Biogas is a simple, inexpensive way for families to create their own energy.
Cassim Bilali, Director at Care for the Earth Centre in the Rarieda district in Kenya, told us about this alternative source of energy they built at the Centre.
Many residents in Kenya – as in South Africa – live below the poverty line. They’re not connected to the national electricity grid, so they rely on firewood for heat and cooking. But most rural families also own at least two head of cattle and, argues Cassim Bilali – who started the Care for the Earth Centre in the Rarieda district of Kenya in 1992 – these families could produce enough biogas from their cattle to cater for their daily cooking needs.
Biogas is a clean alternative to wood or paraffin for cooking, heating and lighting. Using biogas instead of wood would also reduce the pressure on forests. As it is a smoke free energy source, it results in less pollution than fuel from wood and also has no negative impact on the health of children, the elderly and frail people.
In addition, cooking with biogas is much faster than when using wood and needs no preparation – only the strike of a match.
Only a small quantity of manure is needed to produce enough biogas to satisfy the cooking needs of a family, Bilali says. Two cows would provide enough manure to produce 20 minutes of cooking per day. Care for the Earth feeds about 50kg manure into their digester each day and this produces sufficient energy for four to five hours of cooking.
“Families who don’t have access to cattle can collect manure in communal areas. Excrements from any animal and humans can be used – even food scraps, but it would probably be better to use these scraps to make compost,” says Bilali.
To produce biogas, organic waste is washed into an airtight area where it’s fermented. Methane gas, carbon dioxide and effluent are produced in the process. Bilali uses the effluent to fertilise his orchards and vegetable gardens.The methane gas is used for cooking.
A new biogas digester will produce mainly carbon dioxide the first few times it’s used. Once the aerobic bacteria have used up all the oxygen in the tank, anaerobic bacteria (which make methane) will take over.
Depending on the size of the digester, it can take four to nine weeks to create enough biogas to be flammable. But once the process has started, biogas production is quite fast, provided that new stock is regularly fed into the digester. Care for the Earth supplies their digester with feed stock in the evening and this is fermented the next morning.
Bilali is using a middle-of-the-range bio-digester. It has five main components: The digester body, the gasholder, the slurry inlet, the slurry outlet and the gas use point. Brick chambers serve as the digester body, inlet and outlet; and a metal drum is used as the gasholder. A plastic drum could have been used, but this would have reduced the lifespan of the digester.
Bilali uses a fixed pressure digester. The drum floats up and down as the volume of gas inside the gas holder fluctuates, and the weight of the drum creates a fixed pressure in the system. The advantage of this system over fixed-volume digesters is that the digesters can be used to supply gas to appliances such as gas fridges or gas generators, that require constant gas pressure. Having a floating drum is also much cheaper than using high-pressure cylinders, regulators and compressors to regulate the pressure inside the digester.
In 2010, the system cost Bilali about 50 000 Kenyan shillings to put up. He estimates then that a family can build a simple digester, on which they can cook directly, for less than 10 000 shillings.
Digesters must get at least an hour’s sunlight per day to be able to regulate temperatures in the system. The fermentation process slows down in winter, when it’s colder.
Feedstock is washed into the digester with water. Care should be taken not to put too much water into the digester, because this could reduce the volume of gas created.
The value of trees
Bilali plans to collect some of the methane produced at Care for the Earth in tyre tubes so that he can give some to families and so allow them to experience the benefits of biogas first-hand. He also wants to encourage every family that uses biogas to plant at least two trees at their homes.
“People don’t realise the benefit of trees. They never planted trees, because the trees have always been here. But deforestation is becoming a huge problem. In addition to providing shade, trees can play a significant role in reducing the impact of climate change,” he says.
Bilali has planted a garden featuring a huge variety of trees to show others how trees can beautify their surroundings. He adds that, through planning, farmers can generate income from trees – if, for example, the trees have medicinal value or provide fruit. Fast-growing trees can also be sold for their wood to stop people from stealing wood in forests.
Contact: Mr. Cassim Bilali Director Care for the Earth P.O Box 34 – 40602 Ndori, Kenya