Environmental Engineering Reference
In-Depth Information
electricity in engines at the place at which it is produced, the raw biogas
must be cleaned in a first process in which the water vapor saturated biogas
is desulfurized and dried by cooling.
Certain basic conditions must be met to enable the bacteria to degrade the
substrate efficiently. These are: (1) absence of air (anaerobic atmosphere);
(2) uniform temperature; (3) optimum nutrient supply; (4) optimum and
uniform pH. The equipment of a biogas plant should be able to meet these
basic requirements. Therefore, a biogas plant designer should know from
the beginning what kind of substrate the plant will feed on so that the right
equipment for efficient biogas production can be selected.
The methods of biogas production can be characterized by the number of
process steps, the process temperature, the dry matter content and the way
in which the substrate is fed. Biogas plants feeding on agricultural by-
products such as liquid manure, harvest residue and energy crops often
employ a single-step process in the mesophilic (32-42
C) temperature range
with wet fermentation and quasi-continuous feeding. The method can be
varied depending on the requirements the process must meet in terms of
speed, the degree of digestion and the hygienizing action. For example,
hydrolysis as the first step usually accelerates the process and may also result
in a higher degree of degradation. Increasing the process temperature from
the mesophilic (32-42
8
C) level also speeds up
degradation and improves the health status of the substrate (Eder and
Schulz 2006).
Better health results are also reported for the plug flow fermentation
method in which the substrate is mixed by the slow rotation of an agitator
and moved through a long horizontal digester. Because the substrate in the
digester is not mixed in one pass, quick passage from the feed point to the
delivery point is prevented and a minimum dwell time of the substrate in the
digester is obtained. This enforced dwell time of the substrates improves the
hygienizing action of this method. Unlike the full-mix wet digestion process,
a plug flow digester can normally carry a higher volume load of organic
material per cubic meter of digester volume.
If the substrate digested in the biogas plant contains more than 20% dry
matter, so-called dry digestion methods will normally be applied. In these
methods, the digester is charged with stackable substrates. The substrates
are not mixed, but a liquid called percolate runs through them. After a
sufficiently long dwell time, the digester is opened and the digested product
removed. In addition to this batch process, several other methods for
digesting solid substrates have been developed that like the batch processes,
are now primarily used for digesting municipal waste. The different
processes will be dealt with in another chapter of this handbook and so
are not described in detail here.
The purpose of biogas technology is the conversion of organic substances
C) to the thermophilic (45-57
8
8
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