- For all practical purposes, the volume of the source material remains unchanged, since only
some 35 - 50% of the organic substances (corresponding to 5 - 10% of the total volume) is
converted to gas.
- Fermentation reduces the C/N-ratio by removing some of the carbon, which has the advantage
of increasing the fertilizing effect. Another favorable effect is that organically fixed nitrogen and
other plant nutrients become mineralized and, hence, more readily available to the plants.
- Well-digested slurry is practically oderless and does not attract flies.
- Anaerobic digestion kills off or at least deactivates pathogens and worm ova, though the effect
cannot necessarily be referred to as hygienization (cf. Table 3.8). Ninety-five percent of the ova
and pathogens accumulate in the scum and sediment. Plant seeds normally remain more or
less unaffected.
- Compared to the source material, digested slurry has a finer, more homogeneous structure,
which makes it easier to spread.
Table 3.8: Survival time of pathogens in biogas plants (Source: Anaerobic Digestion 1985)
Bacteria
Salmonella
Shigella
Poliviruses
Schistosoma
ova
Hookworm ova
Ascaris ova
Colititre
Thermophilic
fermentation
53-55 °C
Fatality
Days
Rate
(%)
1-2 100.0
1 100.0
hours
100.0
1 100.0
2 100.0
2 10-1 - 10-2
Mesophilic
fermentation
35-37 °C
Fatality
Days
Rate
(%)
7 100.0
5 100.0
9 100.0
7 100.0
10 100.0
36 98.8
21 10-4
Psycrophilic
fermentation
8-25 °C
Fatality
Days
Rate
(%)
44 100.0
30 100.0
7-22
100.0
30
100
40-60
90.0
53.0
10-5 -10-4
Table 3.9: Concentration of nutrients in the digested slurry of various substrates!
(Source: OEKOTOP, compiled from various sources)
Type
of N
substrate
Cattle dung
Pig dung
Chicken
manure
2.3 - 4.7
4.1 - 8.4
4.3 - 9.5
P2O5
0.9 - 2.1
2.6 - 6.9
2.8 - 8.1
K2O
CaO
MgO
—% TS—
4.2 - 7.6 1.0 - 4.2
1.6 - 5.1 2.5 - 5.7
2.1 - 5.3 7.3 - 13.2
0.6 - 1.1
0.8 - 1.1
1.1 - 1.6
25