Flush toilets, Gates wrote in a recent blog, “Are irrelevant, impractical and impossible for 40 per cent of the global population, because they often don't have access to water, sewers, electricity and sewage treatment systems.”

“The topic we are discussing today can rightly claim to be the most neglected thing in all the things that are done to help the poor,” Gates said on Tuesday at a ceremony to announce the top four projects at his foundation's Reinvent the Toilet Fair in Seattle.

“The toilet that was invented 200 years ago, the flush toilet, really hasn't had that many milestone advances.”

Gates had challenged scientists to come up with a lavatory that didn’t use running water, a septic system or electricity, and that didn’t discharge pollutants; it would also only cost 5 cents a day to run.

The competition was won by Caltech, which came up with a solar-powered reactor that broke down human waste into hydrogen.

Other models include one that uses microwave energy to transform human waste into electricity. Another captures urine and uses it for flushing. And still another turns excrement into charcoal.

Gates envisaged the result of this venture would reach beyond the developing world.

“If we do it right, there's every possibility that some of these designs would also be solutions for rich and middle-income countries,” Gates said.