A. Introduction

Biogas is produced by the anaerobic fermentation of organic material. Biogas production can be considered as being one of the most mature biomass technologies in terms of the numbers of installations and years of use in countries such as China and India It has the potential for multiple uses, e.g., cooking, lighting, electricity generation, running pumpsets and other agricultural machinery, and use in internal-combustion engines for motive power (Bhatia, 1990). Biogas technology is currently receiving increasing attention due to a combination of factors. Anaerobic digestion can make a significant contribution to the disposal of domestic, industrial and agricultural wastes which, if untreated, could cause severe public-health and water-pollution problems. The remaining sludge can then be used as a fertilizer (providing there is no polluting contamination). It therefore contributes to control of environmental hazards and recycling of nutrients whilst alleviating dependence on imported fuels (Gunnerson and Stuckey, 1986). When manure is used in digesters, the sludge actually performs better as a fertilizer since less nitrogen is lost during anaerobic digestion, the nitrogen is available in a more useful form, weed seeds are destroyed, and the sludge does not smell and does not attract flies or mosquitoes. Furthermore, it yields more useful energy than when burnt for cooking as is the common practice in many rural regions.

Biogas production systems are relatively simple and can operate at small and large scales in urban or very remote rural communities. Almost all current biogas programmes. however, are based on family-sized plants which lose significant economies of scale, are suited more for cooking than electricity generation, and often do not produce enough output just to supply this need. Community biogas plants are more economical and can provide enough electricity for pumping water lighting etc. However, there are social difficulties of organization and equity in the contribution of feedstock and the distribution of costs and benefits.

The basic designs of biogas plants - fixed-dome (Chinese), floating-drum (Indian), and bag (membrane) - have been used in a number of countries for many years. The designs reflect modest optimization for reduced capital costs and increased volumetric gas yields. Biogas can be used in internal-combustion engines using either the gas alone in an adapted petrol engine, or using a mixture of biogas and diesel in an adapted diesel engine. The main advantage of a diesel/biogas engine is the flexibility in its operation since it can operate as a dual-fuel engine using biogas and/or diesel. Usually, dual-fuel engines are so designed that when biogas is available the engine will utilize it, and when it is exhausted, the engine automatically switches over to diesel without any interruption. Diesel engines are reliable, simple to maintain, have a longer working life and higher thermal efficiency than petrol engines and are also more extensively used in rural areas.

Biogas technology has made some important advances in recent years, e.g., in China, Denmark and the United States. However, the technology of anaerobic digestion has not yet fully realized its promised potential for energy production. In industrialized countries biogas programmes have been hindered by operational difficulties, lack of basic understanding, and innovation. In some developing countries, development of biogas programmes has lacked urgency because of readily available and inexpensive traditional fuels such as fuelwood and residues. Lack of local skills, together with high costs, tend to be a significant deterrent to optimization and widespread acceptance of biogas technology (Hall and Rosillo-Calle, 1991).