Affordable Biogas digester unit

70 percent of Kenya’s total energy demand is met by wood fuel and charcoal. The pressure on our forests for firewood and charcoal has speeded up severe deforestation. This has led to the destruction of so many water catchment areas and in the long term, cause the drying- out of agricultural land, which would further diminish the country’s already limited agricultural potential.

A farmer feeding a polythene digester with mixture of manure and water (slurry). Our photo does not show the roof that protects the digester from sunlight.

One possible alternative to firewood, especially for farmers, is the installation of a biogas unit using the tubular Polythene (plastic) Biogas Digester. It is an efficient and a cost-effective technology. The costs for a biogas digester made from polythene tube vary, but for a 2-cow unit, one can spend between Ksh 5,000 and Ksh 8,000. It lasts for about four to five years, if well maintained. Findings at KARI-Embu have shown that the Polythene Biogas Digester fed with dung from two dairy cows can supply 30 to 50% of the total energy needs of a typical rural household of about 5 to 8 people, with up to 60% saving on wood fuel, which is a substantial saving on costs.

Biogas is clean and relatively simple to generate, but it is an under-exploited source of energy. Small-scale farmers with two cows can produce enough biogas to cook for a family of up to 8 people.

Biogas is a relatively cheap source of renewable energy to meet our requirements for cooking or even lighting. It is a combustible gas that is produced when organic matter such as farm yard manure is digested inside airtight containers called digesters. However, dung from cattle, sheep, goats, pigs and poultry is the most ideal since it is easily available in most rural households in the country.
The black plastic tube

There are many ways of producing biogas. A relatively easy and cost effective biogas unit is a Polythene Biogas Digester. This is a black (or white), 10 m long polythene tube (1000 mm gauge) like the one now being used by farmers to store silage. This is why this type of biogas unit is also called tubular digester. The digester is quite simple to install because the material used is affordable and readily available in most big hardware shops. It is built within a short time. However, the design of the tunnel and the handling of the plastic tube need extra care.
The 10 metre digester is the most ideal as it produces adequate gas that can meet up to 50% of the daily energy needs for cooking for a family of 5 to 8 people. It is particularly suitable for farmers who have a zero-grazing unit with at least two cows because the collection of cow dung is easier. The digester can be directly connected to the animal shed in order to collect adequate manure, urine and water and to reduce the handling. However, farmers with free grazing systems can also adopt the digester since the daily amount of dung required to maintain gas production is low and dung can be collected from the grazing areas.
Famous in many countries


The Polythene Biogas Digester was developed in Colombia; the technology is widely used in Vietnam and Colombia as well as in other countries of Asia and Latin America. Biogas is clean and does not produce smoke; therefore it reduces respiratory eases experienced in households which use firewood or charcoal. After a false start in promoting this energy source in Kenya in the mid-1990s, it was successfully re-introduced by KARI-Embu in 2005. According to Erastus Kiruiro from KARI-Embu there are around 300 tubular digesters in use, mostly in Central Kenya but also around Nairobi.

*Sources: Erastus Kiruir (KARI-Embu), William Ayako (KARI Naivasha). More information you can get from the Kenyan based company JuaNguvu Ltd in Mombasa which is specialized in the building of biogas units and solar energy systems; the company offers trainig courses for building biogas units. Contacts: http://juanguvu.com, e-mail: info@JuaNguvu.com

 

 

 

 

 

 

 

 

 

 

 

I no longer buy firewood

David Muriithi is a small-scale farmer with 1 ¼ acres of land in Kagumo village in the outskirts of Kagumo town. Due to the high population, the area suffers an acute shortage of firewood.
For many years, Muriithi was forced to buy firewood for his family at the local market, where 1 cubic metre cost him between Ksh 800-1200 depending on availability. In the year 2005, Muriithi attended a farmers’ field day near Kagumo town where officials from KARI showed farmers how to produce biogas for cooking using a plastic digester. Seeing how simple it was to produce biogas, he immediately bought the necessary components and with assistance from KARI, he set up his own digester next to his zero-grazing shed with two cows. Within a short time he was already producing his own biogas which met all his family’s cooking needs. The by-product is very high quality manure which he applies on crops on his farm that include tea, maize, potatoes, beans and peas.
Farmers learnt from Muriithi

“Except for some little charcoal to warm the house I no longer buy firewood because my wife now uses biogas to cook all the meals for my family of four. I save a lot of money that I would have used to buy firewood. Besides biogas is safe and clean as it does not produce smoke and we now live healthier”, he says. Many other farmers in the area have learnt from Muriithi and have already set-up their own biogas units.

Biogas is a low-cost energy source

TOF asked Erastus Kirui* a few questions regarding the biogas unit:

What are the reasons for increased adoption of this biogas technology by farmers?

We use a better approach: The farmer based technology transfer that incorporates a component of capacity building and scaling-up based on farmer-to-farmer networks.

Do the farmers share their knowledge?

An ideal biogas digester should be put up next to the zero grazing shed and in close proximity to the house and kitchen to facilitate easy flow of gas. This well done digester should also be covered against sunlight.

Yes, they do. This is possible because of the low technical requirements on the installation and management.
Are many small-scale farmers apprehensive of the costs?
The costs are relatively low. Farmers pay about Ksh 5,000 for a Polythene Biogas Digester, but they can save money for firewood, charcoal and kerosene. In a nutshell, it is a low-cost domestic energy-source.
We understand that one of the disadvantages of this biogas models is the short lifespan of the digester tube. Is this true?

This is no doubt a critical point. We have seen that the digester material (the plastic tube) has a lifespan up to four, even five years. But this needs a good management. When farmers discover the benefits, they really do everything to protect their biogas units.
*Erastus Kiruiro works at KARI-Embu and is co-author of the KARI-brochure “Biogas production”. KARI Technical Note Series No. 24, January 2003. Erastus Kiruiro contact: 0722 30 38 81

Any farmer can afford a biogas unit

Making a plastic digester is easy. But any farmer intending to set up a unit should seek advice from technical personnel. They should explain to them the minor details to ensure the system works efficiently. The plastic digester should be handled with care.
1. Prepare a horizontal trench on the ground in a good part of the farm preferably near the zero-grazing unit. The trench will enable the polythene tube to hold the digester in place. The trench should be trough-shaped with a top width of 65 cm, a bottom width of 50 cm, a depth of 65 cm. The length of the digester is variable depending on the number of animals but a digester measuring 8 to 10 m long is recommended for a 2-cow or 8-pig unit. The trench should have firm sides to avoid soil from collapsing, and a gentle slope on the floor (about 5 percent) to ensure outflow of exhausted slurry.
2. The two sides of the digester are fitted with the 4“PVC pipes measuring about 1 m to serve as inlet and outlet for the slurry. The PVC should be tied to the digester using rubber straps made from old vehicle tubes. Biogas cannot escape through these pipes since the pipes remain immersed in the dung inside the digester.
3. A small hole (about one centimeter in diametre) is punctured through the wall of the digester about 1 metre from the inlet end. A small piece (about 30-cm long) of the normal PVC water pipe (1.2 cm or ½“ diametre) is then inserted into the digester and an air-tight joint made using rubber straps. (You can cut this small piece from the PVC pipes which will eventually be used to deliver the gas from the digester tube to the jiko (kitchen).
4. The digester is then laid horizontally into the trench with the inlet, outlet and gas tube facing upwards. 5. The cow dung accumulated over time is mixed with water at a ratio of 1:2 and the mixture poured into the digester through the inlet pipe until the digester is about three quarters full; this usually occurs when the mixture starts flowing from the outlet pipe.
6. The external end of the 30 cm-long pipe is then fitted to other PVC water pipes using elbow joints. These pipes will eventually deliver gas from the digester to the kitchen.
7. It takes two or three days for the dung in the digester to start producing gas. Once the digester starts swelling, this is an indication that it is producing biogas. To ensure continuous gas production, the system should be fed with about 1 to 2 buckets (the normal 20-lt capacity) with the same mixture of cow dung and water (ratio of 1: 2) daily.
8. The digester should be covered with light materials such as grass straw and maize stalks to protect it from direct sun rays (ultra-violet radiation). A fence of fine wiremesh or closely spaced wooden slats should be used to protect the digester from damage by children, pets and livestock. The plastic digester should be handled with care.

Photos courtesy of JuaNguvu of their demonstration plot in Mombasa. JuaNguvu PO Box 1779 – 80100 GPO Mombasa Tel: 0722 87 37 38 email: info@juanguvu.com

Important tips for biogas users

Careful handling: The plastc digester should be handled with care. It should be covered and protected. The inlet and outlet should be airtight as well as the gas outlet.
Regular feeding: A 5 m3 digester requires 19 kg of cow dung and 47-57 litres of water to produce enough gas for a day. Under-feeding reduces the amount of gas produced. If a green looking slurry comes out of the digester, this is an indication that it is overloaded.
Cow dung: Cow dung is the ideal substrate for bio-digesters because it is not acidic. If livestock wastes and garbage have to be used, cow dung should be used as a starter substrate.
Effluent recycling: Recycling some of the digested slurry improves the performance of the bio-digester. This is important when the digester is still new because the used slurry contains more of the biogas producing bacteria.
Gas pressure: The amount of gas produced depends on the size of the biodigester, its feeding regime, type of substrate and environmental conditions such as aerial temperature (the warmer, the better). The mean volume of a 2-cow bio-digester is about 5 m3. This will produce enough gas to cook for about 3 hours. Within this period, gas pressure drops and there is need to place an object weighing about 3-5 kg at the top the digester to increase the pressure and therefore flow of gas to the kitchen.
Temperature: Maximum gas production will occur at 35-40°C. Gas production declines as temperature drops and will cease at 10°C.