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10.34 CHAPTER TEN

Chlorination System Design for SWTR Compliance
The following simplified steps can be taken to determine probable CT required and the
probable credit:
1. Determine the total removal/inactivation required.
• Determine whether the usual 3-log Giardia and 4-log virus removal/inactivation
applies.
• Determine whether higher removal/inactivation is required because of a vulnerable
source.
2. Determine the credits that will be allowed for physical removal.
• For conventional treatment--2.5-1og Giardia and 2-log virus.
• For direct filtration--2-1og Giardia and l-log virus.
3. Determine credits required for disinfection.
• For conventional treatment--minimum 0.5-log Giardia and 2-log virus.
• For direct filtration--minimum l-log Giardia and 3-log virus.
4. Determine CT required for design conditions, using tables provided in publications de-
tailing the SWTR requirements. Note that the worst case would be a combination of
high pH, low temperature, and high flow; however, these may not occur concurrently,
and several combinations of representative conditions should be considered.
5. Using Figures 10.2, 10.3, and 10.4, determine the hydraulic efficiency of the basins
that the water will pass through from the point of disinfectant application until it en-
ters the distribution system (or at the first customer, if appropriate).
6. Using Table 10.2, compute the total effective detention time through the plant.
7. Multiply the detention time by the chlorine residual at the first customer, and compare
it with the required CT.
If primary disinfection can be applied as the raw water enters the plant, there is usu-
ally not much problem in getting adequate detention time through the process basins in a
full conventional plant. However, many plants now practice zero or minimal prechlori-
nation to control disinfectant by-product (DBP) formation, so the CT credit must be
achieved in the clearwell or a separate contact basin with optimized hydraulic efficiency.
The designer must be particularly careful when relying on a clearwell, or any treated wa-
ter storage basin, for CT credit. Most existing clearwells are poorly baffled or have none
at all. In addition, their basic function as useful storage volume, which may be withdrawn,
may conflict with the need to maintain more than a minimum volume to satisfy contact
time requirements.

CHLORINE DIOXIDE SYSTEMS

In certain circumstances, chlorine dioxide is an excellent choice among disinfectants. In
particular, it is effective in destroying phenols, yet it does not form trihalomethanes in
significant amounts. Chlorine dioxide's disinfectant properties are not adversely affected
by a higher pH, as those of a free chlorine residual are. Consequently, chlorine dioxide
is a much-quicker-acting disinfectant than chlorine is at higher pH levels. In western Eu-
rope, use of chlorine dioxide is increasing, particularly in Holland, Germany, France, and