Environmental Engineering Reference
In-Depth Information
Table 12.4 Comparison of decimation time T 90 (time for destruction of 90% of
pathogens) of some pathogenic bacteria in AD system and untreated animal
slurry system
Bacteria
AD system
Untreated slurry system
53
8
C
35
8
C
18-21
8
C
6-15
8
C
T
90 (hours)
T
90 (days)
T
90 (weeks)
T
90 (weeks)
Salmonella
typhimurium 0.7
2.4
2.0
5.9
Salmonella
dublin
0.6
2.1
Escherichia coli
0.4
1.8
2.0
8.8
Staphylococcus aureus
0.5
0.9
0.9
7.1
Mycobacterium
paratuberculosis
0.7
6.0
Coliform bacteria
3.1
2.1
9.3
Group D streptococci
7.1
5.7
21.4
Streptococcus faecalis
1.0
2.0
Source: Compiled after Bendixen (1994, 1995, 1999).
12.2.9 Controlled inactivation of animal pathogens
The AD process has a sanitization effect, being able to inactivate most of the
common pathogens present in the feedstock mixture inside the digester
(Table 12.4). The inactivation of pathogens occurs as a result of a
combination of constant process temperatures (thermophilic or mesophilic)
and retention times of feedstock, in interaction with other parameters of the
micro-environment inside the digester.
The sanitation efficiency of AD can be measured using indicator
organisms. One of the most common of these is Streptococcus faecalis
(FS) (Bendixen, 1994, 1995, 1999). A reduction of the amount of FS per
gram biomass of 3-4 log 10 units corresponds to the combined effect of
digestion temperature and time of exposure able to destroy most of the
pathogens such as Salmonella sp. and Pestivirus (ssRNA / Flavivirus) and to
reduce the viability of many parasites such as Ascaris.
Operators must select process temperatures and retention times that are
appropriate for the kind of feedstock to be digested. In the case of existing
AD plants, the choice of allowable feedstock depends to a large extent on
the type of process applied (e.g. mesophilic or thermophilic) and the existing
pre-treatment facilities at the plant. The combination of process temperature
and retention time is the most important pathogen inactivation factor.
Martens et al. (1998), Engeli et al. (1993) and Carrington (2001) indicate
that the degree of pathogen inactivation is more complex, occurring from a
combined effect of temperatures and retention time with other process
parameters such as pH, redox potential and NH 3 concentration inside the
digester. Combinations of thermophilic or mesophilic process temperatures
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