Environmental Engineering Reference
In-Depth Information
intensive, but for biogas production, generation of a dry matter content of
around 8% may be sufficient.
There are promising research results and some full-scale applications
around the world using aquatic biomass as a feedstock for biogas
production, but technological solutions to some issues are still
in the
research phase.
2.3
Characteristics of biogas feedstock
2.3.1 Suitability and availability
The substrates used in practice for biogas production are selected based on
their suitability and availability. Suitability in this case is defined by a
number of characteristics and parameters such as the content of easily
digestible organic matter, methane potential, particle size, dry matter
content, pH, C:N ratio, the content of macro- and microelements, etc.
Availability means that the feedstock is easily accessible for biogas plant
operators and can be supplied in sufficient amounts on a renewable basis.
The values of some basic parameters of feedstocks commonly supplied to
European biogas plants are illustrated in Table 2.1. The biomass resources
suitable as feedstocks for biogas production vary significantly in terms of
composition, digestibility, methane potential, dry matter content, content of
nutrients and other characteristics. The importance of these characteristics is
that they can be used to optimize the AD process and methane production.
Most AD processes run optimally at neutral pH (around 7), and a C:N ratio
of the substrate mixture between 20:1 and 30:1. As the anaerobic
microorganisms inside the digester need to be supplied with some basic
'ingredients' necessary for their metabolism, it is common practice to mix
more than one feedstock (co-digestion) with the aim of obtaining a balanced
substrate composition and a synergic effect of improved process stability
and higher methane yield (Angelidaki, 2002). Evaluations of the suitability
of biogas feedstocks are presented in Chapter 3 of this topic.
2.3.2 Digestibility
Digestibility is the main AD feedstock parameter, with direct influence on
methane production, and refers to the ability of the substrate to be
decomposed through AD (see Table 2.2). The digestibility of a certain
material depends on its content of easily digestible compounds like simple
sugars. However, biogas feedstock can also contain various amounts of low
digestible compounds, known as recalcitrant matter, such as lingo-
celluloses. Steffen et al. (1998) noted that the anaerobic degradation rate
varies significantly with feedstock composition. Feedstock composition also
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