Polythene tubular biodigester technology (PTBT) has proved itself as a great help to small-scale farmers as a cheap and simple way both to produce gas and recycle plant nutrients into high quality fertilizer. The technology appeals to rural people because of the low cost of installation and therefore of the gas, and the significant environmental improvements it offers. This article details the benefits of this plastic biodigester and the design of a low-cost model.

New perspective
In the past, biodigesters have been considered mainly as a way to produce combustible gas from waste organic matter. However due to an increasing emphasis on the sustainable use of natural resources in farming systems, biodigesters began to be considered in a much wider perspective, specifically in their potential role for the recycling of plant nutrients. The objective has been to reduce dependence on inorganic fertilizers and make it easier to grow organically.

Value
The introduction of the low-cost plastic biodigester, based on the use of tubular polythene film, has successfully positioned this technology within reach of a greater number of end users (it is estimated that there are more than 30 000 users of this technology in Vietnam).

Developers focused on integrating the biodigester within the farming system and have demonstrated that the biodigestion process leads to major improvements in the value of the livestock manure as fertilizer for crops, as well as for water plants or fish cultivated in ponds. And the simplicity of the installation process has facilitated farmer-to-farmer extension of the technology (see 'installing a biodigester’ for a step-by-step guide).

On farm benefits
For farming systems to be sustainable, there should be a close relationship among the different components that interact in the conversion of solar energy and soil nutrients into food of animal and plant origin. When closely integrated into the farming system, a biodigester can:

• Provide a source of fuel for cooking and lighting, reducing the need for fuel-wood and the work of collecting it. This is particularly important for women and children. In addition, cooking with biogas leaves cooking utensils much cleaner, and the absence of smoke improves the health of women and children who spend much of their time in the kitchen and often suffer from respiratory problems as well as eye irritations

• Improve the quality of the manure that is fed into the biodigester, resulting in high-quality fertilizer for crops, as well as for water plants or fish cultivated in ponds

• Improve the sanitary conditions of the farmyard and reduce the spread of parasites and potentially harmful bacteria, by removing and de-contaminating manure and other organic waste matter from the farmyard

• Improve the environment by reducing dependence on fuel-wood, leading to less deforestation. If the biogas is used this also reduces the emission of methane (a greenhouse gas contributing to global warming) into the atmosphere

Boosting Yield
The study of the changes that take place in the substrate during the digestion process have received relatively little attention and have traditionally been concerned mainly with environmental and health issues. However, recently attention has focused more on the fertilizer value of the effluent. For example, it has been shown that the biomass yield and the protein content of cassava foliage significantly increase when the cassava is fertilized with biodigester effluent from pig or cow manure, as compared with the same amount of nitrogen applied as raw manure.

Similar findings were reported for duckweed grown in ponds fertilized with either the effluent or the raw manure: Reports from China claimed higher productivity in fish ponds when biodigester effluent was used, in comparison with raw manure. In Cambodia, research has confirmed the superior value of effluent from a biodigester fed with pig manure, compared with the same manure applied directly to the pond at comparable nitrogen levels.

The process of fermentation in biodigesters transforms organically bound carbon into gaseous carbon dioxide and methane. The anaerobic (without oxygen) process and the long time in the biodigester kill most organisms, including intestinal parasites, which can cause diseases. In this way, livestock manure is improved chemically as well as biologically through the fermentation process.

Affordable design
Most developmental work with biodigesters has been approached from an engineering viewpoint, aiming to maximize gas production and efficiency by improving the design and construction of the biodigester. There has been very little change in the basic designs of the floating canopy system developed in India or the liquid displacement system developed in China. The relatively high cost of these systems, and the fact that construction can only be carried out successfully by skilled artisans have been major constraints to widespread adoption. Where they have been introduced, these systems have usually had to be subsidized by government or aid agencies.

However subsidies are now no longer needed for the purchase of the construction materials for a low-cost plastic biodigester, as they can be found in most towns in developing countries.

Optimum efficiency
An essential component of the tubular plastic system is the installation of a reservoir for the gas, preferably in the roof space in the kitchen, as close as possible to the stove where the gas will be used. This is because the gas pressure in the biodigester is very low and if the biodigester is situated some distance away from the kitchen, the flow rate along the gas line will be too slow and insufficient to maintain the flame in the stove.

The relatively fragile nature of the polythene film is a weak point in the system and the mode of operation is relatively inefficient compared with more sophisticated biodigesters. However, the construction cost of the plastic biodigester is very low, as are the demands on skills for construction. The cost of the double layer of polythene film is only around US$10.00 and replacement takes about three to four hours. All the other components can be used again when the polythene is changed.

Growth
The increasing emphasis on the need to develop agricultural practices that are in harmony with the environment and to make maximum use of local resources is creating a favourable climate for the promotion of biodigesters. However, much still needs to be done to further our knowledge of biodigesters as an integrated component of the farming system. We need to improve knowledge of the changes that take place in the biological and chemical characteristics of the substrate during the process of biodigestion, in order to make optimal use of the effluent as fertilizer for soil and water plants and for fish ponds.

In addition, the design and construction of the biodigesters can still be improved, and need to be further developed to reduce installation costs and improve the efficiency of converting the input materials into biogas and fertilizers.


Source: Original article published in LEISA, 2005

Author: T.R. Preston.
Finca Ecológica
Tosoly
AA #487, Santander
Colombia
Email: regpreston@utafoundation.org.

See 'installing a biodigester’ for a step-by-step guide (for further information a manual on this type of biodigester, called “Recycling Livestock Wastes” is available from the UTA Foundation.)