Hydraulic Residence Time of CSTRs under Unsteady‐State Condition

Equations to calculate the hydraulic residence time (HRT) of complete‐mix reactors (CSTR), with or without recirculation, under unsteady‐state condition are developed in this study. Recirculation does not affect the hydraulic residence time of a CSTR‐type reactor no matter which state (steady or unsteady) condition the tank is operated under. Response of a CSTR tank to a step change in hydraulic loading is evaluated by using the developed equations. Hydraulic residence time of a CSTR under a periodic influent flow rate is studied, and related normalized curves are proposed to assist design, operation, and evaluation of CSTR reactors. CSTRs can dampen hydraulic fluctuations, especially when the cycle length of influent fluctuations is shorter than one hydraulic residence time (time‐weighted average value) of the tank. The cycle length of the fluctuations of influent is found to be a key parameter in determining variations of hydraulic residence time. Shorter the normalized cycle length, fewer the variations; i.e., an increase of effective volume of the tank will improve its capability in handling influent fluctuations. An example of using the proposed normalized curve in the design of an aerated lagoon, for a given characteristic wastewater cited from the literature, is provided.