Biogas Technology

  Biogas  is a  product  of  anaerobic  fermentation of organic 
  matters, and consists of about  60-70% methane, 30-40% carbon-
  di-oxide, etc. The inputs for biogas digesters are the wastes 
  that are  found  locally; animal  dung, agricultural residues, 
  leaf  litter from  forests, etc. The  residues are introduced 
  into a closed  digester, where,  without the presence of free 
  oxygen the responsible micro  organisms work  successively to 
  convert  complex  organic  matter into CH4, CO2, H2, H2S, etc. 
  The optimum conditions for biogas production are: temperature 
  30-35°C,  pH 6.8-7.5,  Carbon/Nitrogen  ratio  20-30 ,  solid 
  contents  7-9%  and  retention time  20-40 days.  Among these 
  parameters,  temperature is the  most  difficult or costly to  
  control.The gas formation virtually stops when the temperature 
  drops  below  10°C. Retention  time decides the rate at which 
  the waste is digested. The longer the time, the larger is the 
  volume of gas  produced from a given amount of waste and vice 
  versa. Thus, if the  available  amount  of input materials is 
  limited, a bigger digester can be adopted to exploit  the gas 
  potential more fully; and where the waste is abundant, it can 
  be  fed  at a higher  loading  rate  into a small digester to 
  maximise the  gas  production per unit volume of the digester. 
  The  optimum  retention  time  depends on the temperature. In 
  practice, a longer  retention time is usually adopted to cope 
  with  cool  seasons. There  are  various  designs  of  biogas 
  digesters such as:
      (1)   Floating  gas  holder  type  designed by  Kadhi and 
            Village Industries Commission (Directorate of Gobar 
            Gas Scheme, 1979).
      (2)   Optimised design developed by Application of Science 
            and  Technology  to  Rural  Areas (ASTRA) at Indian 
            Institute of Science (Subramanian, 1984).
      (3)   Fixed dome type designed by University of Agricultural 
            Sciences - Bhagyalaxmi design.
      (4)   Raitabandu Biogas Plant - designed by a farmer from 
            Sagar  taluk, Shimoga district to suit the needs of 
            Malnad region.