Rwandans using human waste for cooking

It’s around 11:00am. Students are out for break. Some are playing as they wait for next lessons. The school sits on Shyorongi Hill.

It’s around 11:00am. Students are out for break. Some are playing as they wait for next lessons. The school sits on Shyorongi Hill.

Cool air from the surrounding area and hissing sounds from nearby green trees welcome you to the place. In the kitchen, women and men are boiling sweet potatoes. Their work place is neat and tidy. No heaps of firewood or ash.

This is Stella Matunina Secondary, a school using students’ urine, feces and cow dung to produce power for lighting and cooking.

Sister Marie Josée Nyiramukesha says they no longer spend money on expensive wood fuel and unreliable Electrogaz power.

The move comes at backdrop the amount of fuel wood is diminishing while the demand for more energy is increasing. Now Rwandans are turning to biogas as one of the more efficient ways of supplying energy.


The process works through three digesters, which are connected to the toilets by a big pipe. Reactions within these digesters form gas for cooking and for use in the laboratory.

The final process is taken in the third digester where the gas that is generated goes to the kitchen and laboratory for use through metal pipes.

These digesters are well covered to stop the escape of manufactured gas and they are made so that you can not feel, smell or see any reaction within the digesters.

The reaction is taken inside the digesters. The remaining residues are pushed through a huge pipe to a hole which is 5 metres away.

The pipes from the digesters are connected to coils under three saucepans with a capacity of 100 litres. Food for lunch is ready. Donacian Nkomeje, a cook, is busy putting sweet potatoes in different dishes for the students’ lunch.

Surprisingly, gas from these pipes does not smell at all. The residues which are sent to the hole are black in colour but ready for fertilisers after they dry.

Nyaramukesha, who is in charge of discipline, said they are planning to use these fertilisers in gardens. Beside the hole, there are fresh crops including: tomatoes, cabbages, onions, bananas and many other crops.

She also takes me into the school library. Two teachers are busy teaching students about chemistry particles. The pipes from digesters are connected to a bid coil where students are heating chemicals.

Joselyne Nyirabagina is the head girl and says biogas has helped the school to overcome the problem of power in the laboratory.

Nyirabagina , who is also a science student, adds that before they were facing the problem of power because the school was depending on solar power.

According to Marie Josée Nyiramukesha, a teacher in charge of discipline, the project of biogas was funded with a small amount of money from the school and the remaining was paid under PNUD, the micro grant programme (SGP/MGP).

‘‘Biogas is so cheap in terms of costs and it is also environmentally safe,” she said. “We used to use a chuck of fire wood for cooking but we only use a few now because of biogas.’’

Using biogas digesters to manage animal or human waste is not a new idea, but in Rwanda it has been applied on an enormous scale, and with great success.

According to Innocent Kimenyi, in charge of infrastructure in prisons, biogas plants are now running also in nine prisons from different provinces: Remera, Ntsinda, Ririma, Gisovu, Cyangugu, Gitarama, Mpanga, Butare and Miyove.

Prisoners’ feces are converted into methane gas that can be used for cooking. It has reduced the annual wood-fuel costs by 60 per cent, which would usually reach almost $1 million.
‘‘It also cheap compared to solar and hydro electricity power and environmentally friendly,’’ said Kimenyi.

How bio gas is made

The process requires putting a given amount of human or other animal waste into a digester, which ferments it using bacteria to release methane gas that can be captured and then burned as fuel.

The waste (slurry) is stored in digesters which vary in size from large scale units to smaller units suited to domestic applications.

The digester is shaped like a beehive, and built up on a circular, concrete base using bricks made from clay or sand-cement. The sides taper gradually and eventually curve inward towards a half-meter diameter man-hole at the top.

It is crucial to get the bricks laid in exactly the right shape, and to make the structure water-tight so that there is no leakage of material or water out of the digester. Biogas is stored on the upper part of the digester.

The gas storage chamber is plastered inside with waterproof cement to make it gas-tight. On the outside, the entire surface is well plastered and backfilled with soil, then landscaped.

From the manhole cover, the gas is piped underground towards the kitchen where it is used for cooking and in stoves that are insulated with a brick lining.

A 100m3 plant can store 20m3 of gas, but may generate up to 50m3 per day, so it is important that the gas is consumed regularly.

A particular feature of the plant design is a compensating chamber that acts as a reservoir of methane bacteria for enhanced gas generation.

At first, gas pressure displaces the liquid to the compensating chamber. Consumption of gas leads to backflow of the waste from the compensating chamber into the bio-digester; this agitates the waste, circulates the bacteria, and releases trapped gas.

The continuous inputs of waste, and the gas pressure, push digested effluent out of the bio-digester to a stabilizing tank, and from there, to a solid/liquid separation unit. The stabilising tank allows additional gas production.

It is also advisable to completely dislodge the digesters every seven years. Rwanda’s biogas facilities are among the most ambitious in the world, given their size and scope. They range up to 1,000 cubic meters in something resembling a beehive shape.

Other sectors

Using biogas digesters to manage animal or human waste is not a new idea, but in Rwanda it has been applied on an enormous scale, and with great success.

According to Innocent Kimenyi, in charge of infrastructure in prisons, biogas plants are now running also in nine prisons from different provinces: Remera, Ntsinda, Ririma, Gisovu, Cyangugu, Gitarama, Mpanga, Butare and Miyove.

Prisoners’ feces are converted into methane gas that can be used for cooking. It has reduced the annual wood-fuel costs by 60 per cent, which would usually reach almost $1 million.

‘‘It also cheap compared to solar and hydro electricity power and environmentally friendly,’’ said Kimenyi. Biogas is used in factory boilers and in engine generators to produce electricity and heat.

If internal combustion engines are fuelled with biogas to produce electricity, the factory can use the electricity or export it to the power grid.

The harvesting of biogas is an important role of waste management because methane is a greenhouse gas with greater global warming potential than carbon dioxide.

The carbon in biogas was recently extracted from the atmosphere by photosynthetic plants, thus releasing it into the atmosphere adds less total atmospheric carbon than the burning of fossil fuels.

Ends

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