Environmental Engineering Reference
In-Depth Information
applied as nitrogen-rich fertilizer or further processed and sold as
concentrated liquid fertilizer. The economics of such processes have to be
considered in advance.
For agricultural biogas plants that digest energy crops and/or manure and
vegetal residues from crop production, hygiene and contamination aspects
are usually not a problem for use of digestate as fertilizer. Pathogen
inactivation is important if the produced digestate is also used as fertilizer by
other farmers. In centralized co-digestion plants that co-digest manure from
several farms with various types of wastes and residues, strict hygiene and
other quality assurance measures must be taken in order that no pathogens
are transmitted between farms and the produced digestate is not polluted by
xenobiotic compounds. The tankers used to transport feedstock to the
biogas plant must be cleaned and disinfected before loading with digestate
for subsequent delivery to the farm, using standard procedures for cleaning
biomass transport vehicles. The tanker will service one farm at a time and the
same disinfection procedures must be applied before servicing the next farm
(Al Seadi and Lukehurst, 2012). Depending on the local nutrient situation,
digestate can be sold or given for free to crop farmers. The biogas plant
operator rarely pays a tipping fee when digestate is taken by crop farmers.
Digestate as soil improver
The separated solid fraction of the digestate can be further processed by
composting. The resulting compost can be used as a multifunctional soil
improver in agriculture and horticulture or for topsoil production. The
application of compost from digestate has the same effect on soil as any high-
quality compost, improving soil quality, bringing valuable microorganisms
into the soil and improving the water retention capacity and the pH buffer
capacity of the soil. Digestate contains important amounts of phosphorus
and potassium, covering the crop requirement of such nutrients in many
cases. As with all soil improvers, the quality of the composted digestate used
as soil improver is defined by parameters such as dry matter content, organic
matter content, pH, nutrient content, particle size and bulk density.
12.4.2 Re-utilization of digestate on site
Re-utilization of the liquid fraction
Digestate fractions from solid-liquid separation are frequently re-fed to the
AD reactor. This is common practice for dry digestion processes, either
continuous or batch, where the liquid fraction is recirculated. In continuous
dry digestion processes, the feedstock material (e.g. silage) is mixed with the
liquid digestate fraction and digested. This is often done in a plug-flow
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