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10.4 CHAPTER TEN

TABLE 10.2 Surface Water Treatment Rule Disinfection
Requirements
Log removals
Process Giardia cysts Viruses
Minimum log removal inactivation 3 4
Conventional treatment credit 2.5 2
Remaining for disinfection 0.5 2
Direct filtration credit 2 1
Remaining for disinfection 1 3

system's treatment must be sufficient to ensure that the total process of removal plus in-
activation achieves at least 99.9% (3-log) inactivation or removal of Giardia cysts and
99.99% (4-log) inactivation or removal of viruses. Source waters that are particularly vul-
nerable to microbial contamination may require greater log reductions, at the discretion
of the primacy agency.
Credit for physical removal of pathogenic organisms is given to properly operated fil-
tration processes, as indicated in Table 10.2. The remaining log inactivation is required
to be achieved by the disinfection process. Application of the CT concept is discussed
further under the design consideration section for each oxidant.
Approval of Lower CT Values. The CT values presented in the tables provided by
USEPA are generally considered to be conservative. Each primacy agency may allow
lower CT values for individual systems based on on-site studies showing that adequate
inactivation is achieved under all flow and raw water conditions. Protocols and require-
ments are extensive but may be justified for systems that have unusual circumstances war-
ranting the studies.
Single Point of Disinfection. Systems with only one point of disinfectant application
may calculate the CT that is being achieved by the entire system by measuring the disin-
fectant residual at the exit of the contact volume. The multiplication of this residual con-
centration C and the contact time T through all basins and piping from the application
point to the measurement point will provide a conservative CT value. This is the simplest
calculation, but as indicated in Figure 10.1, this simple CT calculation does not take credit
for the higher disinfectant residual that exists in the contact volume prior to the exit.
An alternate method to calculate the level of disinfection is to use segregated flow
analysis (SFA). SFA estimates disinfection in a contact volume by calculating disinfec-
tion in a number of theoretical "packets" of water that enter the contact volume. Some
packets have very short contact times T but are exposed to high disinfectant concentra-
tions C, while other packets have very long contact times T but are exposed to low dis-
infectant concentrations C. The CT values for each packet are estimated by multiplying
the disinfectant residual with time curve C and the "F curve" T from the tracer test of the
contact volume. The CT value for each packet is used to calculate the log inactivation,
and then all the log inactivations are summed to calculate the overall level of disinfection
in the contact volume. SFA is most applicable to disinfectants with rapid decay rates, such
as ozone, because of the conservative nature of the Tlo concept. (T2o is discussed below.)
However, SFA must be approved for individual systems by each primacy agency. A sim-
ilar comprehensive approach is the Integrated Disinfection Design Framework (IDDF).
Information regarding the IDDF can be found in AWWARF's publication Implementa-
tion of the Integrated Disinfection Design Framework (2001).
Multiple Disinfectants or Application Points. Systems that apply disinfectant at more
than one point will have to profile the system by computing the CT for each section be-