Work out detention time instantly with clear inputs, formula shown and shareable results.
Theoretical detention time is volume divided by flow, but real tanks short-circuit. The baffling factor converts theoretical time to T10, the time in which 10% of the water has passed through — 0.1 for an unbaffled tank, 0.5-0.7 with baffling and 0.7 or better for a serpentine contact tank. Disinfection credit uses CT, residual times T10.
Detention time
Theoretical = volume x 24 / flow; T10 = theoretical x baffling factor
CT value
CT = disinfectant residual x T10 in minutes
Disinfection CT credit is a regulated compliance calculation. Baffling factors must be justified by tracer study or the applicable regulatory default.
Because disinfection must protect the fastest-moving fraction of water, not the average. Using mean time would credit the plant for contact that a tenth of the water never receives.
By a tracer study, which measures the actual residence time distribution. Regulatory guidance provides default factors by tank configuration where no tracer study exists.