Disinfection and Effluent Quality
Chlorine chemistry
Chlorine gas, sodium hypochlorite, and calcium hypochlorite all form hypochlorous acid (HOCl) in water - the dominant disinfectant. HOCl dissociates to hypochlorite ion as pH rises, so disinfection weakens above about pH 8. Free chlorine residual equals HOCl plus OCl⁻. Combined chlorine (chloramines) forms when chlorine reacts with ammonia; it is a weak disinfectant and causes taste complaints. Breakpoint chlorination is the dose at which ammonia is fully oxidized and free residual begins to persist.
CT and contact
Disinfection depends on both residual and time: CT = concentration (mg/L) × contact time (minutes). Baffled, plug-flow contact chambers maximize effective time; short-circuiting cuts it. Chlorine demand is the difference between dose and residual.
Alternatives
UV inactivation depends on dose (mJ/cm²) and water transmittance - solids and color shield organisms. Ozone is powerful but leaves no residual. Dechlorination with sulfur dioxide or sodium bisulfite protects the receiving stream; roughly 0.9 mg/L of SO2 consumes 1 mg/L of chlorine.
Compliance
Effluent limits are usually written for fecal coliform or E. coli as a geometric mean. Use the geometric mean, not the arithmetic mean, and confirm which statistic the permit requires.
Worked example
A baffled contact chamber holds 20,000 gal at 1.0 MGD. Detention = 20,000/1,000,000 × 24 h = 0.48 h = 28.8 minutes. With a measured residual of 1.0 mg/L, CT = 28.8. If the permit requires CT of 25 at 1.0 mg/L residual, the chamber complies.