Disinfection and Disinfection Byproducts
Chlorine hydrolyzes to hypochlorous acid: HOCl predominates below about pH 7.5 and is far more effective than the hypochlorite ion that dominates above it - that is why pH matters in CT questions. Chlorine demand is dose minus residual after the contact time; the difference is what reacted. Breakpoint chlorination describes the curve where dose increases past the demand and free residual appears.
CT = residual concentration x contact time (mg/L x minutes), compared against tables that depend on disinfectant, target organism, temperature, and pH. Lower temperature means a larger required CT. Chloramines give a persistent but weaker residual; the chlorine-to-nitrogen weight ratio controls monochloramine formation, and excess chlorine creates di- and trichloramine with taste complaints. Ozone and UV are strong against Cryptosporidium, which resists chlorine, but neither leaves a lasting residual. Chlorine dioxide can form chlorite.
DBP limits: TTHM 80 µg/L and HAA5 60 µg/L as locational running annual averages, plus bromate and chlorite limits. Enhanced coagulation targets TOC removal to starve DBP formation.
Traps: assuming any detectable residual proves adequate inactivation; forgetting CT tables are temperature- and pH-dependent; measuring total chlorine where free chlorine is required; and sampling DBPs anywhere other than the specified maximum-location sites.
Worked example: free residual 0.9 mg/L after 22 minutes gives CT = 0.9 x 22 = 19.8 mg-min/L; compare that number against the table for the organism, temperature, and pH in the question.