1 .Introduction Biogas technology
provides an alternate source of energy especially in rural parts of India . The
technology meets the basic need for cooking fuel in rural areas by using local
resources, such as cattle waste and other organic wastes. The digested slurry
can be used as manure for enriching soil and for sustaining its productivity.
The potential for this technology is presented in the fact that India supports
a large cattle wealth. This has also led the Ministry of New and Renewable Energy,
Government of India to promote family type biogas plants since 1981-82 under the
National Biogas and Manure Management Programme (NBMMP). 2. Anaerobic
Digestion Anaerobic digestion is a naturally occurring process through
which organic matter such as manure, feed spills, meat processing wastes and crop
residues are stabilized by microorganisms strictly in the absence of air. During
this process, some organic compounds are converted to methane (CH4) and carbon
dioxide (CO2) gases. This mixture of gases is known as biogas. The composition
of biogas is 50 to 75 per cent CH4 and 25 to 45 per cent CO2. Like natural gas,
biogas can also be used as a fuel in power generators, engines, boilers and burners.
Following is the schematic diagram representing the anaerobic process: 
In
practice, specially designed and insulated tanks are used to facilitate the anaerobic
digestion process under a controlled atmosphere. These tanks are known as anaerobic
digesters. The effluent coming out from the digester after the completion of the
digestion process is known as digestate. Digestate has nutrient value and can
be applied on land like manure. Digestate also has much less odour compared to
stored manure. (Source: Mahendran Navaratnasamy
and Brian Koberstein, Agriculture Stewardship Division, and Bruce Partington,
Rural Utilities Division, Alberta Agriculture and Food
University ) 3. Operating parameters Most
anaerobic digesters are operated in the temperature range of 15 to 45°C. The
pH of the slurry in the digester is maintained between 6.5 and 7.5. The typical
retention time of organic matter in the anaerobic digesters varies from 2 days
to 60 days, depending on the type of digester and the concentration of organic
matter processed. 4. Approved technologies for biogas production at farm
level Under NBMMP the following models of family type biogas plants
are being promoted: Fixed dome type of biogas plant: The
fixed dome type is also popularly known as the “Deenbandhu Model”. The fixed dome
type consists of a dome constructed of concrete, known at the digester tank. This
dome is fixed and thus it is called fixed dome type of biogas plant. The feedstock
along with water is mixed in the mixing tank to form the slurry and allowed to
ferment. Once the pressure is enough within the dome, the gas can be taken out
from outlet. The used slurry expands and overflows into the overflow tank. 
Floating
dome type biogas plant The floating gasholder type is also popularly
known as “Indian Khadi and Village Industries Commission (KVIC) Model”. The digester
is constructed, which is divided into two parts. One part is an inlet, from where
the slurry is fed to the tank, the other is the cylindrical dome of the tank,
which is made out of stainless steel that floats on the slurry and collects the
gas generated. Hence it is called floating gas-holder type of bio gas plant. The
slurry is fermented and as gas produced by the bacterial fermentation, the pressure
inside increases. The gas is collected from the outlet pipe and utilized. 
Advantages and disadvantages: In
the floating gas-holder type of plant, the floating chamber is made of stainless
steel. This is expensive and needs continuous maintenance and supervision for
non-rust. This does not arise in the fixed dome type of bio gas plant as it is
constructed of concrete. The volume of fixed dome type of biogas is fixed.
So if the gas pressure increases inside, it may cause damage to the concrete dome.
This does not happen in the floating holder type of bio gas plant. (Source:
Indian Institute of Technology, New Delhi, Indian Institute of Science, Bangalore
and Ministry of New and Renewable Energy- Annual Report 2007-08)
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