Disinfection - Public Water Systems - page 661

In addition to groundwater sources, this distinction may also apply to waters that do
not require filtration (i.e., sources that would qualify for filtration avoidance) or sys-
tems with natural filtration.
The second selection point is for disinfection systems that require only virus in-
activation. This situation is created in groundwater supplies, or treatment systems that
include membrane filtration, which serve as a complete barrier for bacteria and pro-
tozoa.
Selecting a New Primary Disinfectant If it is determined, using the schematic in
Figure 19-18 or Figure 19-19, that a new disinfectant is required, the third phase in
the decision process (Fig. 19-20) addresses the factors concerning selection of a pri-
mary disinfectant:
• TOC Concentration. A high TOC concentration indicates a high potential for
DBP formation. In these cases, the decision tree will favor those disinfectants that will
not produce DBPs or will produce the least amount of DBPs. Note that precursor
removal and enhanced coagulation are used to reduce TOC during treatment optimi-
zation in Step 1 (see Fig. 19-20). ''High TOC'' quantifies the potential to produce
DBPs and is defined as a condition meeting one of the following criteria:
• TOC exceeds 2 mg / L
• TTHM exceeds MCL (0.08 mg / L under Stage 1 D / DBP Rule)
• HAA5 exceeds MCL (0.06 mg / L under Stage 1 D / DBP Rule)
• Bromide Concentration. The reactions of strong oxidants (ozone and peroxone)
with bromide to produce hypobromous acid and bromate precludes their usage with
waters containing high concentrations of bromide. High bromide is defined as con-
centrations exceeding 0.10 mg / L.
• Filtered versus Nonfiltered Systems. Two separate decision trees are presented:
for those systems that include filtration and those that do not filter. This distinction
addresses the degree of inactivation required. A high log inactivation is required for
non-filtered surface waters, requiring a high disinfectant dose or a long contact time
( CT ). Consequently, the potential for DBP formation increases. In addition, some dis-
infectants, such as chlorine dioxide, have an upper-limit dose that can be applied;
therefore, these disinfectants cannot be considered for systems requiring high disin-
fectant doses unless chlorite removal is practiced (e.g., with ferrous salt).
Note that in some instances a clear decision is not possible because the exact
oxidation reactions and site-specific conditions play a significant role. For example, a
decision to use ozone in the presence of bromide (which can form brominated DBPs)
must be evaluated against potential DBPs formed by other disinfectants. In these cases,
a bench or pilot study may be required to determine the extent and nature of DBP
formation using the feasible disinfectants while meeting inactivation requirements.
Selecting a Secondary Disinfectant The selection of a secondary disinfectant de-
pends on the selected primary disinfectant. In addition, Figure 19-21 identifies three
decision points:
• Assimilable organic carbon (AOC) concentration. AOC is produced when a
strong oxidant (e.g., ozone or peroxone) is used as the primary disinfectant in the
presence of high-TOC water. High AOC is defined as concentrations exceeding 0.10