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3.4 FACTORS AFFECTING MICROBIAL ACTIVITIES IN DIGESTER
3.4.1 Slurry
For proper solubilization of organic materials, the ratio between solid and water should be 1:1 when the
domestic wastes are used (see Chapter II).
3.4.2 Seeding or Bacterial Population
Acetogenic (acid forming bacteria) and methanogenic are naturally present in cow dung. However, their
number is quite small. Acid forming bacteria proliferate fast and increase their number, while methanogenic
bactera develop very slowly. Therefore, for die initial reaction, small amount of sludge of another digester is
generally used as seeding or inoculum. This sludge contains high concentration of acetogenic and
methanogenic bacteria, which could enhance the process of anaerobic digestion of organic materials.
Some study has shown that the seeding materials can be mixed with the input slurry up to the ratio of 30 to
50 percent. If inoculum is increased further, less volume of gas is obtained due to reduced inputs fed to the
digester.
3.4.3 Stabilization of pH Value
Methane producing bacteria are very sensitive to pH level. For high amount of methane, optimum pH of
digester should be maintained between 6 and 8. The acidic condition lowers down methane formation
(see Chapter I).
3.4.4 Temperature
Temperature factor is critical value in the beginning of methane formation. Once metabolism occurs
exothermic reaction is helpful for the methane production. In case of mesophilic digestion, temperature range
should be maintained between 30 to 40°C. In case of thermophilic digestion, it should be between
45 to 60 °C. In cold climate, the temperature of digester area should be raised upto 35 °C (see Chapter I).
3.4.5 Nitrogen Concentration
Methane production is the activity of Carbon metabolism, thus excess amount of nitrogen inhibits the
bacterial metabolism and lower down the methane production.
3.4.6 Carbon-Nitrogen (C:N) Ratio
C:N ratio is one of the important factors in digester for methane production. High metabolism occurs when
C:N ratio is 30:1. The ratio is only maintained when other substrates are used ratlier man the cow dung or
other animal dung (see Chapter I).
3.4.7 Maintain of Anaerobiosis
Methanogenic bacteria are anaerobic organisms. In aerobic condition, most of these bacteria are inactive in
metabolism, thus digesters should be totally airtight to maintain strictly anaerobic condition. In many places,
digesters are buried in the Earth to maintain anaerobiosis condition.
3.4.8 Addition of Succulent Plant or Algae
For the effective and high production of biogas from cow dung and animal dung many succulent plants or
algae are added. Green algae, water hyacinth and lemon grass are added in the digester. The amount of
biogas produced from the algae was twice (344 ml/g dry algae) of that obtained from cow dung (179/g dry
cow dung) alone. Also, the duration of gas evolution increased with increasing the proportion of slurry. The
calorific value of the gas was 4800 K cal/m 3 and the percentage of methane was 55.4 percent.
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