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Low cost Biodigesters to produce biogas and natural fertilizer from organic waste
Low-cost Biodigestores in practice

Biodigesters

A biodigester is made of plastic (tubular polyethylene, PVC or polyethylene geomembrane). Other materials include 6" pipes for inputting manure and water, and outputing biol. For biogas ½" or ¾" irrigation tubing can be used, together with common accessories such as taps, elbow joints, tees, etc. All these materials are usually available locally in any country.
Since biodigesters are made of flexible plastic, they must be semi-buried in an open trench in the ground. The size of a family biodigester will vary according to region but an area of at least 1m wide by 8m long will be required. In highlands, a biodigester must be located under a shelter to protect it from the cold, and built in an east-west direction.
The construction and installation of a biodigester takes one morning (if a trench has already been dug), and the installation of all the biogas pipes takes an afternoon.

Biogas

A family biodigester produces around 700 litres of gas per day, enough to cook for about three hours. This gas, called biogas, is produced naturally within the biodigester as the manure ferments in the water. Biogas is very similar to bottle gas and can be used for cooking, lighting gas lamps, heating piglet hoods, etc. Cooking with biogas produces none of the smoke associated with firewood stoves, which is harmful to health.
Biogas is stored in a reservoir made of plastic, which acts as a cylinder. These reservoirs have to be placed near the kitchen and in a protected area under a roof. The reservoirs keep the biogas under pressure and each reservoir has enough for about an hour of cooking. Without reservoirs, the biogas would only last about 20 to 30 minutes.

Biol

Every time a biodigester is loaded with fresh manure and water via the inlet pipe, 80 litres of biol come out at the other end (outlet pipe). Biol consists of a mix of manure and water that has fermented in the biodigester. Biol is a liquid fertilizer that can completely replace chemical fertilizer. After it is filtered, biol can be sprayed on crops as a foliar fertilizer, or can be placed directly on the soil or into irrigation canals. Producers report an increase in crop production from 30% to 50%. In addition, biol protects against insects and helps plants recover from damage by frost.

The basic elements of a biodigester

To use a biodigester it is necessary to have animals, water and appropriate information.
A sufficient number of animals is required to produce a bucket of fresh manure a day (20 litre bucket). It takes two to three cows or six to seven pigs to produce a daily bucketful (20 L) of fresh manure. Manures from different animal (sheep, pig, donkey, etc) can be mixed but half the bucket must necessarily contain cow dung. To facilitate the collection of dung the animals should sleep in a corral or barn near the house. If the animals are loose and not brought in at night, it will be more difficult to collect the manure.
It is also necessary to have access to water, from a river, well, or spring. The ideal is to have a reservoir of water which is full all year, because 60 litres are needed every day. If there is a lack of water for a few weeks, you can still have a biodigester, but it will be more difficult to make it work.

Loading the biodigester

A family biodigester needs to be loaded each day with fresh manure and water, depending on the type of animals on the farm.

Maintenance

Correct maintenance involves looking after the 3 parts of the biodigester
• Safety valve: made using a soda bottle, which fills with water. This bottle lets out excess gas. It is necessary to maintain the water level in the bottle.
• Sulphide Filter: To remove the smell (which is not manure) of biogas, it must be filtered by passing it through steel wool (normally used to wash pots). When the kitchen starts to smell, the steel wool must be changed.
• Water condenses in the biogas pipes, which from time to time must be drained. This water is easily removed through the taps or the joints of the biogas pipes.

Biodigester care

Family biodigesters aren’t expensive but they are delicate. Since they are made of polyethylene, which is thicker than the plastic used for greenhouses, they can easily rip if not protected. Small holes up to 5 cm can be repaired, but not if they are larger. To avoid problems, such as an animal accidentally treading on the plastic tank, it is advisable to place a wall around or near the biodigester. Similar care should be taken of the reservoirs that store the biogas. They should be located in a covered, enclosed area intended only for the reservoirs.

Duration

A well-looked after biodigester can last from 5 to 6 years. After this time the plastic needs to be replaced, while other materials last longer. Some well looked after biodigesters have lasted 11 years without replacing the plastic. However if care is not taken to stop animals from entering, a biodigester can be broken within a few days after installation.