Safe water is a precondition for health and development and a basic human right, yet it is still denied to hundreds of millions of people throughout the developing world. Waterrelated diseases caused by insufficient safe water supplies coupled with poor sanitation and hygiene cause 3.4 million deaths a year, mostly among children. Despite continuing efforts by governments, civil society and the international community, over a billion people still do not have access to improved water sources. The scale of the problem of water quality is even larger. It is increasingly clear that many of the existing improved sources in developing countries do not provide water of adequate quality for domestic purposes. A well-known example of this is the extensive contamination of tubewells with naturally occurring arsenic in Asia. As serious as this and other cases of chemical contamination are, the principal cause of concern is microbiological contamination, especially from faeces. While groundwater is generally of much higher microbiological quality than surface water, an increasing number of sources and systems used by people for drinking and cooking water are not adequately protected from faecal contamination. This is due to a variety of factors, including population pressure, urbanization and the inadequate construction, operation and maintenance of
water systems.