Last year, researchers at Rice University revealed a new way to convert solar energy directly into steam using light-absorbing nanoparticles. At that time, the technology had already been used to create a solar steam-powered autoclave for sterilizing medical and dental equipment and the project had been awarded a grant from the Bill and Melinda Gates Foundation to turn the technology to the task of sanitizing human waste. The researchers have now put both applications to the test.

The heart of both systems are nanoparticles that heat up so quickly when submerged in water and exposed to sunlight that the surrounding water instantly vaporizes to form steam. As an example of its effectiveness, the system even works with icy-cold water and boasts an overall energy efficiency of 24 percent. This is superior even to commercial photovoltaic solar panels that typically have an overall energy efficiency of around 15 percent. Despite the electricity-generation potential of the system, the researchers chose to first focus on sterilization applications for the developing world.

“Sanitation technology isn’t glamorous, but it’s a matter of life and death for 2.5 billion people,” says Naomi Halas, the director of Rice’s Laboratory for Nanophotonics (LANP) and lead researcher on the project. “For this to really work, you need a technology that can be completely off-grid, that’s not that large, that functions relatively quickly, is easy to handle and doesn’t have dangerous components. Our Solar Steam system has all of that, and it’s the only technology we’ve seen that can completely sterilize waste.”

The researchers tested the solar-steam autoclaves in two set ups – one for sterilizing medical and dental equipment and another for sanitizing human waste. They found the heat and pressure generated by the steam was not only sufficient to kill even the most heat-resistant living microbes, but also spores and viruses.

“The process is very efficient,” says Rice graduate student Oara Neumann, the lead author on the PNAS study who created the light-harvesting nanoparticles. “For the Bill & Melinda Gates Foundation program that is sponsoring us, we needed to create a system that could handle the waste of a family of four with just two treatments per week, and the autoclave setup we reported in this paper can do that.”

The research team hopes to partner with waste-treatment company Sanivation in conducting the first field tests of the solar steam waste sterilizer at three sites in Kenya.

The team’s paper detailing the technology appears online in the Proceedings of the National Academy of Sciences Early Edition.

Halas and Neumann describe the technology in the following video.