BIOCHEMICAL CONVERSION: ANAEROBIC DIGESTION - Biomass as a Sustainable Energy Source for the Future - page 465

microbial organic carbon conversions in the absence of strong electron acceptors or
energy sources will eventually lead to methane production. Evolution has taken care
of an enormous microbial diversity that is capable of catalyzing most organic carbon
conversions as long as energy is available for the reaction. This thermodynamic
foundation makes anaerobic digestion very suitable for treatment of heterogeneous
mixtures of substrates since all biodegradable organic compounds are con-
verted to methane and no specific measures need to be taken to avoid side-product
formation.
A second main selling point of anaerobic digestion is the production of a gaseous
end product, minimizing the efforts required for downstream processing. Even though
some degree of upgrading of the biogas will be required for either combustion in a
CHP plant or for distribution into the natural gas network, the phase transition during
anaerobic digestion from solid and water-soluble organic carbon to gaseous organic
carbon is a major advantage compared to, e.g., bioethanol production. The limited
energy requirements for anaerobic digestion compared to bioethanol and biodiesel
production are also the main reasons why anaerobic digestion has a two to three times
higher net energy yield (in MJ per ha) for energy crops compared to bioethanol and
biodiesel production.
Despite these intrinsic advantages of anaerobic digestion, the gaseous end product
also represents the main limitation of the process. Current infrastructure for transporta-
tion fuels relies on liquid fuels, and European legislation aims for replacement of a sig-
nificant fraction of these fuels with biomass-based fuels. This indirect subsidy for
bioethanol and biodiesel has increased the price of these liquid fuels and has stimulated
the production of bioethanol and biodiesel to a much higher extent than biogas produc-
tion. One could easily argue that instead of replacement of a significant fraction of liquid
transportation fuels with bioethanol or biodiesel, part of the infrastructure could be
replaced by biogas or natural gas-powered transportation, but this argument is only
scarcely heard (Tilche and Galatola, 2008). Consequently, anaerobic digestion is cur-
rently primarily used for bioenergy generation from low-value feedstocks like OFMSW,
manure, and agro-industrial residues. These processes can currently only be developed
cost-efficiently due to subsidies for sustainable energy. This has even become more true
in recent years in which the natural gas price has decreased significantly to below US$
0.5 per m 3 due to the discovery of novel natural gas reserves (shale gas).
In general, the anaerobic digestion process itself is not limiting implementation.
Even though it is a relatively slow process, requiring retention times of 10 30 days

and consequently large bioreactors, it is a very robust process with a well-defined end
product. The bottleneck for industrial implementation of anaerobic digestion is related
to the complex infrastructure required. Effective upgrading of the residues that remain
after anaerobic digestion and complex thermal energy integration upon biogas
combustion are prerequisites for successful implementation. Other complications
include the need for nutrient recovery or removal from the digestate and definition of
a destination of the solid fraction of the digestate. A final concern is the large scale
required for effective implementation of anaerobic digestion (typically more than
100,000 tonnes per year), combined with the infrastructure required for feedstock
supply and reuse of the digestate.

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