24 October 2010

Low Cost Biogas Digesters Bring Big Advantages to Farmers

Biogas digesters 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 digesters (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 digester systems 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 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.

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 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.

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.

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 to30 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 digesters, 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 digester systems typically have stable year-round manure production, and collect and feed the digester with manure daily.

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.

Steve Last is also a regular contributor of dog breed related articles at The Dog Breeds Compendium.


wdjoyner said...

Interesting post.

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?

Steve said...

Indeed this is a very sweeping statement and justification is quite hard to do.

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.

Renaldo Little said...

I am an EcoTech Institute student from Mississippi and I am currently studying Renewable Energy Technology. I am only 1/4 of the way through but I have fell in love with biomass. As great of a school EcoTech is, sadly it does not offer a major dedicated to biomass, so I have been doing research on my own. I think the idea of biogas is awesome and the fact that the biproducts are fertiizer and animal feed is perfect for sustainable living. There is a lot of farming in the south and I hope to start a business and make contributions to the advancement of the biogas industry.

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