Environmental Engineering Reference
In-Depth Information
digestate storage capacity needs to be established to accommodate digestate
production, since its land application must be specifically adjusted to the
season of plant growth.
Digestate is applied as fertilizer in the same way and using the same
equipment as the spreading of animal manures and slurries. The most
suitable methods of application are the same as those used to apply raw,
untreated slurry. The equipment used should minimize the surface area
exposed to air and ensure rapid incorporation of digestate into the soil. For
these reasons, digestate is best applied with trailing hoses, trailing shoes or
by injection into the topsoil. Spreading digestate by splash plate is banned in
many countries as it causes air pollution and losses of valuable nutrients.
Digestate has a declared content of nutrients and can be therefore
completely integrated in the fertilization plan of the farm. Due to its higher
homogeneity and flow properties, digestate penetrates soil faster than
untreated animal slurries (Al Seadi et al., 2008). The application of digestate
as fertilizer involves risks of nitrogen losses through ammonia emissions and
nitrate leaching. In order to minimize these risks, the following simple rules
of good agricultural practice should be applied.
.
Digestate should be only applied at the start of vegetative plant growth
(i.e. spring).
.
Optimum weather conditions for the application of digestate are high
humidity but not excessive rain and no wind.
.
Dry, sunny and windy weather increases evaporation and reduces
nitrogen efficiency considerably.
.
Moderate stirring of digestate before application is recommended.
.
Only cooled digestate from the post-storage tank should be applied.
.
Dragging pipes, dragging hoses or direct injection into soil should be
used for digestate application, never broad spreading (splash plate
spreading).
.
If digestate is applied to the surface of soil it should be immediately
incorporated into the soil.
For economic efficiency, the fields where digestate is applied should be
located close to the biogas plant, to avoid increased transport costs. When
digestate has to be transported for longer distances, volume reduction
through solid-liquid separation can be considered. As the liquid and the
solid fraction of digestate are used separately, different characteristics of the
products have to be considered (Fuchs and Drosg, 2010). Both fractions
contain the macronutrients nitrogen, phosphorus and potassium. The liquid
fraction contains higher amounts of potassium and nitrogen whereas the
solid fraction contains fibrous material and higher amounts of phosphorus.
The simplest way of using these fractions is for the solid fraction to be
composted and used as soil
improver, while the liquid fraction can be
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