Design of anaerobic plants
vvss,coo = 1.3-1.7 kg COD/kg VSS
VCH4,oil = 0.84 kg oil/m 3 CH4
VCH4,oil = 1.0 I oil/m 3 CH4
V!ats,CH4 = 0.75 m 3 CH4/kg fats
Vcarbohydrate,CH4 = 0.42 m 3 CH4/kg carbohydrate
Vprotein,CH4 = 0.47 m 3 CH4/kg protein
VCOD,CH4 = 0.25 kg CH4/kg COD
VCOD,CH4 = 0.38 m 3 CH4/kg COD
1m 3 CH4 (standard condition, 25°C, 1 atm.) weighs 0.667 kg
1 m 3 oil (standard condition, 25°C, 1 atm.) weighs 850 kg
1 m 3 CH4 = 35,000 kJ
1m 3 CH4 = 9.7 kWh
{
30% electricity = 2.9 kWh
1 m 3 CH4 converted to electricity
50% heat = 4.9 kWh
20% loss= 1.9 kWh
Methane content in gas
(Yo
so
60
70
80
100
Caloferic value
kJ/m 3 gas
17,500
21,000
24,500
28,000
35,000
{ electricity
kWh/m 3
1.5
1.7
2.0
2.3
2.9
At electr. prod.
heat
-
2.5
2.9
3.4
3.9
4.9
loss
-
1.0
1.1
1.3
1.5
1.9
Table 9.8
Conversion factors/stoichiometric coefficients in connection with gas production in anaerobic plants.
0.38
m 3 CH4/kg COD (removal)
0.30-0.38
m 3 CH4/kg COD (input)
0.46-0.62
a)
m 3 gas/kg COD (input)
0.45-0.60
b)
m 3 CH4/kg VSS (input)
0.69-0.92
a) b)
m 3 gas/kg VSS (input)
a) for 65% CH4 in gas
b) 1 kg VSS = 1.5 kg COD
Table 9.9
Figures found from experience with gas production, anaerobic plants
9 .3.4 Optimization, anaerobic plants
Quite a few operational problems have to be dealt with in anaerobic plants -just as
the case is in other biological plants. Some of these problems as regards start-up,
disturbances and toxic substances are discussed below.
9.3.5 Start-up, anaerobic plants
The slow growth of the methane producing bacteria means that the start-up of new
anaerobic plants may take a long time. By seeding with anaerobic bacteria, for
example from another plant, the start-up period can be reduced.
The starting period will normally be 2-4 times the sludge age. For a plant at 35°C it
means 30-60 d.
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