The CT Framework
The CT (concentration × time) concept is the backbone of regulatory disinfection compliance worldwide. By multiplying the residual disinfectant concentration by the effective contact time, engineers get a single number that predicts pathogen inactivation. The USEPA Surface Water Treatment Rule specifies required CT values for different pathogens, temperatures, and pH levels — typically requiring 4-log virus and 3-log Giardia inactivation.
Chlorine Chemistry in Water
When chlorine gas or sodium hypochlorite is added to water, it forms hypochlorous acid (HOCl) and hypochlorite ion (OCl⁻). The equilibrium between these species depends on pH. HOCl is a small, neutral molecule that penetrates cell membranes far more effectively than the charged OCl⁻ ion. This pH dependence makes water chemistry control essential for reliable disinfection.
Temperature Effects
Disinfection kinetics follow Arrhenius-type temperature dependence. Colder water requires significantly higher CT values — roughly doubling for each 10°C decrease. This is why northern treatment plants in winter face the greatest disinfection challenges, often requiring higher chlorine doses or longer contact times that increase DBP formation potential.
Balancing Disinfection and Byproducts
The fundamental tension in chlorination is that the same chemical reaction that kills pathogens also creates potentially carcinogenic disinfection byproducts (DBPs). Treatment plants manage this by removing organic precursors before chlorination (enhanced coagulation, activated carbon), using alternative primary disinfectants (ozone, UV), and applying chlorine or chloramine as a secondary disinfectant for distribution system residual.