Environmental Engineering Reference
In-Depth Information
12.1
Introduction
Digestate, the digested effluent of the biogas production process, consists of
the feedstock materials after extraction of biogas through anaerobic
digestion (AD). The composition and quality of digestate is therefore
highly dependent on the composition and quality of the feedstock used.
Because of the content of easily accessible macro- and micronutrients,
digestate is a valuable crop fertilizer, suitable to be used in the same way as
raw animal slurries. Recycling as fertilizer is considered to be the most
sustainable utilization of digestate, as it is able to provide benefits for society
in general and for the environment in particular, as well as to help the
preservation of limited natural resources such as fossil resources of mineral
phosphorus. For use as fertilizer, the digestate needs to be of the highest
quality and free of pathogens, chemical and physical impurities and
pollutants; this can be achieved by using AD feedstock of controlled quality.
Regulatory frameworks, aimed to guarantee the production of high-quality
digestate, thus are implemented in countries with developed biogas sectors
such as Germany, Denmark, Austria, Sweden Switzerland and the United
Kingdom. The regulatory frameworks are regularly updated and increas-
ingly restrictive, in line with new knowledge and experience.
Depending on its end use and on the requirements related to it, the
digestate can be used as it is produced (whole digestate) or it can be further
refined through a number of treatments and technologies; this is known as
digestate processing. The most well-known, used for its simplicity and low
cost, is digestate separation into liquid and solid fractions, using decanter
centrifuges and screw press separators. There are also more complex
processing technologies, of various degrees of technical maturity. Common
to all is that they provide volume reduction and separation of the valuable
nutrients and fibers from the high volume of water contained in the whole
digestate. In addition, the nutrients are often concentrated in order to
further reduce transportation costs. When utilization of digestate for
agricultural purposes is not feasible because of unsuitable quality or other
local conditions, the whole digestate or fractions resulting from digestate
processing can be used for other purposes, such as landfill cover in the case
of digestate from municipal solid waste, for energy when dewatered or as a
raw material for industrial processes.
12.2 Digestate quality
The quality requirements for digestate depend to a large extent on how it is
to be used. High quality is the essential pre-condition for use as crop
biofertilizer, which is by far the main utilization of digestate. In order to
provide quality assurance and thus to enhance the use of high-quality
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