Disinfection credit is bought with exposure, and exposure is a concentration multiplied by a time. $\text{CT} = C\,T_{10}$ — read aloud C T equals C times T-ten. $C$ is the disinfectant residual in mg/L, measured at the END of the contact zone, not the dose at the front — the water eats its way through the demand on the journey, and on a coloured surface water it can eat most of it. $T_{10}$ is the effective contact time in minutes. The product $\text{CT}$ comes out in mg·min/L, which is exactly the unit every regulator's table is printed in.

Now the number that separates people who have done this from people who have read about it. $T_{10}$ is not the tank's detention time. Volume over flow gives $\theta$ — theta — the theoretical detention time, which is what the basin would deliver if every drop marched through in step. Real water short-circuits. $T_{10} = \theta \times \mathrm{BF}$ — T-ten equals theta times B F — where $\mathrm{BF}$ is the baffling factor, a bare fraction from 0.1 to 0.7. $T_{10}$ is defined as the time by which only a tenth of a tracer has reached the outlet, so nine tenths of the water has had at least that much contact — the credit is taken on the fast tenth, not the average.

The spread is brutal. An unbaffled clearwell with the inlet and outlet at opposite corners is 0.1: a sixty-minute tank delivering six minutes of credit. A serpentine baffled basin close to plug flow is 0.7. Between those two the same concrete, the same flow and the same chlorine buy seven times the credit, and the difference is where the walls are.

What this page cannot tell you is whether the CT is enough. That comes from your regulator's table for your pathogen, your disinfectant, your temperature and your pH — all four at once. Two habits protect you. Use the worst-case temperature of the day, because the same credit can ask roughly twice the CT at 5 °C as at 15 °C, and a plant that passes comfortably in August fails in February on identical operation. And know the exception: Cryptosporidium shrugs off free chlorine at any CT a plant can deliver. That credit has to come from UV, ozone or filtration, and no amount of chlorine will buy it.