Every basin, tank, and pipe in a water or wastewater system holds water for some length of time before it moves on — and that length of time, called detention time, quietly controls whether disinfection actually kills pathogens, whether solids actually settle out, and whether biological treatment actually has time to work. Get the concept and the formula down cold here, because every later calculation you'll do — sizing a chlorine contact chamber, troubleshooting a short-circuiting clarifier, running a CT compliance check — starts with this one idea.
What Is Detention Time?
Detention time (also called hydraulic retention time, HRT, or hydraulic detention time, HDT) is the average length of time a "parcel" of water spends inside a tank, basin, reservoir, or pipe before it exits. Picture a bathtub: water flows in from the faucet and flows out the drain at the same time