Environmental Engineering Reference
In-Depth Information
Feedstock description
A detailed description of each material supplied as feedstock to an AD plant
is a very important part of quality control. The description must comply
with national regulations in the area (quality protocols and standards that
are set to ensure that digestate is suitable for use as fertilizer) and must also
be made available to digestate customers. The basic information that must
be provided in the feedstock description is specified in Chapter 2 of this
topic. The feedstock producer must provide a complete and accurate
feedstock description and is responsible for providing the quality as
declared. The biogas plant must verify compliance of the documentation
with the actual quality of the feedstock. Based on their characteristics and
properties and on the intended use of the produced digestate, a specific
material can be accepted or excluded as feedstock for AD. For more details
on the characterization of feedstocks see Chapter 3.
12.2.5 Managing feedstock quality
Digestate to be used as fertilizer must be a high-quality product, free of
physical impurities, pathogenic matter and chemical pollutants. The
feedstocks currently used in agricultural biogas plants are usually of high
quality. Most unwanted impurities supplied by agricultural feedstock types
- except for inert materials such as wood particles, gravel and metals - are
normally decomposed or inactivated by AD processes, so they will not affect
the suitability and safety of digestate utilization as fertilizer. However, some
of the digestible materials used as feedstocks in AD plants contain matter or
compounds that are hazardous to living organisms and the environment
because they are able to pass undestroyed through the AD system; this is the
case for heavy metals and persistent organic pollutants (POPs). Feedstock
materials containing hazardous pollutants or other unwanted compounds in
concentrations considered to be hazardous for living organisms or for the
environment must be excluded from the AD processes where digestate or
digestate fractions are to be used for agricultural purposes. Such materials
can be used as feedstocks for other biogas processes, where the produced
digestate is not used as fertilizer but is used for industrial or energy
applications (such as co-combustion for CHP generation) or where the fate
of the hazardous matters and compounds is strictly controlled, so any
further risks are avoided.
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Physical impurities in AD feedstock
Animal manure and slurry, straw, garden waste, crop wastes, energy crops,
source-separated organic household waste and food waste are valuable
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