Environmental Engineering Reference
In-Depth Information
8.2.2 Reactor type
Digestion reactors are characterised by the feeding mode (batch or
continuous) and by the mixing type (CSTR or plug-flow); reactor types
are described in more detail in Chapter 5. The choice of reactor type is
strongly dependent on feedstock characteristics. Batch reactors (usually
garage-type systems) are exclusively used for solid feedstocks. As there is no
mixing, impurities or fibrous substrates do not disturb the process, which is
an advantage of this system. Distribution of micro-organisms happens
through water sprinkling from the digester ceiling. If the substrates are too
compact, dry zones may appear. Structuring materials such as wood chips
and branches help efficient water percolation.
Continuous reactors are either plug-flow or CSTR systems. Plug-flow
reactors are used for solid feedstocks. The entering substrates push material
through the digester, and this plug-flow effect can be achieved when the dry
matter content of the substrate mix is above 20% at the entrance of the
digester. CSTRs are used for low dry matter content substrates. Solid
substrates can be introduced as long as the dry matter content of the
substrate mix in the digester stays below 15% (Go¨ risch and Helm, 2006).
Above this level, complete mixing of the reactor contents cannot be
guaranteed. In plants with more than one digestion tank, plug-flow digesters
and CSTRs can be combined and substrates can go through one or both of
them depending on their dry matter content and degradation rate.
8.2.3 Number of phases
Most biogas plants function within a one-phase system, which means that
all the steps of microbial degradation take place in the same tank. The
advantages of this method are simple processing and lower investment costs.
By contrast, a two-phase system separates the hydrolysis stage from the
process (in a separate tank); pH, temperature and retention time can be
optimised for each phase. This leads to better degradation kinetics and is
recommended for substrates with a high content of sugar, starch or proteins.
During the hydrolytic phase, these easily degradable substances produce
large amounts of acids, which inhibit methane formation in a one-phase
system.
8.2.4 Reactor temperature
According to the temperature in the reactor, there is a distinction between
psychrophilic (10-25
8
C), mesophilic (25-45
8
C) and thermophilic digestion
(50-58
C). The temperature in the reactor affects several parameters, each
having a significant influence on the digestion process.
8
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