CT Value for Disinfection Credit

Disinfection obeys a dose law, and the dose is concentration multiplied by time. The US Surface Water Treatment Rule turned that into regulation: a plant must accumulate a tabulated CT, in milligrams per litre times minutes, to claim a given log inactivation of Giardia or viruses at a given pH and temperature. Free chlorine at 1.2 mg/L held for 45 minutes delivers CT = 54 mg·min/L, comfortably past the roughly 45 required for 3-log Giardia at 10 °C and pH 7.5. Whatever units you type the residual and the time in, the solver returns CT in mg·min/L.

Two things sink real plants. First, the C is the residual at the END of the contact zone, not the dose you fed — demand eats the difference, and on a coloured surface water that can be most of it. Second, the t is T₁₀, the theoretical detention time multiplied by a baffling factor, so an unbaffled clearwell with a 0.1 factor delivers a tenth of the contact you think it does. Temperature and pH swing the requirement hard: the same 3-log Giardia credit needs roughly twice the CT at 5 °C as at 15 °C, and chlorine loses potency as pH rises because the killing species is hypochlorous acid, not hypochlorite ion. Ozone and chlorine dioxide need far less CT; chloramine needs vastly more, which is why it is a distribution-system residual and not a primary disinfectant.