t = V / Q
Detention time (also called hydraulic retention time, HRT) is the average time a parcel of liquid spends inside a reactor or treatment basin. It is calculated as t = V / Q, where V is the volume and Q is the volumetric flow rate. In water and wastewater treatment, detention time determines the efficiency of sedimentation, chemical reactions, and biological processes. Typical detention times: sedimentation tanks 2-4 hours, activated sludge aeration basins 4-8 hours, anaerobic digesters 15-30 days. Too short a detention time means insufficient treatment; too long means oversized infrastructure and higher costs. The concept applies equally in chemical engineering to continuous stirred-tank reactors (CSTRs) and plug flow reactors (PFRs).