Environmental Engineering Reference
In-Depth Information
TABLE 14.2 COD-based stoichiometry and kinetics of the two-step anaerobic digestion
process (hydrolysis and methanogenesis)
Step
P
S
X
CH
4
Rate equation
Hydrolysis
−1
1
0
0
k
h
P
Methanogenesis
0
−1
Y
X
Y
CH
4
q
max
S
S
K
S
+
S
X
Bioreactor setup
Conversion reactions
CH
4
In
Out
Plug-low reactor
P
s
Recycle
X
120
60
S
100
50
CH
4
CH
4
80
40
60
30
40
20
s
P
20
P
10
0
X
X
0
0.05
0.5
5
0
2
4
6
8
10
Time (d)
Recycle ratio R (-)
FIGURE 14.9
Impact of recirculation on a plug flow anaerobic digester. Pictures show the
bioreactor with variable recirculation flow considered and the conversion reactions assumed.
The left graph shows the effluent concentrations of particulate substrate (P), soluble
intermediate (
S
), and the end products methane (CH
4
) and methanogenic biomass (
X
)asa
function of the recirculation ratio
R
. The right graph shows the concentration profile over
the reactor at a recirculation ratio of 1.
14.4.2 Bioreactor Design
14.4.2.1 Reactors
The two-step process proposed to describe the anaerobic diges-
tion process is implemented in a bioreactor model. The two main bioreactor configura-
tions are the plug flow reactor (PFR) and the continuous stirred tank reactor (CSTR). In
a PFR, mixing is assumed to be limited to the radial direction, resulting in a concentra-
tion gradient of the substrate, product, and biomass along the length of the reactor (see
Figure 14.9). In a CSTR, concentrations are homogeneously distributed over the reactor
volume, and the concentrations inside the reactor equal the effluent concentrations.
Effluent recirculation from a PFR results in a hybrid between a PFR and a CSTR.
Derivation of the mass balances and their solution are outside the scope of this
chapter, but a qualitative idea of the impact of the reactor design can be obtained from
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