Recycling the world's wastewater, almost all of which goes untreated, would ease global water shortages while protecting the environment, the United Nations said in a major report on Wednesday.
"Neglecting the opportunities arising from improved wastewater management is nothing less than unthinkable," said Irina Bokova, director-general of UNESCO, one of several UN bodies behind the report issued on World Water Day.
For decades, people have been using fresh water faster than Nature can replace it, contributing in some regions to hunger, disease, conflict and migration.
Two-thirds of humanity currently live in zones that experience water scarcity at least one month a year.
Half of those people are in China and India.
Last year, the World Economic Forum's annual survey of opinion leaders identified water crises as the top global risk over the next decade.
On current trends, the UN Environment Programme forecasts that water demand — for industry, energy and an extra billion people — will increase 50 per cent by 2030.
Global warming has already deepened droughts in many areas, and the planet will continue to heat up over the course of the century, even under optimistic scenarios.
"There is an absolute necessity to increase water security in order to overcome the challenges brought on by climate change and human influence," said Benedito Braga, head of the World Water Council, an umbrella grouping of governments, associations and research bodies.
Wastewater — runoff from agriculture, industry and expanding cities, especially in developing nations — is a major part of the problem.
That is especially true in poor countries where very little, if any, wastewater is treated or recycled.
High-income nations treat about 70 per cent of the wastewater they generate, a figure that drops to 38 per cent for upper middle-income countries.
In low-income nations, only eight per cent of industrial and municipal wastewater undergoes treatment of any kind.
More than 800,000 people die every year because of contaminated drinking water, and not being able to properly wash their hands.
Water-related diseases claim nearly 3.5 million lives annually in Africa, Asia and Latin America — more than the global death toll from AIDS and car crashes combined.
Chemicals and nutrients from factories and farms create deadzones in rivers, lakes and coastal waters, and seep into aquifers.
The 200-page World Water Development Report details a four-pronged strategy for transforming wastewater from a problem to a solution, said lead author Richard Connor of UNESCO's World Water Assessment Programme.