Hydraulic Detention Time

Detention time is the single most quoted number in plant design: how long, on average, a drop of water stays inside a basin. Flocculation wants 20–30 minutes, a primary clarifier 1.5–2.5 hours, a chlorine contact chamber at least 30 minutes at peak hour, and an anaerobic digester 15–30 days. The arithmetic is trivial (a 200,000 gallon basin taking 500 gpm holds each drop 400 minutes, or 6 hours 40 minutes) but the number decides whether the biology, the chemistry and the settling all have time to happen.

The trap is the word "theoretical". Real basins short-circuit: a jet from the inlet can carry a slug of water to the outlet in a fraction of the calculated time, and regulators know it. Drinking-water disinfection credit is therefore taken on T₁₀, the time in which only 10% of a tracer has appeared at the outlet, obtained by multiplying this theoretical time by a baffling factor of 0.1 for an unbaffled tank up to 0.7 for a serpentine one. If you are chasing coliform violations in a plant that looks fine on paper, run a salt or fluoride tracer study before you touch the chemistry: the volume in the equation must be the WORKING volume too, not the tank you can see, since sludge blanket and dead corners are not detention.