Page 526 - Water and wastewater engineering

DISINFECTION AND FLUORIDATION 13-21

Water that spends a long time in the distribution system allows for the reactions that form
THMs to proceed toward completion. A distribution system retention time is considered high if it
exceeds 48 hours (U.S. EPA, 1999).

Selection of Disinfectant. Although there are other disinfectants and combinations of disinfec-
tants, this discussion is limited to chlorine gas, liquid sodium hypochlorite, chloramines, chlorine
dioxide, ozone, and UV radiation.
A summary of disinfectant properties and considerations in their selection are summarized
in Tables 13-6 and 13-7 .

TABLE 13-6
Summary of disinfectant properties (based on typical disinfectant application)
Condition Chlorine Ozone Chlorine dioxide Permanganate Chloramine Ozone/peroxide Ultraviolet

Produce THM with TOC y s n n y s n
Produce oxidized organics s y s s n y s
Produce halogenated organics y s n n y s n
Produce inorganic byproducts n s y n n s n
Produce BOM s y s n n y n
MRDL applies y n y n y n n
Lime softening impacts y n n n y n y
Turbidity impacts n s n n n s y
Meet giardia - 2.0 log y y y n n n y
Meet giardia - 2.0 log n y y n n n y
Meet crypto - 2.0 log n y y n n n y
Meet crypto - 2.0 log n y y a n n n y
Meet virus - 2.0 log y y y n n n y
Meet virus - 2.0 log y y y n n n y
Secondary disinfectant y n s n y n n
Operator skill (1 low; 5 high) 1 5 5 1 2 5 3
Applicable to large utilities y y y y y y n
Applicable to small utilities y y y y y y y

y yes, n no, s sometimes
a
Ct values to achieve 2.0 log inactivation are very high at common water temperatures.

TABLE 13-7
Consideration for selecting disinfectant
Consideration Cl 2 NaOCl O 3 ClO 2 Chloramine UV

Residual persistence Low Low None Moderate Very low None
pH dependence Yes Yes Some Some Yes None
Safety concerns Very high Moderate High Very high Moderate Moderate
Complex equipment Yes No Very Yes Yes Yes

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