Environmental Engineering Reference
In-Depth Information
Europe by the Animal By-product Regulations (ABPR); further informa-
tion given in Chapter 2.
Although most of the common pathogens and common viruses are
inactivated during the AD process (mesophilic and thermophilic) (Bendixen,
1994, 1995, 1999; Lund et al., 1996), supplementary sanitation is required
for some specific feedstock types. Batch pre-sanitation by pasteurization or
pressure sterilization is done for veterinary safety reasons, but also has the
economic advantage of avoiding the high costs of pasteurizing the whole
digester volume.
12.2.10 Inactivation of plant pathogens and weed seeds
Mesophilic digestion offers significant or total destruction of most disease-
spreading spores that could affect crops (Zetterstrom, 2008; Lukehurst
et al., 2010). Overbeek and Runia (2011) indicate that mesophilic digestion
effectively inactivates plant pathogens. Potato nematodes Globoderar
ostochiensis and G. pallida, are destroyed in a few days at 35
C. Tests
made with Fusarium oxysporum, which affects maize and cereal crops,
showed that it was inactivated by mesophilic AD in one day, and no spores
were present in the final digestate (Overbeek and Runia, 2011).
Plasmodiophora brassicae, which is much more difficult to inactivate
according to Engeli et al. (1993), did not survive after 14 days at 55
8
C.
Research results from Denmark (ICROFS/BioConsens, 2011) show that
AD effectively reduces the germination power of plant and weed seeds
present in feedstock.
8
12.3 Processing of digestate
12.3.1 Reasons for the processing of digestate
After removal from the digester, digestate can be used without any further
treatment. The storage, transport, handling and application of digestate as a
fertilizer results in significant costs for farmers compared with its fertilizer
value; this is due to the large volume and low dry matter content. The costs
increase further with investment in slurry storage capacities, which are
required by national environmental regulations in countries like Denmark,
Germany and France where the period of fertilizer application is limited to
the growing season and the amount of nutrients applied per unit of
agricultural land is restricted by pollution control regulations. The
European nitrate directive also limits the annual nitrogen load which can
be applied to agricultural land (European Commission, 1991). As digestate
has a high content of easily available plant nitrogen this influences the
amount of digestate that can be applied. Such strict legislative frameworks,
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