The growth of the earth’s urban population and areas continues as a major demographic trend; it is projected that 70 % of the world’s population will live in urban areas by 2050. Urban growth today is most rapid in developing countries, where cities gain an average of 5 million residents each month. Megacities and metacities – defined by UN Habitat as cities with more than 10 million inhabitants or 20 million inhabitants respectively – are gaining ground in Asia, Latin America and Africa. In most developing countries, urban growth is inextricably linked with slum expansion and poverty; in 2000, nearly one third of the world’s urban dwellers lived in slums. As city infrastructure cannot keep pace with massive urban growth, many people are left without adequate access to drinking water and sanitation.