Environmental Engineering Reference
In-Depth Information
handling large volumes of low dry matter biomass can benefit from a
volume reduction by separation. By mixing different digestate fractions (e.g.
ammonia liquid from stripping), the concentration of some nutrients can be
adjusted and this can be a considerable advantage for the marketing of the
product as biofertilizer.
12.4 Utilization of digestate and digestate fractions
Digestate or digestate fractions can be used as agricultural fertilizers or
high-quality compost and soil improvers, re-digested in the biogas plant,
used for energy production or other applications (e.g. industrial products)
or even as animal feed. The suitability of digestate for various uses is highly
dependent on its composition and quality and the local nutrient situation.
Digestate is, first of all, a valuable biofertilizer, rich in plant nutrients and
with great potential worldwide as a substitute for mineral fertilizers. The
quality requirements related to digestate use as fertilizer were addressed in
Section 12.2. Apart from unsuitable quality, specific local conditions (e.g.
regional excess of nutrients in intensive agricultural areas or in highly
populated areas) can make land application of digestate problematic as it
would add further nutrients to the local environment. In such cases, other
solutions for digestate processing and utilization must be applied.
12.4.1 Digestate as agricultural fertilizer and soil improver
Digestate as fertilizer
Recycling as a crop fertilizer or soil improver is the most sustainable
utilization of digestate. The concept of closing the nutrient cycle and
substitution of fossil fertilizers are the main drivers. Safe agricultural
recycling requires digestate to be of highest quality. The simplest way to use
digestate as fertilizer is to apply the 'whole digestate' as it is removed from
the digester onto crop fields, without further treatment. The amounts and
timing of application are regulated by national fertilizer management
legislations and good agricultural practices, and depend largely on the soil
properties and the crops being cultivated. To prevent loss of nutrients and
pollution problems, most European countries have adopted the prescrip-
tions of the European nitrate directive (European Commission, 1991) and
have taken measures to limit nitrate input onto land to a maximum amount
of 170 kg/ha per year and implement bans for fertilizer application during
the winter season. The seasonal restriction means that, in Europe, digestate
must be stored for four to nine months, depending on the national
requirements. In other climates, with continuous plant growth throughout
the year, storage requirements may be shorter. In all cases, sufficient
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