Bio-gas is a mixture of gases, usually carbon dioxide and methane, produced when microbes act upon organic waste in anaerobic conditions (in the absence of oxygen). Bio-gas provides a clean, easily controlled source of renewable energy, which may replace the use of firewood or fossil fuels.
In recent years, bio-gas generators have attracted the interest of bio-fuel researchers as they harness methane to produce energy, which would otherwise lead to the depletion of the ozone layer and consequent global warming. They have also attracted the interest of development scientists for their potential to address the energy requirements of rural populations.
 Bio-gas in Nature
Animals that eat a lot of plant material, particularly grazing animals such as cattle, generate large amounts of bio-gas. This is produced not by the animals themselves, but by billions of micro-organisms in their guts. Bogs and lake bottoms, where decaying organic matter builds up under wet and anaerobic conditions, are another source of bio-gas.
Bio-gas also builds up in the biomass present in landfills, where large mounds of garbage are buried under the surface. Bacteria break some of the garbage down and produce quantities of bio-gas. This is sometimes collected and burned to heat buildings near the landfill. bio-gas can contain traces of hydrogen sulphide (H2S) gas, particularly in the case of landfill gas. Care must be taken to deal safely with this gas because H2S can be fatal even in small amounts. To calculate how much waste is needed to power a bio-gas generator, use the bio-gas calculator at Biogas Calculator.
 Did You Know?
- People have been using bio-gas as an energy source for over 200 years. In the days before electricity, bio-gas was drawn from the underground sewer pipes in London and burned in street lamps (which is why they were known as gaslights).
- Using bio gas generators have two benefits -- they consume methane a greenhouse gas, and are a source of fuel.
 How Bio-gas Generators Work
Bio-gas generators are basically large tanks. Shredded plant materials and animal waste are put in them and mixed with water to form a slurry. Many kinds of naturally-occurring anaerobic bacteria arrive with the shredded plant material. These are known as "methanogenic", because they produce methane, the main ingredient in bio-gas. The tank is then sealed so no air can get in. Bio-gas begins to be produced by the anaerobic bacteria once all the oxygen in the generator is used up. The bio-gas forms bubbles in the mixture, and collects at the top of the tank. It is piped to a large balloon-like bag where it is stored until needed.
Eventually, the production of bio-gas in the generator slows down. The mixture of water and manure is replaced with a fresh supply to start the process again. Although the spent material in the generator is unable to produce more bio-gas, it still contains large amounts of plant material and other organic matter. Dried to form a rich black soil, it is spread on fields as a fertiliser.
 Why Bio-gas?
The United Nations Development Programme (UNDP) 1997 report, Energy After Rio: Prospects and Challenges, identified community bio-gas plants as one of the most useful decentralised sources of energy supply. Bio-gas plants have the following advantages:
- They destroy methane, the highly destructive greenhouse gas, so it does not go into the atmosphere and contribute to global warming.
- They provide clean-burning fuel for stoves and lamps instead of using wood or dung.
- They reduce labour, eliminating the back-breaking task of collecting firewood which a majority of rural women have to do on a daily basis.
- They protect the remaining forests by reducing the dependence on firewood.
- They reduce respiratory disorders caused by smoke from cooking with firewood, which especially affects women and their small children.
- They improve village hygiene, since the attached toilets lead to the pathogen-destroying digestion tanks of the systems.
- They yield more potent fertiliser than the original constituent wastes.
- They provide lighting needed for evening study, literacy classes, home and community activities.
 Bio-gas Applications
As methane is hard to compress, its best application is as a stationary fuel, rather than mobile fuel. Bio-gas is best used directly for cooking/heating, light or even absorption refrigeration. It may also be efficiently used to run pumps and equipment.
 Bio-gas Initiatives in India
Bio-gas plants in India were experimentally introduced in the 1930s, and research was principally focussed around the Sewage Purification Station at Dadar in Bombay. The early bio-gas plants were, however, expensive and not very efficient in terms of gas output. Some early models were also prone to burst, so overall, the technology was not viable for dissemination.
However, in 1981, with the beginning of the 6th Five-Year Plan and the formation of the National Project for Bio-gas Development (NPBD), bio-gas plants were set up all over the country, where gas was extracted from cow dung and agricultural waste. Currently, there are about 3.9 million family-type bio-gas plants in the country.
- The Modern Importance of Biogas
- Beginners Guide to Biogas
- India Biofuels Annual