Following the 1994 genocide in Rwanda, the country’s prisons are now full to bursting with men and women indicted for their part in the terrible events. Among many others, this presents three very distinct challenges: how to deal hygienically with the large quantities of human waste generated by inmates, how to provide prisoners with a source of cooking fuel that minimises depletion of already stretched resources from local woodlands and how to provide them with sufficient food to survive?

The Kigali Institute of Science and Technology has developed a revolutionary method for dealing with all these practicalities simultaneously – a series of giant biogas plants that collect and treat human waste, turn the bi-product into fertiliser and at the same time generate natural gas for cooking.

“Following the genocide, [prisons] are accommodating five times more prisoners than planned for,” says the system’s designer, Ainea Kimaro. “These numbers of inmates have resulted in overloading the toilet waste treatment system, a situation that risks the health of the community and damages the environment. In addition, large quantities of wood are needed for cooking, and more food stock is required.”

The system, says Kimaro, was adapted from one he developed in the 1980s for converting cow dung. It operates through a series of beehive shaped ‘bio-digesters’, huge underground chambers made of clay or cement bricks.

“The bio-digesters take organic matter such as manure into an air-tight tank, where bacteria break down the material and release biogas – a mixture of mainly methane with some carbon dioxide,” Kimaro explains. “The biogas can be burned as a fuel, for cooking or other purposes, and the remaining material can be used as organic compost for crop production.”

Although the digesters can generate up to 50m3 a day of gas, they can only hold 20, so gas is regularly piped into the prison kitchens where it fires a series of brick-lined stoves. These heat huge 500litre pots that cook beans, maize and porridge for prison inmates.

Meanwhile, to ensure it is safe to use, the solid waste from the system is held for three months in the chambers before being pumped into the prison gardens, where it fertilizes crops such as papaya, maize, bananas, and tomatoes. Regular tests are carried out on the fertilizer to ensure it contains no harmful bacteria.

So far, the system has proved highly effective. And Kimaro claims it has already reduced demand for local firewood by as much as 60 per cent. The plants are now running across six prisons covering a population of 30,000, and KIST expects to install three more each year.

In 2005, the project caught the attention of judges of the Ashden Awards, the international ‘green Oscars’ that each year give prizes to the world’s most innovative projects in the field of sustainable technology. According to Anne Wheldon, the awards’ technical director and one of the judges that selected the scheme, one of the initiative’s key advances is its scale. “The use of biogas digesters is not new, but technically to make them on this scale is in its own right an enormous challenge,” she says.

Significantly, Wheldon believes the scaling-up of technology that previously only existed at a single household scale has important implications for the safe treatment of human waste in the developing world, particularly as population growth leads to sanitation problems.

“Increasingly in Africa and the developing world, there are going to be large concentrations of people without proper sewage systems where health, odour and pollution of watercourses will be a real problem,” she says. “This system could be used anywhere where there isn’t mains sewage. And it could be scaled down to a family-sized development, or scaled up for a conurbation of say 500,000.”

Kimaro agrees, adding that the technology would be particularly suited to tropical climates where warm temperatures make it more viable. “Topical heat makes generation of biogas faster, and in that sense, free of cost, so you may ask why more African countries aren’t using it. But to be fair, the [technology] that make biogas systems more efficient came on only recently, and the word hasn’t passed around enough.”

Once the technology catches on though, Kimaro believes that in addition to cooking fuel it could have broader application as a fuel for generating electricity. “I want to make more and more electricity out of the biogas plants,” he says. “Most diesel generators can use biogas up to 80 per cent diesel substitution, and sole biogas generators for generation of electricity are already on the market.”