2. Global Wastewater and Sludge Production, Treatment and Use

Cities produce large amounts and very diverse types of waste including wastewater. The quality of these wastes depends on their source, the way in which they are collected and the treatment they receive. The final fate of these wastes is also very diverse. To better understand these systems this chapter provides definitions and reuse typologies and describes common reuse patterns and their driving factors. The chapter also shows that, while the prospects for resource recovery from wastewater and sludge are promising the potential is still largely untapped, except in the informal sector. The resources embedded in the approximately 330 km³/year of municipal wastewater that are globally generated would be theoretically enough to irrigate and fertilize millions of hectares of crops and to produce biogas to supply energy for millions of households. However, only a tiny proportion of these wastes is currently treated, and the portion which is safely reused is significantly smaller than the existing direct and especially indirect use of untreated wastewater, which are posing significant potential health risks. The chapter ends with a call for standardized data collection and reporting efforts across the formal and informal reuse sectors to provide more reliable and updated information on the wastewater and sludge cycles, essential to develop proper diagnosis and effective policies for the safe and productive use of these resources.