Environmental Engineering Reference
In-Depth Information
earthworms. In general, composting of the solid fraction increases the
concentration of nutrients in the solid fraction, but also may result in
nitrogen loss.
Drying
Besides composting, solid digestate can be also dried by different
technologies such as solar radiation or excess heat from a CHP plant. The
dried material can be used in the horticultural and gardening sectors, as is or
in pelletized form. The material can be used also in nurseries or for special
cultivation systems such as mushroom production.
Local policy and markets influence the marketability of compost or dried
digestate. Quality standards and legislation on fertilizers and compost
products need consideration. Especially for waste digestate, concentrations
of heavy metals and other chemical pollutants may be a barrier to the sale of
digestate products. Legal frameworks in most countries stipulate the quality
conditions for the marketing of waste-based digestate products.
12.3.5 Further processing of the liquid fraction
The liquid fraction can be applied on soils as nitrogen-rich liquid fertilizer
without any further processing. It can also be re-fed to the AD reactor, as
described in Section 12.4.2. The most important technologies for further
treatment of the liquid fraction of digestate are ammonia removal,
membrane filtration, aerobic treatment and evaporation.
Ammonia stripping
Among the intermediate processing steps to reduce nitrogen concentration
in digestate, ammonia stripping seems to be one of the preferable
technologies. Compared to struvite (magnesium ammonium phosphate)
precipitation, which requires the addition of magnesium and phosphate to
allow a maximum of 88% ammonia removal, a pH of 9.5 and an
ammonium: magnesium:phosphate molar ratio of 1:1.25:1 (Miles and Ellis,
1998), ammonia stripping has the advantage that it does not require
expensive chemicals. Due to the clogging problems in state-of-the-art
packed stripping columns, the trend seems to be towards more simplified
stripping vessels. The big advantage of ammonia stripping is that a
standardized nitrogen fertilizer product can be recovered. In addition, such
a fertilizer liquid can be used to enrich other digestate fractions in digestate
processing to a standardized nitrogen concentration, which can increase
their marketability.
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