Environmental Engineering Reference
In-Depth Information
(Uludag-Demirer et al., 2005; Marti et al., 2008) have been proposed.
Whatever process is applied, advanced digestate processing in most cases
requires high chemical and energy inputs. Together with increased
investment costs for appropriate machinery, considerable treatment costs
may result. In the following text, distinctions will be made between
unprocessed digestate and digestate fractions (co- and by-products from
digestate processing). An overview of digestate processing technologies is
given in Fig. 12.3.
12.3.3 Solid-liquid separation of digestate
The first step in any digestate processing system is solid-liquid separation:
the partitioning of liquid digestate into high dry matter solid material (fiber,
sludge) and low dry matter liquid. Both fractions can be used without
further treatment as fertilizer. Solid-liquid partitioning separates most of
the phosphorus with the solid fraction and most of the nitrogen with the
liquid fraction, which helps the management of plant nutrients in digestate
by enabling separate dosage of phosphorus and nitrogen and transport and
application of the phosphorus to other areas.
The phosphorus-rich fiber fraction can be applied or sold as a
phosphorus-rich fertilizer; it can be dried and pelletized, composted and
used as soil improver, used for industrial purposes (composite materials) or
even incinerated for energy recovery. As shown in Fig. 12.4, the liquid
fraction contains the main part of nitrogen (N) and potassium (K) and this
fraction can be applied as liquid fertilizer or mixed with a high solids
feedstock and re-fed to the digester. In countries where phosphorus is the
rate limiting nutrient for eutrophication (e.g. Ireland) solid-liquid separa-
tion is advantageous as the solid fraction, may be exported and the liquid
fraction containing little phosphorus, may be land applied. The various
technologies for further processing of the solid and liquid fractions are
described in Sections 12.3.4 and 12.3.5.
A variety of solid-liquid separation technologies are available on the
market; these include decanter centrifuges, screw press separators, bow
sieves, double circle bow sieves, sieve belt presses and sieve drum presses.
The decanter centrifuge and the screw press separator have gained
popularity, especially among farmers who need to export their excess of
nutrients to other areas. Screw press separators are mainly used in energy
crop digestion, where the digestate is rich in fibers. Decanter centrifuges are
used in manure co-digestion (as in Denmark) and also in municipal or
industrial waste treatment plants.
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