Environmental Engineering Reference
In-Depth Information
8.2
Feeding system: holding tank.
drops and digestion perturbations can be a consequence. Odour emissions
are another disadvantage of these systems. Nowadays, screw or piston
systems are normally employed, which allow automation and regular
feeding in small amounts. They may enclose intermediate storage volumes,
allowing automated feeding. Mixers and cutters for homogenising the
material or weighing devices are further options to complete the system. The
latter are especially useful when precise dosing or tracing of substrates is
required.
Screw conveyors bring solid substrates into the digester through the
ceiling or the upper part of the side wall, where no hydraulic pressure is
present (Fig. 8.3). They are often used in biogas plants, but are prone to
abrasion when long fibre substrates or impurities are present
in the
feedstock.
In piston systems, substrates are pressed through a delivery cylinder into
the digester (Fig. 8.4). Feeding into the bottom of the digester is possible
using a hydraulic actuator, but backflow of digestate must be prevented by
valves. A disadvantage of this system is compaction of the substrates by the
piston force, which makes it less accessible to the microbes.The formation of
sinking layers is also possible.
More than one feeding system can be used to take account of different
feedstock types. Feeding management has a significant influence on the
fermentation process. Sudden high loads of organic matter or abrupt
substrate changes disturb the microbial community and result in a reduction
in gas production. Optimal feeding is done regularly in small doses, with a
homogenous substrate mix.
In general, the more elaborate and automated a system, the higher its
price. At the same time, these systems allow efficient operation with less time
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