Disinfection and Disinfection Byproducts
Disinfection is the primary public-health barrier. Know CT (disinfectant concentration × contact time), the concept of C·T required to achieve a target log inactivation, and how pH, temperature, and disinfectant type change effectiveness. Chlorine is weaker against Giardia and Cryptosporidium at high pH; ozone and UV are strong against protozoa, but UV provides no residual and ozone leaves little residual. Understand primary vs secondary disinfection, residual maintenance, and chlorine decay in the distribution system.
DBP chemistry: chlorine reacts with NOM to form trihalomethanes (THMs) and haloacetic acids (HAAs). Know that TOC removal before chlorination, enhanced coagulation, lowering pH, and moving the chlorine application point downstream reduce DBP formation. Traps: assuming more chlorine always means better disinfection (it can raise DBPs without improving CT if contact time is short); confusing free vs total chlorine; forgetting that temperature and pH alter CT credit; and treating UV as a residual disinfectant. Also watch Stage 1/2 DBP rule concepts (LRAA, locational running annual average) and precursor control.
Rule of thumb: CT = residual concentration (mg/L) × contact time (minutes); if you halve contact time, you need double the residual to hold CT.
Worked example: Required CT is 30 mg·min/L; contact time is 20 min. Required residual = 30 ÷ 20 = 1.5 mg/L. If the measured residual after contact is only 1.0 mg/L, CT = 20 mg·min/L and you fall short-raise residual or extend contact time.