Anaerobic Wastewater Treatment
The production of methane and carbon dioxide is absolutely dominant compared
to other gasses.
The produced amount of methane can be determined by means of a COD-balance
as methane represents a certain amount of COD:
that is, 1 mol CI-4 ~ 2 mol Oz or 16 g methane ~ 64 g Oz = 64 g COD
Hence the conversion factor between COD and methane is
vcoo,CH4= 0.25 kg CH4/kg COD.
At 25°C it corresponds to a methane gas volume of 0.38 m 3 C~/kg COD.
From the COD balance, Expression (9.1), we find the methane production, rv,CH4 · Vz:
(9.1)
and on the assumption that Q1 ~ QJ, the result is:
(9.15)
For most anaerobic processes, 90-95 per cent of the removed COD can be recovered
as methane, the rest is found in the form of sludge production.
The methane gas production can be used for heating or electric power generation.
In Table 9.8 we have a number of conversion factors for methane gas.
The carbon dioxide content in the gas from the anaerobic plants depends on the
composition of the organic matter (its oxidation step) and the buffer characteristics
of the wastewater. The COz content is often 20-30 per cent. Figures found from
experience are shown in Table 9.9.
Example 9.5
How big is the methane production in the plant from Example 9.3?
The organic matter in the influent and the effluent, respectively, is as follows:
C, = 3.0 kg COD/m 3
C3 = 0. 7 kg COD/m 3
The volume of wastewater, 0 1, is 1,200 m 3 /d.
From Expression (9.15) we find the methane production:
rv,cH4 · V2 = 1,200 m 3 /d (3.0-0.7) kg COD/m 3 · (0.25 kg CHJkg COD) = 690 kg CH~d
corresponding to 1,030 m 3 CHJd (1 atm., 25°C).
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The production of methane and carbon dioxide is absolutely dominant compared
to other gasses.
The produced amount of methane can be determined by means of a COD-balance
as methane represents a certain amount of COD:
that is, 1 mol CI-4 ~ 2 mol Oz or 16 g methane ~ 64 g Oz = 64 g COD
Hence the conversion factor between COD and methane is
vcoo,CH4= 0.25 kg CH4/kg COD.
At 25°C it corresponds to a methane gas volume of 0.38 m 3 C~/kg COD.
From the COD balance, Expression (9.1), we find the methane production, rv,CH4 · Vz:
(9.1)
and on the assumption that Q1 ~ QJ, the result is:
(9.15)
For most anaerobic processes, 90-95 per cent of the removed COD can be recovered
as methane, the rest is found in the form of sludge production.
The methane gas production can be used for heating or electric power generation.
In Table 9.8 we have a number of conversion factors for methane gas.
The carbon dioxide content in the gas from the anaerobic plants depends on the
composition of the organic matter (its oxidation step) and the buffer characteristics
of the wastewater. The COz content is often 20-30 per cent. Figures found from
experience are shown in Table 9.9.
Example 9.5
How big is the methane production in the plant from Example 9.3?
The organic matter in the influent and the effluent, respectively, is as follows:
C, = 3.0 kg COD/m 3
C3 = 0. 7 kg COD/m 3
The volume of wastewater, 0 1, is 1,200 m 3 /d.
From Expression (9.15) we find the methane production:
rv,cH4 · V2 = 1,200 m 3 /d (3.0-0.7) kg COD/m 3 · (0.25 kg CHJkg COD) = 690 kg CH~d
corresponding to 1,030 m 3 CHJd (1 atm., 25°C).
303