Disinfection and Disinfection Byproducts
Inactivation, not just residual
Disinfection has two jobs: inactivating pathogens and leaving a residual for the distribution system. The CT concept combines residual concentration (mg/L) and contact time (minutes) as CT = C × T, then compares the result with a table value for your target log inactivation, pH, and temperature. CT calculations use peak hourly flow and the baffling factor of the contact basin - not average flow.
Chlorine chemistry drives everything. Free chlorine exists as HOCl and OCl⁻; HOCl is the far stronger disinfectant, so effectiveness drops as pH rises above roughly 7.5. Chlorine demand is the chlorine consumed by water before a residual persists; breakpoint curves show the shift from combined to free chlorine.
Chloramines provide a persistent residual with lower trihalomethane formation, but they nitrify in warm, long-detention systems and are weaker against viruses. Ozone is powerful but forms bromate when bromide is present. Chlorine dioxide forms chlorite. UV inactivates Crypto well but leaves no residual.
DBP control: remove precursors (TOC) before chlorination, reduce dose, adjust pH, manage bromide, and control water age. California MCLs are 80 µg/L for TTHM and 60 µg/L for HAA5, both as running annual averages; bromate is 10 µg/L and chlorite 1,000 µg/L.
Traps
- Reporting total chlorine when the rule requires free chlorine.
- Using average flow instead of peak hourly flow in CT.
- Assuming UV or ozone alone satisfies the distribution residual requirement.
Worked example
A clearwell gives 15 minutes of contact at peak flow, baffling factor 0.7, and free chlorine of 1.2 mg/L. Effective CT = 1.2 × (15 × 0.7) = 1.2 × 10.5 = 12.6 mg·min/L. Compare that to the required table value for your log credit and temperature.