Discussion by Otis J. Sproul,
University of Maine, Orono, Maine,
Department of Civil Engineering.
Inactivation in Anaerobic Digestion
The inactivation of viruses in an anaerobic digester is probably caused by one or more of the
following mechanisms:
1. Heat denaturation of the protein coat
2. Enzymatic action on the protein coat
3. "Natural dieoff".
Dr. Palfi has shown quite conclusively that the anaerobic digestion system is not one which gives a
high order of inactivation even though the sludge is exposed to temperatures of 30°C or more for
extended periods of time.
The reasons for the relatively poor inactivation which Dr. Palfi and others have noted is perhaps
caused by one of the following factors:
1. In completely mixed digesters a significant fraction of the influent sludge appears in the effluent in
times as short as one day.
2. Short circuiting causes early sludge loss even in plug flow reactors.
3. All portions of the anaerobic digestion tank do not reach the digestion temperature.
4. Adsorption of the virus to a surface at a specific chemical site on the virus may have "removed"
this site from the inactivating effects of the heat.
Inactivation as a Function of Time and Temperature
A convenient way of expressing the inactivation effects of temperature and time has been shown in
Figure 1. This data was obtained in our laboratories for poliovirus Type 1 in a 0.0005 M CaCl
2
distilled water solution at 20°C and 30°C. The inactivation has been plotted as a function of the
product of the detention time in days and the temperature in °C. While this sort of analysis is fraught
with some hazards the data does show a fair agreement and is suitable for purposes of this discussion.
The fraction of material which passes out of a completely mixed digester is given by:
- t
— = ( 1 - e ^ )
c o
Where c = Amount of incoming liquid passing from the digester after a retention time, t
c 0 = Total amount of liquid entering the digester in time, t^
t = Retention time, days
td = Detention time of digester at flow rate c 0 , days
e = Napierian log base.
The digester which Dr. Palfi reported on for 1969 was a completely mixed reactor with a detention
time of 21 days. Approximately 9 percent of the influent sludge passed from the digester after a
retention of only one day. Increasing retention times produced increasing fractions of the sludge
which appeared in the effluent which have not been exposed to a lethal combination of time and the
30°C temperature.
This method of analysis does not consider the dynamic nature of a completely mixed system. That
is, for example, the amount of material leaving the reactor after two days of retention is in part made
up of material which came into the digester during the second day and which had a retention time of
only one day. The approach taken here represents a conservative one in that the virus inactivation is
actually less than what would be predicted by a more exact mathematical formulation.
Dr. Palffs 1970 data showed a reduction of 95 percent in the digestion process consisting of
completely mixed 20 day and 10 day primary digesters in series and an unmixed 10 day secondary
digester. The data in Figure 1 indicates that a (Time) (Temp.) product of about 500 would be required
for 95% inactivation of Polio Type 1. With the 33°C temperature in Dr. t*alfi's digesters, and assuming
the same inactivation rates as in Figure 1, then a retention time in excess of 15 days in the digesters
would have been required for the 95 percent inactivation. Using equation 1 and 10 day and 5 day
retention times in the 20 and 10 day primary digesters then about 4.4 percent of the raw sludge would
appear in effluent from the primary digesters after 15 days. The mixing characteristics in the
secondary digester are not known but some amount of short circuiting probably occurred. Some
unknown amount of virus inactivation in the secondary digester did occur in the 4.4 percent leaving
the primaries in less than 15 days. The inactivation which probably occurred, however, is not
105
University of Maine, Orono, Maine,
Department of Civil Engineering.
Inactivation in Anaerobic Digestion
The inactivation of viruses in an anaerobic digester is probably caused by one or more of the
following mechanisms:
1. Heat denaturation of the protein coat
2. Enzymatic action on the protein coat
3. "Natural dieoff".
Dr. Palfi has shown quite conclusively that the anaerobic digestion system is not one which gives a
high order of inactivation even though the sludge is exposed to temperatures of 30°C or more for
extended periods of time.
The reasons for the relatively poor inactivation which Dr. Palfi and others have noted is perhaps
caused by one of the following factors:
1. In completely mixed digesters a significant fraction of the influent sludge appears in the effluent in
times as short as one day.
2. Short circuiting causes early sludge loss even in plug flow reactors.
3. All portions of the anaerobic digestion tank do not reach the digestion temperature.
4. Adsorption of the virus to a surface at a specific chemical site on the virus may have "removed"
this site from the inactivating effects of the heat.
Inactivation as a Function of Time and Temperature
A convenient way of expressing the inactivation effects of temperature and time has been shown in
Figure 1. This data was obtained in our laboratories for poliovirus Type 1 in a 0.0005 M CaCl
2
distilled water solution at 20°C and 30°C. The inactivation has been plotted as a function of the
product of the detention time in days and the temperature in °C. While this sort of analysis is fraught
with some hazards the data does show a fair agreement and is suitable for purposes of this discussion.
The fraction of material which passes out of a completely mixed digester is given by:
- t
— = ( 1 - e ^ )
c o
Where c = Amount of incoming liquid passing from the digester after a retention time, t
c 0 = Total amount of liquid entering the digester in time, t^
t = Retention time, days
td = Detention time of digester at flow rate c 0 , days
e = Napierian log base.
The digester which Dr. Palfi reported on for 1969 was a completely mixed reactor with a detention
time of 21 days. Approximately 9 percent of the influent sludge passed from the digester after a
retention of only one day. Increasing retention times produced increasing fractions of the sludge
which appeared in the effluent which have not been exposed to a lethal combination of time and the
30°C temperature.
This method of analysis does not consider the dynamic nature of a completely mixed system. That
is, for example, the amount of material leaving the reactor after two days of retention is in part made
up of material which came into the digester during the second day and which had a retention time of
only one day. The approach taken here represents a conservative one in that the virus inactivation is
actually less than what would be predicted by a more exact mathematical formulation.
Dr. Palffs 1970 data showed a reduction of 95 percent in the digestion process consisting of
completely mixed 20 day and 10 day primary digesters in series and an unmixed 10 day secondary
digester. The data in Figure 1 indicates that a (Time) (Temp.) product of about 500 would be required
for 95% inactivation of Polio Type 1. With the 33°C temperature in Dr. t*alfi's digesters, and assuming
the same inactivation rates as in Figure 1, then a retention time in excess of 15 days in the digesters
would have been required for the 95 percent inactivation. Using equation 1 and 10 day and 5 day
retention times in the 20 and 10 day primary digesters then about 4.4 percent of the raw sludge would
appear in effluent from the primary digesters after 15 days. The mixing characteristics in the
secondary digester are not known but some amount of short circuiting probably occurred. Some
unknown amount of virus inactivation in the secondary digester did occur in the 4.4 percent leaving
the primaries in less than 15 days. The inactivation which probably occurred, however, is not
105