Biogas systems use bacteria to break down wet organic matter like animal dung, human sewage or food waste. This produces biogas, which is a mixture of methane and carbon dioxide, and also a semi-solid residue. The biogas is used as a fuel for cooking, lighting or generating electricity. Using biogas can save the labour of gathering and using wood for cooking, minimise harmful smoke in homes, and cut deforestation and greenhouse gas emissions. Biogas plants can also improve sanitation, and the residue is useful as a fertiliser.
Individual biogas systems are already benefitting several million households in Nepal, India, China and elsewhere. Larger systems are also used, for instance to process farm waste in Germany, and at sewage treatment works in the UK. A simple biogas plant has a container to hold the decomposing organic matter and water (slurry), and another to collect the biogas.
There must also be systems to feed in the organic matter (the feedstock), to take the gas to where it will be used, and to remove the residue.
In fixed dome biogas plants (the most common type), the slurry container and gas container are combined, so that the gas collects under a rigid dome over the slurry. As the slurry breaks down, the biogas which is produced pushes some of the slurry into a separate reservoir. When the biogas is taken off, the slurry flows back.
A biogas plant needs some methane-producing bacteria to get it started. Once the plant is producing biogas, the bacteria reproduce and keep the process going. Cattle dung contains suitable bacteria, and a small amount of cattle dung is often used as the ‘starter’ for a biogas plant, even when it is not the main feedstock.
