a biogas plant uses cattle dung, human excreta and other organic material as raw material. The organic material has to be thoroughly mixed with an equal quantity of water. The amount of water used makes it an unviable proposition for areas with little water to spare. This is one reason why the number of biogas users in the country has remained small. Many are dissuaded from using biogas plants because of the water intensive nature of the technology.

"The bacterium that breaks down the organic material into gas requires specific temperature, moisture, and acidity. This makes it is difficult to alter the water content in the input,' says A K Tripathi, principal scientific officer in the Union ministry of non-conventional energy sources (mnes). Biogas plants that are capable of solid state fermentation of cattle dung and agro-residues are being looked into by different agencies across the country. They could provide the answer to the long-standing problem.

"The solid state digestion of cattle dung means that the fresh cattle dung is fed into the biogas plant with limited dilution by water,' says N Shyam, project coordinator (renewable energy) at the Central Institute of Agricultural Engineering (ciae), Bhopal. Compared to the small-scale biogas plant that requires equal amounts of water and dung, solid-state digestion requires a very little quantity of water. "Little or no water is required to operate the solid-state digester.

Another advantage that solid state fermentation gives is that the digested slurry can be can be readily transported to fields for use as compost. In the common small-scale biogas plants, the effluent discharge needs to be spread on the floor and dried for a month before it can be transported to the fields to be used as manure. The effluent of the soild state fermentation biogas is, therefore, much more eaily managed.

Prototypes of the plant have been working satisfactorily for the last two years at three sites in Haryana. The gas yield from a 2 cubic metre (cum) fixed dome type of biogas plant modified for solid-state digestion operation is around 60 per cent more than that for the common plant. A biogas plant of this size suffices for a household.

Commercial production of the solid-state fermentation-based plant is yet to begin. Research is being carried out on improving the technology and making it commercially viable under the Indian Council of Agricultural Reseacrh sponsored All India Coordinated Research Project on Renewable Sources of Energy for Agriculture and Agrobased Industries. Several research institutes are working on the project, including Chaudhury Charan Singh Haryana Agricultural University. Scientists at the university are studying, among other things, the efficacy of using cattle dung as an input at varying temperatures. At the Himachal Pradesh Agricultural University, based in Palampur, scientists are working on the solid-state fermentation of various mixtures of cattle dung and aerobically decomposed Lantana camara, a exotic plant species that has turned into a menace through out the country .

Biogas is a clean source of energy since it is non-polluting and is soot free. It contains upto 70 per cent of non-flammable methane, which helps to reduce the use of 30 lakh tonnes of fuelwood per year. At the same time, biogas plants in India produce 700,000 tonnes of urea per year.
Source of biogas Operating a biogas plant can be troublesome if there is a shortage in the supply of cattle dung. "Biogas can be obtained from any degraded material, but the gas yield differs,' mentions N P Singh, adviser at mnes and editor of Bio Energy News , a mnes publication. Many crop residues, vegetable market waste, aquatic weeds, forest residues, can all be converted into biogas.

Distillery waste gives the best yield because the by-product contains plenty of organic matter. Besides, it is not necessary to add more water since the waste already contains sufficient amount of it. But the biogas plants, quite obviously, need to be in close proximity to the distilleries. Currently there are around 125 distilleries that have installed biogas plants, but most of them use it only for the boilers. The blood and hide from the slaughterhouse also makes for a rich source. A huge biogas plant has been set up in a slaughterhouse in Hyderabad. Biogas is also obtained from pharmaceutical wastes and paper mills. The Biogas Development Programme, under the mnes, is researching the use of various raw materials. "While some of the research is confined to research laboratories, research has also been undertaken in collaboration with various industries,' says Singh.

Despite its tremendous potential, the technology is yet to gain favour with people. Some vexing problems need to be sorted. "There are a few important problems that require concerted effort to be solved by the scientific community,' says Shyam. These include, the lower gas yield in colder climes. The non-availability of easy to operate and low cost designs based on use of agro-residues is another.