Biogas digesters, also known as Biogas Plants, Anaerobic Digestion Plants, are often considered to be something new by those in the developed nations, however, they have been widely used for many years in developing countries, especially India and China, as firewood for cooking becomes scarce. Other countries from Honduran farmers to the tiny South Pacific island nation of Tuvalu, are able to harness the methane gas created naturally from decomposing manure and other organic materials. Besides producing the fuel gas, these Biogas Plants using the process of anaerobic digestion, have the added potential benefit of producing a high nutrient slurry fertilizer and providing better sanitation on farms.
The win-win process goes even further though because the emission of pollution from the digester is very much lower than without the digester, as well, so they can help to reduce river and groundwater pollution at the same time. A functional biogas digester system applies the science of microbiology and involves the development of renewable energy.
With biogas technology, the farm or community/human waste (called slurry) is stored in specially constructed containers while being digested. Biogas Plants can accommodate manure handled as a liquid, slurry, or semisolid depending on the type of reactor design used. Biogas digesters take the biodegradable feedstock, and convert it into two useful products: gas and digestate (solid and liquid). Both of these are useful, and this is certain to be one of the main reasons why more and more farmers around the world are beginning to install a biogas digester.
Biogas Plants are usually built underground to protect them from temperature variations and also to prevent accidental damage. They not only perform the necessary actions required to keep the bacteria happy and creating the biogas, but designs suitable for farms and communities have been devised for the developing nations which are able to be replicated using items that are cheap, easy to source, and easy to assemble.
The typical developing nation biogas digester is constructed in a pit which is excavated by a trained labourer with assistance from one or more members of the household or community. A very common design for biogas digesters has a volume of 8m3, some are larger at 10-15m3, and provide enough gas for a two-ring stove and a light.
A biogas digester can function well on human and animal waste. We know of one Anaerobic Digestion Plant which is based upon pig farming, and is built below the pigsty. The pigsty is insulated, and the plant produces some heat as well, which helps to keep both the pigs and the biogas digester in the warm conditions both need during cold winter weather.
A biogas plant consists of one or more airtight reservoirs into which a suitable feedstock such as cow dung, human waste, or even abattoir waste, is placed, either in batches or by continuous feed. It is mixed and the solid and liquid digestate is removed on a regular basis. The methane bubbles to the surface, and in the simplest systems, it is stored as a big bubble above the liquid. In more sophisticated systems the methane is stored in separate tanks, for use when needed.
There is an enormous potential for benefit from the increased use of anaerobic digestion in Africa. An AGAMA Energy fact sheet estimates that in South Africa there are 400,000 households with two or more cows and no electricity that could make use of biogas digesters. An article dating back to 30 November 2005, in the Rwandan newspaper, The New Times, states that the Institute for Scientific Research and Technology in Kigali plans to install some 1,500 biogas digesters by 2009 in the imidugudu settlements. These are villages where rural Rwandans were relocated after the genocidal wars of the mid-1990s.
Worldwide, about 16 million households use small-scale biogas plants, according to Renewables 2005: Global Status Report, a study by the Worldwatch Institute.
It has been reported that in India a domestic biogas digester unit capable of producing energy for an entire family’s cooking needs can be installed for between R5 000 and R8 000, that is less than 200 US Dollars. Facilities best suited for biogas plants typically have stable year-round manure production, and collect and feed the digester with manure daily.