A parallel drive to improve sanitation, especially in India where Prime Minister Narendra Modi has made basic toilets a national priority, would also yield strong returns without even considering improved human dignity.
"Provision of basic water and sanitation facilities ... would be a good investment in economic terms," Guy Hutton, senior economist at the World Bank's Water and Sanitation Program, wrote in a report.
Universal access to basic drinking water at home would cost $14 billion (9 billion pounds) a year until 2030 and yield benefits of $52 billion (34 billion pounds), or about $4 for every dollar spent, according to the preliminary findings that will form part of a wider review.
The benefits were twice those estimated in a previous global study Hutton led in 2012, he told Reuters, partly because of larger than expected falls in diarrheal disease and lower costs of digging wells or boreholes.
Overall, building toilets to eliminate defecation outside in rural areas would cost $13 billion a year to 2030 and give benefits of $84 billion, a return of $6 for every dollar spent. The benefits were slightly less than in a previous study.
Investments in better water could mean 170,000 fewer deaths a year while basic sanitation would cut 80,000 deaths, mostly from infectious diarrhoea.
Water and sanitation have long been U.N. priorities. In the past 25 years, more than two billion people of a world population now totalling about 7.3 billion have gained access to better water and almost two billion to sanitation.
The findings are also part of a series for the Copenhagen Consensus Center, which is looking at costs and benefits of everything from crop research to fighting AIDS as part of new U.N. development goals for 2030.
"We can save a lot of people" with clean drinking water and sanitation, Bjorn Lomborg, head of the Center, told Reuters. Even so, rates of return were "not as spectacular" as investing in nutrition or ending malaria.
Still, Hutton said the study estimated only health benefits and time saved, such as from walking to a river to fetch water.
"They hide intangible impacts such as dignity, social status and security," he said. The United Nations in 2010 defined improved sanitation and water as fundamental human rights.
(Reporting By Alister Doyle; editing by Ralph Boulton)
By Alister Doyle