BIOCHEMICAL CONVERSION: ANAEROBIC DIGESTION - Biomass as a Sustainable Energy Source for the Future - page 456

these models are very complex with six to eight groups of microorganisms that are
characterized with at least four or five kinetic and stoichiometric parameters for each
group of microorganisms.
Here, we describe a strongly simplified version of the anaerobic digestion model
that focuses on the most common rate-limiting steps in the process. For anaerobic
digestion of particulate substrates like those considered here, there are two potentially
rate-limiting steps in the overall process and one boundary condition that need to be
fulfilled.

14.4.1.1 Rate-Limiting Step 1 Hydrolysis of complex particulate organic matter
(first step in Figure 14.2), typically described as a first-order process in the concen-
tration of the biodegradable particulate substrate (P, g P-COD

L −1 ) under formation of a

L −1 ): soluble substrate (S, g S-COD

r h =k h P ð Eq : 14 : 12 Þ

This process step is generally catalyzed by extracellular enzymes that are excreted by
bacteria growing on the soluble substrate. Hydrolysis is lumped with the (non-rate-
limiting) second step in Figure 14.2, the microbial growth reaction (acidogenesis) that
converts monomeric carbohydrates, amino acids, and fatty acids to mainly VFA.

14.4.1.2 Rate-Limiting Step 2 Methanogenesis of VFA, which includes the acet-
ogenesis and methanogenesis steps (last two steps in Figure 14.2). This process is
described as a typical microbial growth process, in which the microbial substrate con-
version is a first-order process in the concentration of biomass ( X ,g X-COD L − 1 ). It

h −1 ),
which in turn is a function of the maximum biomass-specific substrate uptake rate
( q ma S ,g S-COD

g −1

also depends on the actual biomass-specific uptake rate ( q S ,g S-COD

X-COD

h −1 ) and the substrate concentration ( S ,g S-COD L − 1 ) in a so- g −1

X-COD

called Monod term, S /( K S + S ).
The rate of this step can be expressed as

S
K S + S X

r met = q S = q max
S

ð Eq : 14 : 13 Þ

14.4.1.3 Boundary Condition A certain fraction of the particulate substrate is not
susceptible to biodegradation and is therefore referred to as the fraction of inert sub-
strate ( f I ). The initial concentration of biodegradable substrate (P 0 ,g P-COD L − 1 ) can

be calculated from the total particulate substrate concentration (P T0 ,g P-COD L − 1 )

using P 0 =(1 f I )P T0 .

The resulting process stoichiometry and kinetics are shown in Table 14.2. In
Section 14.4.2, this process model is implemented in a bioreactor model to clarify
the impact of the two rate-limiting steps on the overall design of an anaerobic digester.

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