One approach from the California Institute of Technology that Mr Gates said he found "super interesting" integrates an electrochemical reactor to break down water and human waste into fertiliser and hydrogen, which can be stored in hydrogen fuel cells as energy.

'Substantial market'

Without cost-effective alternatives to sewers and waste-treatment facilities, urbanisation and population growth will add to the burden. In some cities, more than half the volume of human waste escapes into the environment untreated. Every dollar invested in sanitation yields about $US5.50 in global economic returns, according to the World Health Organisation.

"Human waste that is properly handled can be a very economically attractive investment due to the health benefits," said Guy Hutton, a senior adviser for water, sanitation and hygiene with Unicef in New York, in an interview. "Given the unmet need of 2.3 billion people still without basic sanitation, there is a potentially very substantial market and economic gain to be had."

The reinvented toilet market, which has attracted companies including Japan's LIXIL Group, could generate $US6 billion a year worldwide by 2030, according to Mr Gates.

'Golden opportunity'

"Innovative companies have a golden opportunity to do well by doing good," LIXIL president Kinya Seto said in a statement. "We can help jump-start a new era of safe sanitation for the 21st century by developing solutions that can leapfrog today's existing infrastructure, functioning anywhere and everywhere."

Companies displaying their sanitation technologies included China's Clear, CRRC and EcoSan; Sedron Technologies from the US; SCG Chemicals, a unit of Thailand's Siam Cement; and India's Eram Scientific Solutions, Ankur Scientific Energy Technologies, and Tide Technocrats, the Gates Foundation said in an emailed statement.

The initial demand for the reinvented toilet will be in places like schools, apartment buildings, and community bathroom facilities. As adoption of these multi-unit toilets increased and costs declined, a new category of reinvented household toilets would become available, the Gates Foundation said.

"Our goal is to be at 5 cents a day of cost," Mr Gates said in a telephone interview before the exhibition. Small-scale waste treatment plants, called omni-processors, might be suited for uses beyond human waste management - such as for managing effluent from intensive livestock production - because of their low marginal running costs relative to the value of the fertiliser and clean water they produced, he said.

I definitely never thought that Melinda would have to tell me to stop talking about toilets and fecal sludge at the dinner table.

Bill Gates

"The value of those outputs exceeds the operating cost," Mr Gates said. "So you'll actually be looking for sources of biomass that keep it fully busy."

Mr Gates, who with wife Melinda has given more than $US35.8 billion to the foundation since 1994, said he became interested in sanitation about a decade ago after he stopped working full-time at Microsoft.

"I never imagined that I'd know so much about poop," Mr Gates said in remarks prepared for the Beijing event. "And I definitely never thought that Melinda would have to tell me to stop talking about toilets and fecal sludge at the dinner table."