Biogas refers to a gas made from anaerobic digestion of agricultural and animal waste. The gas, a mixture of methane and CO₂, is used

1. for direct combustion in cooking or lighting applications;

or

2. to power combustion engines for motive power or electricity generation.

The technology is particularly valuable in agricultural, waste treatment or animal processing units where there is excess manure (e.g. pig, cattle, chicken, human) or farm waste.

With biogas technology, waste (called slurry) is stored in specially constructed containers while being digested. There are a number of technologies used to accomplish this:

• Batch type digesters treat a large amount of material at once. They are used for large scale application

• Continuous flow units add and remove waste material on a daily or regular basis. They are best suited for small-scale domestic applications.

Besides producing the fuel gas, biogas digesters have the added benefit of producing a high nutrient slurry fertilizer and encouraging better sanitation on farms.

Digester temperature is an important factor in maintaining the bacteria necessary for digestion.

This is one reason why fixed dome digesters tend to be more successful in areas with extreme temperature fluctuations.

Gas production is dependent upon digester temperature, fermentation or retention time and the feedstock material.

Small scale farmers that keep zero-grazed pigs (>10) or dairy cows (>3) are good candidates for installing household biogas units. However, a biogas unit will only yield good results if it is properly planned, constructed, operated and maintained. Regular supply of water is essential for operation of biogas plants.

Information required to design and install a biogas digester include:

• Size of family and daily cooking (and lighting) requirements.
• Availability and amount of feedstock (water, number and type of cows, pigs etc)
• Materials available on site (bricks, etc) for construction of digester.

Using this information, the type of digester, the required digester volume and retention time can be determined.