1979) suggested that HRT depends upon the interior temperature of the digester.
Higher the temperature of the digester, lower is the retention time. HRT varies
between 20 and 120 days, depending upon the design and operating temperature
of the digester. For digesters operating in countries of tropical region, HRT is usually
taken as 40–60 days. In countries of colder region, digesters are designed for HRT of
about 100 days (UN Guidebook on Biogas Development 1980).
6.1.2.7 Toxicity
Mineral ions, heavy metals, and detergents are toxic materials that inhibit the normal
growth of pathogens in the digester. Small quantity of minerals (e.g., sodium,
potassium, calcium, magnesium, ammonium, and sulfur) also stimulates the growth
of bacteria, while high concentration leads to toxic effects. For example, presence of
ammonium from 50 to 200 mg/L stimulates the growth of anaerobic microbes,
whereas its concentration above 1500 mg/L produces toxic compounds. Similarly,
heavy metals, such as copper, nickel, chromium, zinc, lead etc., in small quantities,
are essential for the growth of bacteria, but their higher concentration has toxic
effects (Moharao 1975). Detergents, including soap, antibiotics, organic solvents,
etc. also inhibit the activity of methane-producing bacteria, and hence, addition of
these substances in the digester should be avoided.
6.1.2.8 Agitation
Agitation or mixing of digester contents significantly helps to ensure intimate
contact between microorganisms, which leads to improved fermentation efficiency.
Coppinger (Coppinger 1979) suggested that effect of varying degrees of mixing of
digester contents improves biogas production. The major problem associated with
the different designs of biogas plant is that a thick layer of scum formation appears at
the top of the digester which blocks the gas from coming out of the upper free portion
of the digester. Thus, no gas is available at the utility point. The effects of recirculation of gas to break the scum formation were investigated by Mahanta et al. They
found that recirculation of gas improves the biogas yield. The recirculation of gas
increases the biogas production by three times. The gas production with circulation
is much more than that without recirculation at the same pH.
6.1.2.9 Additives
The additives seem to play an important role in biogas yield. Addition of 5%
commercial charcoal to cattle dung slurry on dry-weight basis raised the yield by
17 and 35% in batch and semicontinuous processes, respectively. Madamwar and
Mithal (Madamwar and Mithal 1986) performed two sets of experiments: one at
controlled temperature of 38
C and the other at ambient temperature of 15
C to find
the impact of adding pectin to cattle dung slurry as feed on biogas yield. Pectin not
only enhances gas yield but also imparts process stability during the periods of
fluctuating temperature. The impact of adding inert materials, such as vermiculite,
charcoal, and lignite bovine excreta, as feed on biogas yield has also been reported
(Geeta et al. 1986). These additives increased biogas yield by 15–30%. Pebbles,
146
S. Sivamani et al.
Higher the temperature of the digester, lower is the retention time. HRT varies
between 20 and 120 days, depending upon the design and operating temperature
of the digester. For digesters operating in countries of tropical region, HRT is usually
taken as 40–60 days. In countries of colder region, digesters are designed for HRT of
about 100 days (UN Guidebook on Biogas Development 1980).
6.1.2.7 Toxicity
Mineral ions, heavy metals, and detergents are toxic materials that inhibit the normal
growth of pathogens in the digester. Small quantity of minerals (e.g., sodium,
potassium, calcium, magnesium, ammonium, and sulfur) also stimulates the growth
of bacteria, while high concentration leads to toxic effects. For example, presence of
ammonium from 50 to 200 mg/L stimulates the growth of anaerobic microbes,
whereas its concentration above 1500 mg/L produces toxic compounds. Similarly,
heavy metals, such as copper, nickel, chromium, zinc, lead etc., in small quantities,
are essential for the growth of bacteria, but their higher concentration has toxic
effects (Moharao 1975). Detergents, including soap, antibiotics, organic solvents,
etc. also inhibit the activity of methane-producing bacteria, and hence, addition of
these substances in the digester should be avoided.
6.1.2.8 Agitation
Agitation or mixing of digester contents significantly helps to ensure intimate
contact between microorganisms, which leads to improved fermentation efficiency.
Coppinger (Coppinger 1979) suggested that effect of varying degrees of mixing of
digester contents improves biogas production. The major problem associated with
the different designs of biogas plant is that a thick layer of scum formation appears at
the top of the digester which blocks the gas from coming out of the upper free portion
of the digester. Thus, no gas is available at the utility point. The effects of recirculation of gas to break the scum formation were investigated by Mahanta et al. They
found that recirculation of gas improves the biogas yield. The recirculation of gas
increases the biogas production by three times. The gas production with circulation
is much more than that without recirculation at the same pH.
6.1.2.9 Additives
The additives seem to play an important role in biogas yield. Addition of 5%
commercial charcoal to cattle dung slurry on dry-weight basis raised the yield by
17 and 35% in batch and semicontinuous processes, respectively. Madamwar and
Mithal (Madamwar and Mithal 1986) performed two sets of experiments: one at
controlled temperature of 38
C and the other at ambient temperature of 15
C to find
the impact of adding pectin to cattle dung slurry as feed on biogas yield. Pectin not
only enhances gas yield but also imparts process stability during the periods of
fluctuating temperature. The impact of adding inert materials, such as vermiculite,
charcoal, and lignite bovine excreta, as feed on biogas yield has also been reported
(Geeta et al. 1986). These additives increased biogas yield by 15–30%. Pebbles,
146
S. Sivamani et al.