service deficit only dropped from 23 percent to 22 percent. Achieving the Millennium Development Goals sanitation target of cutting in half the service deficit by 2015 and achieving sanitation for all by 2025, will require significant ramping up of the pace of improvements. At the current rate of investment, the urban service deficit will only drop to 18 percent by 2015. Nevertheless, in absolute terms wastewater flows in urban areas will continue to grow substantially, and make improved wastewater management a necessity.

_Growing agricultural activities in and near urban areas_ . The growth in the urban population in the developing world will not only lead to an increasing supply of urban wastewater, but also to a growing demand for irrigation water for agricultural activities in and near urban areas.

Estimates on the current extent of urban agricultural activities are limited. UNDP (1996) estimated that in the early 1990s more than 800 million people (or a third of all urban households) were involved in some way in urban agriculture, producing about 15 percent of the world's food. More recent estimates for West Africa suggest that about 20 million people—of an urban population of 100 million—live in households engaged in urban agriculture; in many cities they produce 60 to 100 percent of the consumed perishable vegetables (Drechsel _et al_ ., 2006). Table 2.6 shows the contribution of urban agriculture to food consumption in selected Asian and African cities. Even in high-income cities, such as Singapore and Hong Kong, the share of certain products supplied from urban agricultural activities is relatively high.

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