Calculation of mean cell residence time for unsteady-state activated sludge systems

A formulation to calculate the mean cell residence time (MCRT or sludge age) of unsteady-state activated sludge systems is presented. The formulation was studied by applying it to data generated by computer simulation and to data obtained from an actual wastewater treatment plant. The computer simulation study allowed the effects of step and pulse changes in biochemical oxygen demand (BOD) loading, and step changes in a control variable, waste sludge flow rate, to be studied independently of each other and of other disturbances. The unsteady-state MCRT formulation (herein called the dynamic sludge age, or DSA) was found to be an improvement over the traditional steady-state calculation, both for process control, and for research into activated sludge dynamics.