Environmental Engineering Reference
In-Depth Information
o
=
224ODM
−
56COD
−
304N
org
ð
Eq
:
14
:
5
Þ
49COD
−
64N
org
+ 112ODM
352N
org
:
:
n
=
ð
Eq
14
6
Þ
49COD
−
64N
org
+ 112ODM
From the stoichiometric coefficients of organic feedstock degradation to methane
(
λ
CO
2
) in the overall stoichiometric equation for anaerobic
digestion, the biogas composition can be estimated:
λ
CH
4
) and carbon dioxide (
λ
CH
4
λ
CH
4
+
λ
CO
2
λ
CH
4
+
%
CH
4
=
λ
CO
2
100
%%
CO
2
=
λ
CO
2
100
%
ð
Eq
:
14
:
7
Þ
The stoichiometric equation furthermore allows for calculation of the biogas produc-
tion upon full organic carbon degradation, using the definition of the normal volume
of 1 mole of gas (NVMG ~22.4 l
mol
−1
):
=
ODM
12 +
h
+16
o
+14
n
kg
−1
Biogas L
n
:
ð
1
−
n
Þ
NVMG
ð
Eq
:
14
:
8
Þ
Besides the derivation of the elemental composition of the organic substrate, these
basic three measurements also enable identification of the composition of the substrate
in terms of the principal building blocks, i.e., proteins, carbohydrates, and lipids, as
outlined in Table 14.1.
From the elemental composition derived earlier, the carbon mole fraction (
)
of CHO, PR, and LIP in the organic substrate can readily be calculated from the
elemental compositions shown in Table 14.1. The derivation of the equation for
the PR fraction is straightforward since PR is the only nitrogen-containing com-
pound assumed and VFA is the only charged compound considered. Therefore,
the fraction of proteins (
η
η
PR
) in the waste follows directly from the N-content of
TABLE 14.1 Principal compounds considered as general organic feedstock constituents
Oxidation state
b
(
γ
)
N-content
Compounds
a
mol
e
mol
−1
C
mol
N
mol
−1
C
Abbr.
Chemical composition
Proteins
PR
C
1
H
2.52
O
0.87
N
0.26
4.0
0.26
Carbohydrates
CHO
C
1
H
2
O
1
4.0
0.0
Lipids
LIP
C
1
H
2.85
O
0.575
5.7
0.0
The elemental composition of proteins is based on the generalized amino acid composition of proteins as
proposed by Batstone et al. (2002). Carbohydrates are based on the composition of glucose C
6
H
12
O
6
.
a
The model lipid chosen is composed of one glycol molecule and three n-palmitic acid side chains, resulting
in C
51
H
98
O
6
as elemental composition.
b
The oxidation state of the substrate is defined as the number of electrons liberated per C-mol substrate
upon full oxidation to carbon dioxide, equivalent to the COD definition described earlier (Heijnen and
Kleerebezem, 2010).
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