Environmental Engineering Reference
In-Depth Information
G
01
Δ
standard Gibbs free energy, corrected for pH = 7
[kJ per electron] or
[kJ per reaction]
K
a
equilibrium constant for acid
-
base equilibrium
[
−
]
K
h
equilibrium constant for gas
-
liquid partitioning
[
−
]
[s
−1
]
k
h
hydrolysis rate coefficient
[s
−1
]
k
L
a
volumetric gas
-
liquid mass transfer coefficient
L
− 1
]
K
S
half-saturation coefficient in the Monod equation
[g
S-COD
kg
− 1
]
ODM organic dry matter content
[g
ODM
L
− 1
]
P
particulate substrate concentration
[g
P-COD
P
energy production
[MW]
q
S
actual biomass-specific substrate uptake rate
h
−1
]
[g
S-COD
g
−1
X-COD
q
max
S
maximum biomass-specific substrate uptake rate
g
−1
h
−1
]
[g
S-COD
X-COD
L
− 1
h
− 1
]
r
h
rate of hydrolysis
[g
P-COD
r
met
rate of methanogenesis
h
−1
]
g
−1
X-COD
[g
S-COD
R
recirculation ratio
[
-
]
L
− 1
]
S
soluble substrate concentration
[g
S-COD
kg
− 1
]
TDM total dry matter
[g
TDM
L
− 1
]
X
concentration of methanogenic biomass responsible
for converting VFA to biogas
[g
X-COD
Y
stoichiometric factor
[typically
g
COD
g
−1
COD
]
γ
oxidation state of the substrate
mol
−1
C
[mol
e
]
η
fraction of a given compound in substrate mixture
[
−
]
λ
CH
4
stoichiometric coefficients of biomass degradation
to CH
4
[
−
]
λ
CO
2
stoichiometric coefficients of biomass degradation
to CO
2
[
−
]
14.1
INTRODUCTION
14.1.1 What Is Anaerobic Digestion?
Anaerobic digestion is a biotechnological process by which a complex organic feed-
stock is first converted into a range of simpler water-soluble organic compounds that
are subsequently converted into methane-containing biogas. The process is anaerobic
(literally without air), meaning here that it occurs in absence of oxygen or any other
external electron acceptor like nitrate or sulfate. In nature, anaerobic digestion occurs
in environments rich in organic carbon and limited by input of electron acceptors and/
or energy sources such as light. Example natural environments include wetlands, rice
paddies, and the rumen of animals and insects.
One of the main conceptual advantages of anaerobic digestion as bioenergy-
producing bioprocess is the gaseous end product that evidently implies directly
in situ
product separation. The biogas generated typically consists of 50
-
70 vol.% methane
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