when ambient temperature ranges between 18 and 20
C, it takes approximately
14–18 days for pH to attain a stable value.
6.1.2.4 Dilution and Consistency of Feed
All waste materials fed to a biogas plant consist of solid substance, volatile organic
matter and nonvolatile matter, and water. During anaerobic digestion process,
volatile solids undergo digestion and nonvolatile solids remain unaffected.
According to a finding by The Energy and Resources Institute (TERI), fresh cattle
waste consists of approximately 20% total solids (TS) and 80% water (TERI 1987).
TS, in turn, consists of 70% volatile solids and 30% fixed solids. For optimum gas
yield through anaerobic digestion, normally, 8–10% TS in feed is required. This is
achieved by making slurry of fresh cattle dung in water in the ratio of 1:1. However,
if the dung is in dry form, the quantity of water has to be increased accordingly to
arrive at the desired consistency of the feed (i.e., ratio could vary from 1:1.25 to even
1:2). If the dung is too diluted, the solid particles will settle down into the digester
and if it is too thick, the particles impede the flow of the gas formed at the lower part
of the digester. In both cases, gas production will be less than optimum. It is also
necessary to remove inert materials such as stones from the feed before sending the
slurry into the digester. Otherwise, the effective volume of digester will decrease.
6.1.2.5 Loading Rate
Loading rate is defined as the amount of raw materials fed per day per unit volume of
digester. It is an important parameter that affects gas yield. If the plant is overfed,
acids will accumulate and methane production will be inhibited since bacteria cannot
survive in acidic conditions. Similarly, if the plant is underfed, the gas production
will also be low because of alkaline solution, which is also not a favorable condition
for anaerobic bacteria. The effect of daily and alternate day loadings on biogas yield
was also studied (Gore 1981). It was found that a 50 kg charge on daily basis and
100 kg charge on alternate day basis produced 2.9043 and 2.9285 m
3 of gas,
respectively. Also, for a particular size of plant, there is an optimum feed of charge
rate that will produce maximum gas and further quantity of charge will not proportionately produce more gas. A daily loading rate of 16 kg of volatile solids per m
3 of
digester produces 0.04–0.074 m
3 of gas per kg of raw dung fed (Moharao 1975). The
recommended loading rates for plants working on night soil range from 1.04 to
2.23 kg of volatile solids per m
3 of digester (Moharao 1974). Higher loading rates
are recommended only in cases where mean ambient temperature is high.
6.1.2.6 Hydraulic Retention Time
Hydraulic retention time (HRT) is the average period that a given quantity of input
material remains in the digester to be acted upon by the methanogens. In a cattle
dung plant, the retention time is calculated by dividing total volume of the digester
and volume of feed per day. The rate of gas generation is initially high and then,
gradually, declines as the digestion approaches completion. Thus, the time required
for 70–80% digestion is considerably less than that needed to achieve complete
digestion. HRT is chosen to achieve at least 70–80% digestion. Langrage (Langrage
6 Stoichiometric Analysis of Biogas Production from Industrial Residues
145
C, it takes approximately
14–18 days for pH to attain a stable value.
6.1.2.4 Dilution and Consistency of Feed
All waste materials fed to a biogas plant consist of solid substance, volatile organic
matter and nonvolatile matter, and water. During anaerobic digestion process,
volatile solids undergo digestion and nonvolatile solids remain unaffected.
According to a finding by The Energy and Resources Institute (TERI), fresh cattle
waste consists of approximately 20% total solids (TS) and 80% water (TERI 1987).
TS, in turn, consists of 70% volatile solids and 30% fixed solids. For optimum gas
yield through anaerobic digestion, normally, 8–10% TS in feed is required. This is
achieved by making slurry of fresh cattle dung in water in the ratio of 1:1. However,
if the dung is in dry form, the quantity of water has to be increased accordingly to
arrive at the desired consistency of the feed (i.e., ratio could vary from 1:1.25 to even
1:2). If the dung is too diluted, the solid particles will settle down into the digester
and if it is too thick, the particles impede the flow of the gas formed at the lower part
of the digester. In both cases, gas production will be less than optimum. It is also
necessary to remove inert materials such as stones from the feed before sending the
slurry into the digester. Otherwise, the effective volume of digester will decrease.
6.1.2.5 Loading Rate
Loading rate is defined as the amount of raw materials fed per day per unit volume of
digester. It is an important parameter that affects gas yield. If the plant is overfed,
acids will accumulate and methane production will be inhibited since bacteria cannot
survive in acidic conditions. Similarly, if the plant is underfed, the gas production
will also be low because of alkaline solution, which is also not a favorable condition
for anaerobic bacteria. The effect of daily and alternate day loadings on biogas yield
was also studied (Gore 1981). It was found that a 50 kg charge on daily basis and
100 kg charge on alternate day basis produced 2.9043 and 2.9285 m
3 of gas,
respectively. Also, for a particular size of plant, there is an optimum feed of charge
rate that will produce maximum gas and further quantity of charge will not proportionately produce more gas. A daily loading rate of 16 kg of volatile solids per m
3 of
digester produces 0.04–0.074 m
3 of gas per kg of raw dung fed (Moharao 1975). The
recommended loading rates for plants working on night soil range from 1.04 to
2.23 kg of volatile solids per m
3 of digester (Moharao 1974). Higher loading rates
are recommended only in cases where mean ambient temperature is high.
6.1.2.6 Hydraulic Retention Time
Hydraulic retention time (HRT) is the average period that a given quantity of input
material remains in the digester to be acted upon by the methanogens. In a cattle
dung plant, the retention time is calculated by dividing total volume of the digester
and volume of feed per day. The rate of gas generation is initially high and then,
gradually, declines as the digestion approaches completion. Thus, the time required
for 70–80% digestion is considerably less than that needed to achieve complete
digestion. HRT is chosen to achieve at least 70–80% digestion. Langrage (Langrage
6 Stoichiometric Analysis of Biogas Production from Industrial Residues
145