Microsoft co-founder Bill Gates has challenged scientists to develop waterless toilets for the 2.5 billion people around the world without access to modern sanitation.
The billionaire philanthropist has announced $3.4m (£2.2m) in new funding for its "Reinvent the Toilet Challenge", with $100,000 (£64,000) going to the California Institute of Technology for its work on a sun-powered system that recycles water and breaks down human waste into storable energy.
About 1.5 million children under five-years-old die every year - mostly in sub-Saharan Africa and south Asia - because of sanitation problems.
But Mr Gates said modern flushable toilets were not the answer as they need a complex sewer system and use too much water.
"The flush toilets we use in the wealthy world are irrelevant, impractical and impossible for 40% of the global population," he said in a statement.
"Beyond a question of human dignity, this lack of access also endangers people's lives, creates an economic and a health burden for poor communities, and hurts the environment."
To solve the problem, his charitable foundation handed grants to eight universities around the world to develop a toilet that operates without running water, electricity or a septic system.
It was also to be designed to not discharge pollutants, preferably capture energy or other resources, and operate at a cost of just five cents a day.
Some of the prototypes have gone on display in the open courtyard of the foundation's Seattle headquarters this week.
They include the one produced by the California Institute of Technology (Caltech), which has an electrochemical reactor to transform the waste into a hydrogen gas
The hydrogen gas produced in the Caltech design can be stored in hydrogen fuel cells to provide a back-up energy source for night operation or use in low-sunlight conditions.
Mr Gates also handed out a prize to Loughborough University for a toilet that transforms waste into biological charcoal, minerals, and clean water.
Another project on display was from the London School of Hygiene and Tropical Medicine. It sends black soldier fly larvae into latrines and even home toilets to process waste, resulting in high quality, environmentally friendly animal feed.
The fly larvae project is already being tested in Cape Town, South Africa, and the inventors are working on a kit to sell to entrepreneurs. They have had inquiries from Haiti, Sudan, Kenya and Ghana about adopting the approach.
The foundation, which Gates co-chairs with his father and wife, Melinda, is the world's biggest private philanthropic organisation with an endowment worth more than $33bn.
It is spending about $80m a year on water, sanitation and hygiene issues.