Environmental Engineering Reference
In-Depth Information
digester. In batch dry digestion processes, the liquid fraction, which is also
called percolate, is used to recirculate microorganisms and nutrients in the
process. In wet digestion (wet fermentation), a share of the liquid fraction can
be reused as mashing liquid for the feedstock. This is the case in agricultural
AD plants digesting very little or no manure, where the liquid fraction can be
used to achieve the desired dry matter content (usually around 10%) of the
feedstock mixture. In waste treatment processes, the liquid fraction can be
used to solubilize the organic wastes in, for example, pulpers.
Re-utilization of the solid fraction
The solid fraction of digestate can be mixed with the other feedstocks and
re-fed to the digester in order to enhance the dry matter content, as is often
the case with pig slurries. There are also practices where the solid fraction of
digestate is treated on site by fungi or other treatments, with the aim of
making the recalcitrant matter in the solid fraction more available to
anaerobic microorganisms and thereby increasing methane. The separated
fibers can be used in storage bunkers to cover silage, such as corn or grass
silage, replacing the foil cover which reduces decomposition. The fibers are
re-fed to the digester together with the silage used as AD feedstock.
A recently investigated method for seasonal AD plants is the storage of
microorganisms in digestate pellets, used to inoculate the new AD processes
of the next season. Speetzen et al. (2011) investigated storage opportunities
for microorganisms from biogas plants treating wastewater from potato and
sugar beet industries. The production and AD of these wastewaters is
seasonal, and thus digestate pellets are used to inoculate the new process and
shorten the start-up time (with lower methane production) of the AD plants,
required by the adaptation phase of the microorganisms. Speetzen et al.
(2011) indicate that the pellets can be stored at 4
8
C for a year, at low cost
and without problems. However, for this application, only a very small
fraction of the digestate will be recirculated and it will be a niche application
to seasonal plants only.
12.4.3 Digestate as an energy carrier
Whenever possible, digestate should be used as fertilizer or soil improver, as
its application adds nutrients and organic matter to the soil, with positive
impacts on overall fertility and carbon storage. Exceptions are situations
when the quality of digestate is not suitable for use as fertilizer (e.g. high
concentrations of chemical pollutants), when such utilization is prohibited
by national legislation, as is often the case for digestate from AD plants
treating sewage sludge from wastewater treatment plants or unsorted
municipal solid waste, or when use as fertilizer is not feasible, as is the case
Search WWH ::
Custom Search