Environmental Engineering Reference
In-Depth Information
Box 5.2 Biogas production based on volatile solids (VS) destruction
Energy value (kJ/kg) = 337C þ1419(H1419 8 ) þ 93S þ 23.26N [5.2]
where C, H, O and N are the proportions of dry solids which are carbon,
hydrogen, oxygen, sulphur and nitrogen respectively. C is 48.3% of dry
solids and is thus 50.84% of volatile solids (48.3/0.95); similarly H is 8% of VS,
O is 37.7% of VS and N is 3.47% of VS.
Energy value of 1 kgVS = 337 (50.84)
þ
1419 [8
(37.7/8)]
þ
23.26(3.47)
Energy value = 17133
80.7 = 21878 kJ/kgVS = 21. 9MJ/kgVS
300 kgDS/t OFMSW; 285 kgVS/t OFMSW; 200 kgVS destroyed/t OFMSW
(70% destruction)
4.38GJ converted/t OFMSW = 116m n 3 CH 4 /t OFMSW or 116m n 3 CH 4 /
285 kg VS = 406 l CH 4 /kgVS
þ
4665
þ
Box 5.3 Relationship between VS and COD in a two-stage process
1 kg of COD generates 350 l CH 4 (Nizami et al., 2009; Sperling and
Chernicharo, 2004)
Thus 1 kg COD generates 13.22MJ
1 kg VS = 20.74MJ
Thus 1.57 kg COD may be generated from 1 kg VS.
The basis for the COD test is that nearly all organic compounds can be fully
oxidised to carbon dioxide with a strong oxidising agent under acidic
conditions (Nizami et al., 2009).
The amount of oxygen required to oxidise an organic compound to carbon
dioxide, ammonia and water is given by
O 2
H 2 O
a
4
b
2
3c
4
a
2
3c
2
C n H a O b N c þ
n þ
! nCO 2
þ
þ cNH 3
½
5
:
3
n = 17, a = 32, b =9,c =1
C 17 H 32 O 9 N
þ
19.75O 2
17CO 2
þ
14.5H 2 O
þ
NH 3
394
þ
632
748
þ
261
þ
17
￿ ￿ ￿ ￿ ￿ ￿
1026
1026
394 kg VS: 632 kgO 2
1.6 kgO 2 to oxidise 1 kgVS
kilogramme of COD generates 350 l CH 4 . Thus, in a two-stage system, the
maximum production of methane is of the order of 560 l CH 4 /kg VS. The
values for methane production per kilogramme of VS for OFMSW may be
compared with documented values for crop digestion (Table 5.2).
Gases dissolve at different rates (see Table 5.3). Carbon dioxide is much
more soluble than methane. At lower temperatures, more CO 2 goes into
solution, causing a higher percentage of CH 4 in the gas. This temperature
 
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