Environmental Engineering Reference
In-Depth Information
which seek to protect the environment, may necessitate transport and
redistribution of nutrients away from intensive areas. These conditions
make digestate processing attractive.
Digestate processing involves the application of different technologies to
the effluent from anaerobic digesters. The technologies applied are mostly
comparable to existing technologies for manure processing, sewage sludge
treatment or wastewater treatment. Digestate processing can be approached
in two ways. The first is digestate conditioning, which aims to produce
standardized biofertilizers (solid or liquid) in which the quality and
marketability of the digestate is improved. The second can be described as
digestate treatment; similar to wastewater treatment, it is applied in order to
remove nutrients and organic matter from the effluent and allow discharge
into a sewage system, an on-site wastewater treatment plant or a receiving
stream. In most cases it will be necessary to carry out both conditioning and
treatment in order to establish a viable digestate processing concept.
12.3.2 Overview of technologies for the processing of
digestate
Digestate processing can be partial, usually targeting volume reduction, or it
can be complete, refining digestate to pure water, fibers/solids and
concentrates of mineral nutrients. The first step in digestate processing is
to separate the solid phase from the liquid. The solid fraction can
subsequently be directly applied as fertilizer in agriculture or it can be
composted or dried for intermediate storage and enhanced transportability.
To improve solid-liquid separation, flocculation or precipitation agents are
commonly applied.
Partial processing uses relatively simple and cheap technologies. For
complete processing, different methods and technologies are currently
available, with various degrees of technical maturity and requiring high
energy consumption and high costs. For nutrient recovery, membrane
technologies such as nano- and ultra-filtration followed by reverse osmosis
are used (Fakhru'l-Razi, 1994; Diltz et al., 2007). Membrane filtration
produces a nutrient concentrate and purified process water (Castelblanque
and Salimbeni, 1999; Klink et al., 2007). The liquid digestate can also be
purified through aerobic biological wastewater treatment (Camarero et al.,
1996). However, because of the high nitrogen content and low biological
oxygen demand (BOD), addition of an external carbon source may be
necessary to achieve appropriate denitrification. A further possibility for
concentrating digestate is evaporation with waste heat from the biogas
plant. For reducing the nitrogen content in the digestate, stripping (Siegrist
et al., 2005), ion exchange (Sa´ nchez et al., 1995) and struvite precipitation
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