RWAMAGANA — They're sitting on the floor in close ranks, facing the door. There's about a hundred of them. In their orange uniforms, most of them barefooted, the convicts are waiting under a scorching sun, waiting to be counted and re-counted before entering Rwamagana's penitentiary, the biggest prison in Rwanda.

Among the 8,597 prisoners crammed inside its high walls, more than half are still being imprisoned for crimes they committed during the genocide that killed more than 800,000 people, most of them Tutsi, between April and July 1994. During the commemorations that will officially end on July 4 but also on billboards and on television, the word kwibua, or "remember," is everywhere.

As soon as it emerged from its genocidal hell, Rwanda had to manage an unprecedented influx of prisoners. Between 2001 and 2012, the 12,100 gacaca — popular jurisdictions inspired by ancient assemblies where wise men of the village would solve disputes — judged more than two million people. According to government figures, 65% of them were sentenced to prison or community service.

Although several penitentiaries were expanded or renovated, all rapidly reached their limit. "The smell of feces was unbearable inside the walls, and even around the prison," recalls Vincent, a convict in his sixties. "There have been more than 12,000 of us here. Since the septic tanks were often full, we would suffocate. Visitors used to come with a mask."

In the late 1990s, the Rwandan government launched a significant investment plan in renewable energies, including biogas (producing gases from the breakdown of organic matter) for large schools and, most importantly, for prisons. Of the 14 penitentiaries across Rwanda, 13 are now equipped with organic waste breakdown systems where waste is turned into energy.

In the city of Rwamagana, located about 50 kilometers west of the capital Kigali, the system began operating in 2010. "It enabled us to sanitize the buildings, to get rid of the putrid smells and to save money since electricity costs for cooking dropped 85%," says prison warden Moise Ntawiheba. "On top of that, the forest is thankful."

Before the system was in place, the prison needed 35 cubic meters of wood every month to heat the water and the food. Now, it needs just seven. Environmental protection is a major issue in Rwanda, where not a piece of paper or cigarette butt litters the streets, where energy-saving light bulbs are commonplace, and where plastic bags have been banned since 2006.

The biogas system is buried opposite from the door to the facility's main building. On the outer wall, pipes connect the latrines to a tank. Excrement feeds 12 digesters of 100 cubic meters each. That's where the fermentation process takes place, producing 500 cubic meters of gas every day, 60% of it methane, that is then transported to the kitchens' gas burners. At the other end of the system, a cavity allows for the collection of the odorless organic residue that is used as fertilizer for the crops.

The Rwamagana prison spreads over 415 acres, beyond the first hills we see around it. On about 20% of the property, 274 convicts — 164 have been sentenced to life, including 157 for their roles in the genocide — are allowed to work in the fields, where they grow beans, soy, potatoes and more. Vincent, who is serving a 30-year sentence, is one of them. He also takes care of the biogas system and participated in its realization. "Thanks to this energy, we've saved a lot of time for cooking," he says. "And as far as maintenance goes, we just need to check the pipes aren't leaking."

In the kitchens are huge cooking pots filled with uncooked corn. Some 30 convicts are moving about the burners fed with the biogas. "We're preparing the second meal, our last one of the day," one of them says. He shows the corn they harvested in a field outside the prison. It will be served to the prisoners and will end up, as in an endless circle, fertilizing other crops.