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Biogas plants use bacteria to turn wet organic matter into methane and carbon dioxide. A biogas plant consists of a container to store a large volume of slurry (finely divided solid organic waste in water) and a means to collect the gas and exclude air. Air must be excluded because the methane-producing bacteria are anaerobic, i.e. they do not work when oxygen is present. The most common source of these bacteria is a cow’s gut, so cattle dung has been a favoured feedstock. A small amount of cow dung is often used to start the digestion process if other types of organic matter, such as other animal dung or food wastes, are used. The bacteria work best at 35°C, so in colder climates the container must be insulated or heated to maintain this temperature.

Sewage plants use anaerobic digestion to reduce the volume of solid wastes and reduce the smell. In recent years, many UK water companies have recognised the potential of the gas produced and are generating electricity from it to sell to the grid. Food wastes in landfill also generate biogas and this is now also being captured and used to generate electricity on many sites.

Biogas was originally developed in India in 1859 and inspired the use of sewage plants in the UK and elsewhere. In the 1950s, Ram Bux Singh designed a biogas plant that could be used by farmers using cattle dung as the feed material. These early domestic models used a floating steel gas drum to collect the gas. In the 1970s, people in China developed the displacement gas collecting system, in which the slurry is displaced from a fixed dome into a separate liquid reservoir as gas collects This model is now the most widely used in most places. A typical plant requires a container between 3 and 6 m3 in volume to contain the slurry and gas .

The large-scale biogas programmes plants to individual farmers, who feed them every day with a mixture of water and dung from their own animals (two to four cows or five to ten pigs), so they can use the gas produced for their own cooking. Many plants, especially those in China, have a latrine attached, so help to provide sanitation as well as cooking gas. The semi-solid residue that comes from the plant is a good fertiliser with minimal smell, so is used to improve crop production and reduce the use of artificial fertilisers. The fertiliser value can be enhanced by mixing the residue with crop waste to make compost. This can be further improved with vermi-culture - feeding it to earth worms for additional processing.

The main benefit of a biogas system is that other fuels for cooking are replaced. In rural areas, biogas usually replaces fuel-wood, which is often in short supply. In addition, people do not have to collect firewood and light a fire for cooking, which can save up to four hours of work per day. Collecting dung and feeding it to the plant takes less than half an hour and does not need to be done early in the morning, so there is time to give children a meal before school. Biogas burns with a very clean flame, so women do not have to breathe wood smoke, which is a major cause of respiratory and eye disease. In urban areas, biogas replaces LPG or kerosene saving fossil carbon dioxide.

Food wastes and sewage can be used to generate biogas. If a family or community (school or hostel) rely on their own wastes, they can save between 25% and 50% of the LPG they use for cooking. If the users can supplement the feed with other materials, such as the leftovers from a food processing enterprise, they can completely replace LPG. Food waste produces gas more quickly than dung, so these plants are smaller (typically 1 m3 in volume), and thus suitable for urban homes.

Community biogas schemes, in which people provide dung or their own food waste to a central plant and share the gas, have seldom been successful. People seem to be unwilling to share the work and benefits equitably. However, the development of systems that use food waste as the main feedstock and supply gas to fuel an engine to generate electricity have potential, especially if the scheme can be financed through the avoided cost of waste disposal.

The cheapest biogas plant designs are made mainly of masonry, either brick or concrete. The key to strength and gas containment is using curved shapes - cylinders and domes - and making sure that the plant is impermeable to gas. Steel and plastic (high-density-polythene, HDPE and glass-reinforced-plastic, GRP) are used in some designs. Such designs can be pre-fabricated and thus installed very quickly, but they cost more.

Another design that has proved popular in Vietnam and a few other places is the bag digester. The simplest version is a long plastic tube, with a feed pipe at one end and a pipe for the semi-solid residue to come our at the other. The slurry in the tube generates gas, which inflates the tube. Other versions use a plastic tent over a slurry pit to collect the gas. This design is very cheap, but the plastic can fail after only a few months. Other materials, such as butyl rubber, last longer but are more expensive.

The number of domestic biogas plants in the world is very difficult to estimate. In the early 1980s, China boasted of the installation of 8 million plants, but these were of low quality and appear not to have lasted. The programme in China has continued, with a better quality design but at a slower rate. India has moved more slowly and an estimate of plants built was over 2 million in the year 2000. Nepal, with 156,000 biogas plants in 2005, claims to have more per capita than India. There are programmes in many other parts of the world, such as Vietnam, Brazil and Tanzania, but they are smaller. People have attempted to set up projects in Kenya, for example, but have met with limited success.

The economic viability of biogas technology depends on the cost of the fuel being replaced, and whether there are other economic benefits (for instance, avoided waste disposal costs). Government schemes enabled the programmes in India, China and Nepal to succeed. With the increase in costs of fossil fuels and the potential for carbon-offset finance, biogas is becoming more viable