Environmental Engineering Reference
In-Depth Information
In addition to the hazards that can occur in connection with the
mechanical operation of biogas plants or the uncontrolled escape of biogas,
the risk that pathogenic germs may spread with the digestion residue in
fields is a topic often discussed by the general public. Because the substrates
in biogas plants can contain bacteria, viruses and parasites that may cause
disease in man, animals or plants, this fear is not at all unfounded at first
sight. Generally, the digestion process has an impact on the health risks of a
substrate. Different scientific studies have shown that germs in a substrate
are reduced by digestion in a biogas plant (Bagge et al. 2010, Lebuhn et al.
2007). Hence, when a substrate is digested that normally would have served
as organic fertilizer without any further treatment, the biogas process
generally reduces the risk of pathogenic germs. If a biogas plant processes
waste that contains an additional health risk, the additional conditions that
must be met before the digestate can be returned to the fields should be
considered. For example, if animal by-products are processed, European
Regulation 1069/2009 stipulates health rules as regards animal by-products
not intended for human consumption (European Commission 2009). The
regulation defines three categories of substrate with different risk levels,
specifies substrates that may be spread and those that must not, and also
spells out the health measures to be taken when the digested product is to be
used as fertilizer.
Factors affecting the health status of the substrate include temperature,
retention time, pH and ammonia content (Eder and Schulz 2006). These
parameters are defined by the texture of the substrate, the method of
digestion and a hygienization unit that may be installed. At the European
level, the requirements on biowaste treatment are laid down in the EU
Waste Framework Directive (European Commission 2008) and Regulation
1069/2009. Concrete actions are defined in national ordinances regarding
the treatment of biological waste but also the use of organic fertilizer.
Regulation 1069/2009 requires that category 3 material should be treated at
70
C for 1 hour. Category 3 materials include, for example, food remains,
kitchen and canteen waste, content of fat separators and flotate.
In addition to treatment in a hygienization unit connected upstream of the
biogas plant, thermophilic digestion at 55
8
C and at least 24 hours dwell in
8
the digester achieve the equivalent of 70
C for 1 hour. In that case, however,
the process owner must demonstrate that the health effect is not impaired by
short-circuits in the flow. In addition to the possible transmission of disease
to humans or animals, the use of organic digestate as a fertilizer also raises
the question of how far weed seed can be spread. In this respect, Schrade
et al. (2003) were able to show that all weed seeds examined were not
germinable after thermophilic digestion and most seeds had lost their
germination capacity after 24 hours under mesophilic (35
8
C) conditions and
8
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