Disinfection and Residual Maintenance
Chlorine chemistry
Chlorine gas hydrolyzes to hypochlorous acid: Cl2 + H2O → HOCl + HCl, and HOCl partially dissociates to OCl− near pH 7.5. HOCl is the far stronger disinfectant, so pH control is a disinfection issue, not just a corrosion issue. Free chlorine is HOCl plus OCl−; total chlorine adds combined chloramines, which are weaker but far more persistent and are used deliberately for residual maintenance where DBP formation is a concern. Breakpoint chlorination describes the dose where combined chlorine is destroyed and free residual begins to rise.
CT and log credit
Inactivation is judged by CT = disinfectant concentration (mg/L) × contact time (minutes). Contact time is not theoretical detention: T10 = volume ÷ flow × baffling factor. Required CT rises as water gets colder and, for free chlorine, as pH rises, so tables must be supplied. Log credits are additive across processes only for the same organism.
Residuals and limits
Surface water systems must maintain a detectable disinfectant residual in the distribution system; MRDLs are 4.0 mg/L for chlorine and chloramines and 0.8 mg/L for chlorine dioxide, while TTHM and HAA5 are held to 80 and 60 µg/L as locational running annual averages. Chloramines can nitrify in warm, long-detention mains.
Traps
Questions mix free with total chlorine, MRDL with MCL, and theoretical detention with T10. Never use a memorized CT table - read the one supplied.
Worked example
An 80,000-gallon clearwell at a 0.40 MGD flow: detention = 0.08 MG ÷ 0.40 MGD = 0.20 day = 4.8 hours. With a 0.60 baffling factor, T10 = 2.88 h = 173 min. At 1.2 mg/L free chlorine, CT = 207 mg·min/L.