Disinfection and Disinfection Byproducts
Disinfection and Disinfection Byproducts tests whether you can inactivate pathogens while controlling DBPs. Kentucky systems use chlorine, chloramines, ozone, UV, or chlorine dioxide, so expect questions on CT, demand, residual, contact time, and Stage 2 DBP compliance.
Core rules and concepts
- CT is disinfectant residual concentration multiplied by effective contact time (t10), adjusted for pH and temperature. Higher pH and lower temperature generally require more CT.
- Chlorine demand is the difference between dose and residual. Free chlorine is more reactive than combined chlorine.
- Chloramination reduces TTHM and HAA5 formation but can cause nitrification; monitor ammonia and residual.
- Stage 2 DBPR uses locational running annual averages: TTHM MCL 80 µg/L and HAA5 MCL 60 µg/L.
- DBP samples are collected at locations with maximum residence time, not at the plant.
- UV and ozone are strong disinfectants but provide no residual; a secondary disinfectant is needed.
Worked example
A clearwell has t10 = 30 minutes. The free chlorine residual at the outlet is 1.0 mg/L. CT = 1.0 × 30 = 30 mg/L·min. If the required CT for the measured pH and temperature is 25 mg/L·min, the system meets the requirement with a small margin. If the residual falls to 0.7 mg/L, CT = 21 mg/L·min and the system does not meet the required CT, even though a residual is still present.
Traps
Using theoretical detention time instead of t10; measuring total chlorine when free chlorine is required; assuming a detectable residual always means adequate CT; sampling DBPs at the plant instead of max-residence locations; ignoring nitrification in chloraminated systems.