Anaerobic digestion
In a digestion process, organic matter from human, animal or vegetable waste is broken down by microbiological activity, in the absence of air. This anaerobic process produces a combustible gas, methane, a source of (biogas) energy. The digestion process takes a couple of weeks to a couple of months after which the remaining slurry can be removed, either continuously or batch-wise. Several options are available, ranging from simple digestion techniques to technologically complex designs on a household or municipal scale. A domestic anaerobic digestion technique `fixed dome type' consists of a simple biogas tank with a flat bottom and a round chamber covered with a dome shaped concrete gasholder. The gas is captured in the upper part of the digester. Gas pressure increases with the volume of gas stored, pushing the slurry into a separate outlet tank (see illustration).
| Advantages | Disadvantages |
|---|---|
| Excreta `out of sight'. Net production of clean renewable biogas. |
Gas safety risk. Slurry from digesters has to be removed and treated. |
Costs
- Investment costs vary greatly depending on the overall plant concept. Costs for biogas production increase with decreasing climatic temperatures. Life expectancy ranges from about 20 - 25 years.
- Investment domestic biogas plant in Nepal, (fixed dome types from 4-20m³) US$ 300 400.
- Maintenance costs (8m³ digester) US$ 5.50 US$ 8.50 per year.
- Operational costs - Negligible.
Applying conditions
- Digesters are best suited to warm climates.
- They are most appropriate in rural areas where animal manure can be added to the process.
- The digestion process is sensitive to both temperature and materials. Both need to be controlled.
- Relatively high skills are needed for construction. Operation and maintenance, however, are simple for batch systems.
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External links
- General information about Anaerobic digestion www.itdg.org
- General information www.snvworld.org
- General information http://www5.gtz.de/gate/techinfo/biogas
