Disinfection and Effluent Quality
Chlorination chemistry
Free chlorine (HOCl and OCl⁻) is a much stronger disinfectant than combined chlorine (chloramines). HOCl dominates at lower pH, so disinfection is more efficient near pH 6–7. The breakpoint curve explains dose versus residual: the initial dose satisfies chlorine demand (organics, ammonia, reduced compounds), a combined residual forms, then the breakpoint is crossed and free residual appears. CT is residual concentration multiplied by contact time; cold water and high pH require higher CT.
Dechlorination and alternatives
Sulfur dioxide or sodium bisulfite quenches residual chlorine before discharge. UV disinfection is dose-based (mJ/cm²) and depends on transmittance, TSS, and lamp fouling; it leaves no residual. Ozone is a powerful oxidant with no persistent residual, requires off-gas destruction, and can form bromate. Trap: assuming a disinfection setpoint is a dose - dose, demand, and residual are different quantities.
Effluent monitoring
Coliforms (total coliform, fecal coliform, E. coli) gauge disinfection efficiency. Methods include membrane filtration (reported cfu/100 mL), multiple-tube fermentation (MPN/100 mL), and enzyme substrate tests. Coliform samples must be dechlorinated with sodium thiosulfate and iced; holding times are short. Trap: comparing a cfu result directly against an MPN limit without noting the reporting basis.
Rule of thumb
A well-baffled contact basin with 1.5 mg/L residual and 30 minutes of contact provides a CT near 45 mg·min/L, but actual credit uses T10 (the time for 10% of flow to pass), computed as detention time × baffling factor (0.1–0.7).