Civil Engineering Reference
In-Depth Information
Dissolved organics.
Dissolved organics can interfere with disinfection by consum-
ing the disinfectants to produce compounds with little or no microbiocidal activity.
These reactions also produce undesirable disinfection by-products (DBPs).
•
Because of the difficulties inherent in timely measurements of pathogens, regulatory
compliance for disinfection requirements is specified as a
CT
value.
CT
is defined as
the product of the residual disinfectant,
C,
in mg / L, and the contact time,
T
, in minutes.
Because disinfectant concentrations decrease as the liquid passes through the contact
basin, the disinfection residual is typically the average or effluent disinfectant residual.
In order to address nonperfect plug flow conditions in full-scale contact basins, the
contact time used for this calculation is specified as the T
10
value that represents the
time after which 10 percent of the water has passed through the contact basin—in
other words, the time that 90 percent of the liquid is retained in the contact basin.
The EPA developed
CT
values for the inactivation of
Giardia
and viruses for various
disinfectants. Table 19-1 presents illustrative
CT
values for virus inactivation using
chlorine, chlorine dioxide, ozone, chloramine, and ultraviolet light disinfection. Table
19-2 shows illustrative
CT
values for inactivation of
Giardia
cyst using chlorine,
chloramine, chlorine dioxide, and ozone.
REGULATORY REQUIREMENTS
Disinfection practice must balance the need to inactivate pathogens and eliminate the
formation of undesirable disinfection by-products (DBPs). Disinfection must achieve
the following:
Inactivate the target organisms
•
Meet DBP requirements
•
Meet disinfectant residual requirements
•
Maintain the required disinfection residual in the distribution system
•
TABLE 19-1.
CT
Values for Inactivation of Viruses
Inactivation
Disinfectant
Units
2-log
3-log
4-log
Chlorine*
mg
min/L
3
4
6
Chloramine†
mg
min / L
643
1,067
1,491
Chlorine dioxide‡
mg
min / L
4.2
12.8
25.1
Ozone
mg
min / L
0.5
0.8
1.0
UV
21 36 Not available
Source:
See Reference 2. Adapted from
Guidance Manual for Compliance with the Filtration and Disinfection
Requirements for Public Works Systems Using Surface Water Systems,
by permission. Copyright
mW
s/cm
2
1991, Amer-
ican Water Works Assocation.
* Values are based on a temperature of 10
C, pH range of 6 to 9, and a free chlorine residual of 0.2 to 0.5
mg/L.
† Values are based on a temperature of 10
CandapHof8.
‡Values are based on a temperature of 10
C and a pH range of 6 to 9.
