Environmental Engineering Reference
In-Depth Information
12.6
Set-up of a screw press separator.
Solid-liquid separation of digestate by screw press separator
Screw press separators are often used in medium to large-scale biogas plants
with high fiber content in the digestate, as is the case for energy crop
digestion. Figure 12.6 shows the set-up: a screw presses fibers against the
cylindrical sieve; the liquid fraction drains through the sieve; because of the
increasing diameter of the screw, the pressure increases with the advance of
fibers in the separator; finally, the solid fiber fraction exits at the end of the
separator, where the resistance can be adjusted mechanically.
Unlike decanter centrifuges, screw press separators cannot separate
sludge fractions from the digestate. If the digestate contains mainly fiber
fractions, the amount of solid fraction that will accumulate is dependent on
the dry matter content of the digestate. Bauer et al. (2009) found a
correlation between dry matter content in the digestate and the amount of
solid fraction accumulated (Fig. 12.7).
The separation efficiency of different components in the digestate has
been investigated, as shown in Table 12.9. As mentioned earlier the
separation efficiency will always depend on the dry matter and fiber content
in the digestate. The advantages of the screw press separator over the
decanter centrifuge are low investment costs (approx. 20,000
for a 500 kW
el
plant) (Bauer et al. 2009) and low energy consumption (0.4-0.5 kWh/m
3
)
(Fuchs and Drosg, 2010).
€
Use of precipitating agents for enhancement of separation
The use of chemicals for enhancement of separation is a relatively new
approach for treating digestate or animal manure, though it has become a
widely accepted method for the treatment of municipal and industrial
wastewater. The chemicals most commonly used for phosphorus removal
from wastewaters are aluminum sulfate (Al
2
(SO
4
)
3
), ferric chloride (FeCl
3
),
ferric sulfate (Fe
2
(SO
4
)
3
)) and lime (Ca (OH)
2
). For further agglomeration
of coagulated particles, the addition of polymers may be needed under
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