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 (CH4) or natural gas and carbon dioxide (CO2) , and a liquid 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 to process farm waste in Germany and at sewage treatment works in the UK.
How biogas works
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.
How biogas plants are used
Rural families often use animal dung as the feedstock for a biogas plant. The dung from two to four cows (or five to ten pigs) can produce enough gas for all cooking, and sometimes lighting too. The family needs to feed the plant once each day with a mixture of dung and water.
Food waste can also be used as the feedstock. Food waste breaks down and produces gas more quickly than dung, so the slurry does not need to be held for as long; these plants are therefore smaller and more suitable for urban homes. A family or community using just their own food waste can replace between 25% and 50% of their cooking fuel.
Larger-scale biogas schemes can produce sufficient gas to generate electricity. This is frequently done in sewage treatment plants in the UK, and there are a number of large farm-based plants in Germany and elsewhere.
Biogas plants can work well for many years, provided that they are constructed well and checked regularly. If the plant is made from masonry, care must be taken to make sure that the structure is water-tight and gas-tight. For this reason a prefabricated system where the quality assurance takes place in a factory is preferred.
What are the benefits of using biogas? Instant heat, Cleaner cooking energy, saves time, money and environment
The primary benefit of biogas is replacing other fuels for cooking. In rural areas biogas usually replaces wood fuel , which is often in short supply. Studies show that households with biogas plants save three hours per day.
Collecting dung and feeding it to a biogas plant takes much less time than collecting wood and preparing a cooking fire. Biogas is available whenever it is needed and cooks food quickly, so it is quick and easy to prepare hot food before children go to school.
The actual details of the above will vary largely between rural and urban applications, and the size of the interventions. In summary, however, biogas systems offer multiple benefits. The digester-effluent is usually a good fertiliser, and, if connected to latrines, biogas plants can provide valuable sanitation services. For cooking and other thermal household tasks, it is simple and reasonably efficient to use the gas directly in conventional low-pressure gas burners. Biogas can also provide lighting when used in mantle lamps. In societies where suitable feedstocks are readily available, small family-sized biogas digesters have considerable potential.