Various types of biogas digesters have been developed including floating drum, fixed dome, and plastic bag models. The most reliable design is that of the fixed dome type, made of masonry and/or concrete, largely underground, and installed in the yard of the family.
Natural cycle
Biogas optimizes farm economy. The biogas plant is a sustainable system in which all the components form a natural cycle. The biogas is used for energy emitting CO2 and H2O, the minerals are going back into the soil and the organic waste will improve the structure of the soil.
What are biogas digesters about?
A biogas digester takes animal waste and turns it into a useful fuel (methane) for cooking, lighting, and heating. Biogas originates from the anaerobic digestion of organic material. The daily operation of a biodigester mainly consists on feeding the plant with a mixture of organic waste (dung) and water. Through the digestion process, the waste not turned into biogas is sanitized through: predation by anaerobic microbes; and the absence of oxygen. This sanitized waste effluent can be used as an organic fertilizer when diluted.
Benefits
Substituting conventional cooking material such as, fuelwood, briquettes, and dung cakes, not only saves fuel costs, but also reduces the workload of women and children. Equally importantly, the indoor air pollution associated with cooking on inefficient wood stoves is virtually eliminate with biogas. In addition, the biogas process itself is carbon neutral, but substituting unsustainable woodfuel (deforestation) contributes to the global reduction of greenhouse gas emissions.
Users
On average, farmers with at least 2 cows or 7 pigs, or a flock of 170 poultry, can generate sufficient gas to meet their daily basic cooking and lighting needs. Within the same design, different plant sizes can be constructed to allow for the actual livestock holding and biogas requirement of the family.
source: (Sasse, L. et al. (1991) Improved Biogas Unit for Developing Countries)
Usage
Operation mainly includes daily feeding of the installation with a mixture of dung and water. This is not very time consuming, usually taking up to 20 to 30 minutes per day, for farmers with livestock on-site and access to water. Removing the fermented material takes place automatically as the bio-slurry is discharged into a compost pit through a channel. These types of (fixed dome) biogas plants have a shelf life of over 15 years. Maintenance is restricted to occasional checking and, when necessary, repair of piping and fittings. The plant itself, when operated properly, needs little care. Total investment costs range from $600 to $900 in Africa. The costs in Africa are substantially higher than in Asia ($300-400) because of higher transport costs and costs of cement.
