Hi-tech loos that use little or no water and can recycle waste products safely and sustainably promise to give billions of people around the world access to much-needed sanitation. So why do so many still lack this basic amenity?
About 2.3 billion people still lack basic toilets, according to the World Health Organization (WHO). And 4.5 billion don't have safely managed sanitation, with waste disposed in a way that won't contaminate drinking water.
Each year contaminated water kills half a million children under five through diarrhoeal diseases, the WHO says.
So many inventors, entrepreneurs and research institutions around the world have been working on hi-tech loos that can function without the need for expensive mains sewerage systems.
One approach is taking chloride from urine, turning it into chlorine with electricity, and using that as a disinfectant, says Dr Brian Hawkins, a research scientist in nanomaterials at Duke University, North Carolina.
Activated charcoal can remove organic material and nano-membranes replace the need for septic tanks, he says.
A solar-powered toilet using this approach, developed at Duke and nearby universities, is being tested at a cotton mill in Coimbatore, India and a township in South Africa.