Disinfection and Disinfection Byproducts
Disinfection questions mix chlorine chemistry, CT concepts, disinfection byproduct limits, and the practical safety of chlorine gas and hypochlorite.
Chemistry and residuals
Free chlorine exists as hypochlorous acid (HOCl) and hypochlorite ion (OCl⁻). HOCl is far the stronger disinfectant, and the split shifts toward OCl⁻ as pH rises above the pKa of about 7.5. Chlorine demand is satisfied first; beyond the breakpoint, free residual appears. Chloramines are weaker but more persistent and can nitrify in the distribution system. Chlorine dioxide and ozone are strong but leave no lasting residual; UV inactivates without residual.
Rules to remember
- CT = residual (mg/L) × T10 (min). Compare the calculated CT with the required CT for the target log inactivation, temperature, and pH.
- Maximum residual disinfectant levels: chlorine 4.0 mg/L, chloramine 4.0 mg/L, chlorine dioxide 0.8 mg/L.
- Stage 2 DBPR limits use locational running annual averages: TTHM 80 µg/L, HAA5 60 µg/L.
Worked example
A plant feeds 2.5 mg/L to 1.0 MGD. Pounds per day = mg/L × 8.34 × MGD = 2.5 × 8.34 × 1.0 = 20.9 lb/day of chlorine. At about 1.25 lb of available chlorine per gallon of 12.5 percent sodium hypochlorite, that is roughly 17 gallons per day.
Traps
Reporting DBPs in mg/L instead of µg/L, or averaging every sample instead of using the LRAA. Treating the MRDL as an MCL. Using theoretical detention time in CT instead of T10. Forgetting that higher pH weakens free chlorine at the same residual.