7–10%. Even in situations where C/N ratio is close to 30:1, the biomass can undergo
efficient anaerobic digestion only if waste materials are also biodegradable at the
same time (Singh 1974; SPOBD 1979).
6.1.2.2 Temperature
Most digesters installed in the field lack mechanisms for temperature control and
removal of dissolved oxygen. Hence, efficiency of these digesters is reported to be
low, particularly during the winter months. There are different temperature ranges
during which mesophilic and thermophilic bacteria are most active causing maximum gas yield. Generally, mesophilic bacteria are most active in the temperature
range 35–40
C and thermophilic bacteria in the range 50–60
C. Choice between
the mesophilic and thermophilic digestions is governed by the natural climatic
conditions in which the plant is located. Though, it is possible to create conditions
for thermophilic digestion by external heat, but such a method is generally uneconomical. Length of digestion period is linked with the digester temperature.
The methanogens are inactive in extreme high and low temperatures, while the
optimum temperature is 35
C. When the ambient temperature decreases to 10
C,
gas production virtually stops. Satisfactory gas production takes place in the
mesophilic range (30–40
C). Proper insulation of digester helps to increase gas
production during the cold weather. When the digester operates at a temperature of
15
C, it takes nearly a year for the digestion cycle to complete. However, if the
temperature is approximately 35
C, the cycle can be easily completed in less than a
month. When the digester temperature is maintained at 25
C, it takes approximately
50 days for digestion of cattle waste. But, if the temperature ranges between 32 and
38
C, digestion is complete within 28 days. Mahanta et al. carried out experiments
to analyze the effect of temperature variation on anaerobic digestion of cattle wastes
(NAS 1977). Smith et al. suggested that at low temperature, biogas plants with some
design modifications could also function quite effectively as in a warm climate
(Mahanta et al. 2004a).
6.1.2.3 pH
During anaerobic digestion, microorganisms require a natural or mildly alkaline
environment for efficient gas production (Smith et al. 1982). An optimum biogas
production is achieved when the pH of feed mixture in the digester is between 6.25
and 7.50 (Mahanta et al. 2004b; Wise 1987). The pH in a biogas digester is also a
function of retention time. In the initial period of digestion, as large amounts of
organic acids are produced by acidogenic bacteria, the pH inside the digester can
decrease below 5. This inhibits or even stops the process. Methanogenic bacteria are
very sensitive to pH and do not thrive below 6.5. Later, as the digestion process
continues, concentration of NH 3 increases due to the digestion of N 2 , which can
increase the pH to above 8. When the CH 4 production level is stabilized, the pH
range remains between 7.2 and 8.2. According to studies in China, during the period
when ambient temperature varies between 22 and 26
C, it takes approximately
6 days for pH to acquire a stable value (SPOBD 1979). Similarly, during the period
144
S. Sivamani et al.
efficient anaerobic digestion only if waste materials are also biodegradable at the
same time (Singh 1974; SPOBD 1979).
6.1.2.2 Temperature
Most digesters installed in the field lack mechanisms for temperature control and
removal of dissolved oxygen. Hence, efficiency of these digesters is reported to be
low, particularly during the winter months. There are different temperature ranges
during which mesophilic and thermophilic bacteria are most active causing maximum gas yield. Generally, mesophilic bacteria are most active in the temperature
range 35–40
C and thermophilic bacteria in the range 50–60
C. Choice between
the mesophilic and thermophilic digestions is governed by the natural climatic
conditions in which the plant is located. Though, it is possible to create conditions
for thermophilic digestion by external heat, but such a method is generally uneconomical. Length of digestion period is linked with the digester temperature.
The methanogens are inactive in extreme high and low temperatures, while the
optimum temperature is 35
C. When the ambient temperature decreases to 10
C,
gas production virtually stops. Satisfactory gas production takes place in the
mesophilic range (30–40
C). Proper insulation of digester helps to increase gas
production during the cold weather. When the digester operates at a temperature of
15
C, it takes nearly a year for the digestion cycle to complete. However, if the
temperature is approximately 35
C, the cycle can be easily completed in less than a
month. When the digester temperature is maintained at 25
C, it takes approximately
50 days for digestion of cattle waste. But, if the temperature ranges between 32 and
38
C, digestion is complete within 28 days. Mahanta et al. carried out experiments
to analyze the effect of temperature variation on anaerobic digestion of cattle wastes
(NAS 1977). Smith et al. suggested that at low temperature, biogas plants with some
design modifications could also function quite effectively as in a warm climate
(Mahanta et al. 2004a).
6.1.2.3 pH
During anaerobic digestion, microorganisms require a natural or mildly alkaline
environment for efficient gas production (Smith et al. 1982). An optimum biogas
production is achieved when the pH of feed mixture in the digester is between 6.25
and 7.50 (Mahanta et al. 2004b; Wise 1987). The pH in a biogas digester is also a
function of retention time. In the initial period of digestion, as large amounts of
organic acids are produced by acidogenic bacteria, the pH inside the digester can
decrease below 5. This inhibits or even stops the process. Methanogenic bacteria are
very sensitive to pH and do not thrive below 6.5. Later, as the digestion process
continues, concentration of NH 3 increases due to the digestion of N 2 , which can
increase the pH to above 8. When the CH 4 production level is stabilized, the pH
range remains between 7.2 and 8.2. According to studies in China, during the period
when ambient temperature varies between 22 and 26
C, it takes approximately
6 days for pH to acquire a stable value (SPOBD 1979). Similarly, during the period
144
S. Sivamani et al.