Wastewater from households, industries and agriculture should not be seen as a problem but a valuable resource which could help meet the demands for water, energy and nutrients from a growing global population, the UN says.
Globally, more than 80 per cent of wastewater is released into rivers and lakes without treatment with a negative impact on health and the environment, according to the 2017 UN World Water Development Report published on Wednesday.
Pollution from human and animal waste affects nearly one in three rivers in Latin America, Asia and Africa, putting millions of lives at risk, it said.
But wastewater contains nutrients such as phosphorus and nitrates which can be turned into fertiliser, said Richard Connor, editor-in-chief of the report.
Treated sludge can be turned into biogas that could power wastewater treatment plants or be sold on the market, he added.
"Wastewater itself is a valuable resource, even the term wastewater is an oxymoron," Connor told the Thomson Reuters Foundation.
"We need to stop seeing it as a burden to be dealt with. It's not a waste and should not be a waste, especially in this world of water scarcity," he said by phone from Perugia.
With the world's population expected to grow by one third to more than 9 billion by 2050, the world will need 55 per cent more water and 70 per cent more energy, the United Nations says.
Population growth will also lead to a 70 per cent increase in demand for food, putting more pressure on water through farming, which is already the biggest consumer of water.
More people also means more wastewater, including from sanitation, which governments have pledged to improve as part of development goals agreed by UN member states in 2015.
Increased wastewater is one of the biggest challenges associated with the growth of informal settlements in rapidly expanding cities in developing countries, the report said.
Connor said even though wastewater is a valuable resource, what often stops governments from investing in treatment plants is the cost while what puts people off using it is the "yuck factor".
Yet the International Space Station has been using the same water for 17 years, Connor noted.
"One morning it's tea, by the afternoon it's pee and then the next morning somebody is shaving with it," he said.
A solution for governments is to invest in smaller, decentralised treatment systems, which cost a fraction of conventional plants and require less maintenance, Connor said.
