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.