Biogas digesters 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.
Typical Developing Nation Biogas Digester
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.
Human and Animal Waste Usage
A biogas digester can function well on human and animal waste. We know of one Anaerobic Digester which is based upon pig farming, and is built below the pigsty. The pigsty is insulated, and the digester 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.
Composition and Operation
A biogas digester 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.
Potential in Africa
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.
Rwandan Initiative
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.
Global Usage
Worldwide, about 16 million households use small-scale biogas digesters, according to Renewables 2005: Global Status Report, a study by the Worldwatch Institute.
Domestic Biogas Digesters in India
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 digester systems typically have stable year-round manure production, and collect and feed the digester with manure daily.
Additional Resources
Thinking more about building a digester? Steve Last is web master for the fact filled Anaerobic Digestion Community web site where much more biogas digester information is available.
3 Comments
You say "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." This sounds reasonable. However, do you have any data from nitrogen-level and phosphorus-level testing in the groundwater to support this?
Clearly, if without the presence of an AD Plant the slurry from milking herds, were allowed to flow onto nearby land in an uncontrolled manner and directly enter surface waters and the ground, then the digester would have at that level have "reduced groundwater and river pollution".
However, let's not suggest that such an illegal discharge would reasonably be made by all but a timny minority of farmers, who would end up in court mostly anyway.
When Considering AD there is the compost (fibrous output) and the liquid (digestate) to consider.
There has been UK work on the benefits of compost by WRAP and thie subject is also covered by The Organics Recycling Association.
The benefits of compost and low risk in its use are I think covered by the following link: http://www.wrap.org.uk/farming_growing_and_landscaping/farming_and_growing/farmers/why_use_compost_and_digestate/index.html
However, although the above reference covers compost I didn't see any explanation of the advantages to the environment of digestate spreading, which I had expected to see given the name of the page "why_use_compost_and_digestate".
In fact spreading digestate in large quantities with its high ammonia content could, in my view, be more risky to the environment than if no digestion had taken place.
This is hinted at in a (forthcoming?) piece of WRAP research here: http://www.organics-recycling.org.uk/page.php?article=1948&name=WRAP+research+to+maximise+agronomic+benefit+of+anaerobic+digestate
It says: "The study will explore the impacts of digestate separation on nutrients and odour levels which will be particularly relevant to both AD operators and to farmers.
"The process will involve evaluating a technique for transforming the ammonium in digestate into nitrate and in turn exploring whether the digestate stability test can be made cheaper for the industry as a whole."
So, it seems that the jury is still out on this one, in fact?
Interested to hear other comments.