The introduction of low-cost tubular plastic bio-digesters to rural
Thailand: A proposal
Introduction
The production of biogas, a mixture of predominately methane and carbon dioxide
occurs by the process of anaerobic digestion. Biogas can be efficiently produced in
digesters from pig and cow manure and is a comparatively clean fuel source.
The Renewable 2005 Global Status Report states that 16 million households now use
bio-digesters. Most of them are located in China, India and Nepal.
Benefits of household bio-digesters
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Improve indoor air quality. Many households in Thai villages use open indoor
wood fire pits for cooking and light. Chronic exposure to poor indoor air has
been linked to acute lower respiratory infection which is the leading cause of
death in children under the age of five. Tuberculosis and low birth weight
have also been implicated as consequences of poor indoor air.
Reduction of workload on women and girls. Three hours per household per
day has been measured as the amount of time spent looking for and gathering
firewood by the females in families.
Biogas is a sustainable energy source and would reduce the environmental
damage associated with overuse of forested land and watersheds. As sources
of fuel-wood become scarcer, steep hillsides are being cut resulting in an
increase in loss of topsoil, danger of landslides, silting of streams and rivers
and a decrease in reforestation.
Improve community health and sanitation. In many villages in Thailand,
livestock ranges freely. Villagers and their water supplies are exposed to
pathogens found in animal feces. Building household bio-digesters would
require that animals be penned so that the manure can be fed to the inlet pipe.
Using bio-digesters prevents the survival of most pathogenic organisms.
Effluent has a 70% decrease in chemical oxygen demand (COD). There is
more than a 99% decrease in E. Coli. Families with bio-digesters report fewer
insects, cleaner water and a cleaner environment.
Reduction of greenhouse gases would occur from two fronts. Less fuel-wood
would be harvested from forests and the methane from animal waste would be
converted to carbon dioxide and water. Methane is a greenhouse gas which,
according to the International Panel on Climate Change, contains 23 times
more warming capacity than carbon dioxide. This action would indicate
participation in the Clean Development Mechanism (CDM) of the Kyoto
protocol.
In some communities, the implementation of bio-digesters has created
enterprise at several different levels. Local sales and distribution of
components of digesters improves the economy. Biogas technology is an
employable skill. In a number of communities, the effluent has been used as a
rich fertilizer. Some users of digesters have produced enough excess gas to
engage in small business ventures.
Results from a bio-digester program in Viet Nam
Tubular plastic biogas digesters have been installed in Columbia, Ethiopia, Tanzania,
Viet Nam, Cambodia and Bangladesh. 800 plastic digesters were installed in Viet Nam in
3 years. These digesters are 10.2 meters long, have a volume of 5.1 cubic meters and cost
under $100 USD.
From 16 kilograms of pig manure per day mixed at a 5/1 water to manure ratio, 1.24
cubic meters of biogas was produced. A family of six was able to cook three meals per
day.
Bio-digester components
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Transparent tubular polyethylene. The diameter will vary according to the
capacity of the local producing plants, usually in the range of 80 to 125 cm
(equivalent to a circumference of 2.5 to 4 m). The calibre (thickness) should be
between 800 and 1 000 (200 to 250 microns). The length of the tube is determined
by the size of the bio-digester. The most appropriate material is that used for
greenhouses since this usually contains an ultraviolet (UV) filter which helps to
prolong the life of the plastic when it is fully exposed to the sun.
Two ceramic tubes, 75 to 100 cm long with an internal diameter (id) of 15 cm.
Can be concrete tubes or plastic buckets with the bottoms cut out glued end to end.
Plastic (PVC) hosepipe of 12.5 mm id (the length depends on the distance to the
kitchen).
Two PVC adapters (male and female) of 12.5 mm id.
Two rubber washers (from inner tubes of cars) of 7 cm diameter and 1 mm
thickness, with a 12.5 mm diameter central hole.
Two rigid plastic (perspex) washers of 10 cm diameter and a central hole of 12.5
mm. Although perspex is best, they can also be cut from old plastic buckets or
other items made from strong plastic.
2 m of PVC pipe of 12.5 mm id.
Four used inner tubes (from bicycles, motorcycles or cars) cut into 5 cm wide
strips.
One transparent plastic bottle (capacity 1.5 litres).
One PVC elbow of 12.5 mm id.
Three PVC "T" pieces of 12.5 mm id.
One tube of PVC cement.
The burner in the kitchen can be an elbow of galvanized iron pipe or a metal
bucket lined with furnace cement or clay with an iron grill.
Figure 1. Digging the trench for the digester.
Figure 2. A tubular plastic biogas digester, the inlet pipe from the animal pen is in the
foreground and a protective roof has been constructed over the digester.
Figure 3. A diagram of a household biogas digester with reservoir bag.
The recommendation of the University of Agriculture of Viet Nam is to choose
farmers as the initial population to receive digesters. They have the resources and the
motivation to use the technology. 12% had technical problems but most were resolved by
the farmer owners.
The tubular plastic bio-digesters were found to be inexpensive with a fast payback,
only 4-5 months in most cases. The owners appreciated that the technology is simple and
the digesters are constructed quickly.
Some questions to be resolved in order to implement a biogas digester
program in Thailand
 Is the farming population willing to learn and maintain a new technology?
 Biogas digesters will require some new manual labor on a daily basis however
this is balanced against the reduction in fuel-wood labor.
 Livestock would have to be corralled and fed by hand.
 Does useable space for household digesters exist?
 How would the biogas be equitably distributed if more than one household’s
livestock contributes manure?
 Is there sufficient water supply throughout the change of seasons?
 How will the initial expanse be funded?
 Are local people available who could be trained as technical staff?
Commitment would be required from the local renewable energy community, the
homeowners and the Thai government.
Recommendations
 Research should move forward first in the form of a feasibility study on the
introduction of household biogas digesters to rural Thailand.
 The renewable energy community and policy makers in sustainable rural
development should promote the use of low cost locally available materials.
 The Thai government should consider the benefits of providing subsidies to
manufacturers and distributors, financing to households, funding for training,
public awareness and technical support.
References
1. Bui Xuan An et. al. Installation and performance of low-cost polyethylene tube
bio-digesters on small scale farms. www.fao.org/ag
2. Biogas technology in agricultural regions, Tanzania. ces.iisc.ernet.in
3. Bu Xuan An, Biogas technology in developing countries; Viet Nam case study.
www.fao.org/ag
4. Bu Xuan An, Role of low cost plastic tube digesters in integrated farming
systems in Viet Nam. www.fao.org/ag