A Comparison of Several Control Algorithms for Activated Sludge Waste Rate

Abstract

Several algorithms for the control of the waste flow rate of the activated sludge process were compared by computer simulation. The algorithms included: constant waste flow rate; constant proportion of the mass; proportional-integral-derivative (PID) control of the dynamic sludge age (DSA); PID control of mass inventory; and feedforward ratio control based on B.O.D. loading. The study examined the effect of each algorithm on variability in DSA, biomass, and F/M ratio. PID control of DSA minimized variability in sludge age; PID control of biomass minimized variability in biomass; and constant flow rate minimized variability in F/M ratio when mixed liquor was wasted; when return sludge was wasted then constant proportion wasting was significantly better than constant flow wasting in terms of sludge age variability. It is concluded that until the relationship between sludge age, F/M and biomass and effluent quality in an unsteady-state process is better understood, a conservative control strategy is recommended: plants that waste mixed liquor should use constant flow wasting; those that waste return sludge should use constant proportion wasting.