Chlorination & Disinfection
CT values, breakpoint chlorination, and dechlorination fundamentals.
CT value = Disinfectant Concentration (mg/L) × Contact Time (minutes). Regulators use minimum CT tables to confirm adequate pathogen inactivation.
Breakpoint chlorination is the dose at which all chlorine-demanding compounds (ammonia, organics) have been satisfied and free chlorine residual begins to rise sharply.
Chlorine demand must be satisfied before a measurable free residual appears — know the difference between total, combined, and free chlorine residual.
Dechlorination (commonly with sodium bisulfite or sulfur dioxide) is required before discharge to remove residual chlorine toxic to aquatic life. Exams often ask for the stoichiometric dose ratio.
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