The criteria set out by the Gates Foundation are pretty strict. The toilets have to be hygenic and sustainable, discharge no pollutants, generate energy, recover nutrients and only need a tiny amount of water. Oh yeah, and they also need to have a cost of operation of a nickel per person per day.

From the Delft University of Technology in the Netherlands comes a toilet that uses microwave technology to transform human waste into electricity.

A Swiss team has a working model of a urine-diverting toilet (seen here) that recovers the water used to allow flushing, thanks to a gravity-driven biological membrane.

A team at Cal Tech is proposing a solar-powered toilet that generates hydrogen and electricity, with the solar panel powering an electrochemical reactor to break down water and human waste into hydrogen gas, which in turn can serve as a backup power source.