Environmental Engineering Reference
In-Depth Information
Table 12.5 Combinations of temperatures and MGRTs for sanitation, equivalent
to 708C for 1 hour. Example from Denmark
Temperature
(8C)
MGRT at
thermophilic
temperature (hours)
a
MGRT by treatment in
batch sanitation tank (hours)
Before thermophilic
digestion
a
Before mesophilic
digestion
b
52.0
10
53.5
8
55.0
6
5.5
7.5
60.0
2.5
3.5
a
Hydraulic retention time (HRT) in the digester must be at least 7 days.
b
Temperature of 20-328C; HRT minimum 14 days.
Source: Compiled after Bendixen (1994, 1995, 1999).
and retention time inside the digester can provide pathogen reduction in
animal manure/animal slurries and in vegetable wastes equivalent to the EU
sanitation standard of 70
C for 1 hour (Bendixen, 1994, 1995, 1999;
European Commission, 2009). The hydraulic retention time (HRT)
represents the average time (usually expressed in days) the feedstock resides
inside the digester, at process temperature, and is given by
8
m
3
HRT
ð
hours or days
Þ¼
Digester volume
ð
Þ
m
3
horm
3
=
Influent flow rate
ð
=
=
day
Þ
The HRT influences digestate quality and depends to a large extent on the
digestibility of the feedstock mixture.
In continuous flow stirred digesters, it is possible that portions of the
feedstock mixture pass through the digester by short cuts. The minimum
guaranteed retention time (MGRT) is therefore the minimum time (usually
measured in hours) that any portion of the feedstock resides inside the
digester at constant process temperature. Depending on the feedstock
materials digested, combinations of process temperatures and MGRT are
allowed (Table 12.5) to provide sanitation equivalent to pasteurization
(70
C for 1 hour) for feedstock types where other specific pathogen
reduction measures are not required by other regulations. Table 12.5
shows that a sanitation equivalent to 70
8
C and a
MGRT of 10 hours, when the HRT is a minimum of 7 days. The specific
combinations of temperatures and MGRT/HRT must be respected.
Occurrences of bovine spongiform encephalopathy (BSE) and foot and
mouth disease (mononucleosis) have led to the enforcement of strict rules on
the treatment and further use of animal by-products, aiming to prevent
further spread of such diseases. Since 2002, the utilization of animal by-
products not intended or suitable for human consumption is regulated in
8
C for 1 hour occurs at 52
8
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