The biodigester solution began as a means of managing food waste. In 2011 HECAF360 built a single-chamber, underground biodigester at Bir Hospital, Kathmandu. A culture of biogas has existed in Nepal for more than 40 years, according to a nationwide survey, which suggests 69% of the country's total energy demand is met by biomass energy. Homes and farms dispose of their animals' manure into small biodigesters on their land, which releases methane gas to fuel cooking.

At Bir hospital, methane released by digested food waste was piped into the staff tearoom. But Nakarmi faced a new challenge at Kathmandu Medical College and Teaching Hospital, where he installed a biodigester in 2016. "After Bir, hospitals we worked with had maternity services which produce placentas as well as food waste," he says. "We modified the biodigester to make it suitable for both types of waste."

Before hospital staff began adding waste, constructors put in cow dung to "seed" the chambers. This contains bacteria the mix needs to digest the hospital waste and generate methane. Hospital workers throw pathological and some food waste into the first underground chamber through an inlet above the ground. The majority of the hospital's food waste goes into the second inlet and chamber. "You feed placentas and some food to balance that carbon and nitrogen into the first chamber," says Stringer. "Food, which is much bulkier but doesn't need such a long residence time – because there's no potential for infection – is fed into the second."

The team trained hospital staff to segregate waste and ensure they only fed suitable organic materials into the digester. Staff also regularly pour water into the inlets to keep the mixture fluid. Gravity slowly moves it though the system. The digestate ends its passage by tipping out of the second chamber into a sewer, from where it safely washes away. By this time, any pathogens have died. "Most viruses can last maximum a week outside of the body," says Stringer. "There's no risk."

The methane gas produced by the biodigester at TUTH is piped out of the chambers into the staff room. It fuels a stove used for cooking, and has replaced some of the liquefied petroleum gas (LPG) the hospital used to buy. The biodigester in Tribhuvan produces 1.5 cubic metres a day of methane gas – the equivalent of five standard-size LPG gas cylinders. One cylinder is enough for a family of five to cook two meals a day for a month and costs about £15 ($20), according to Nakarmi.

By replacing the incinerator, 4.6 tonnes of CO2 emissions were avoided in 2019, as well as all diesel fuel and methane gas emissions, the assessment estimates.