Feedback Control of Activated Sludge Waste Rate

An algorithm for feedback control of activated sludge waste rate has been developed. It uses proportional-integral-derivative (PID) control with waste flow rate as the manipulated parameter and an unsteady-state calculation of sludge age (the dynamic sludge age, or DSA) as the process variable. PID control can be implemented either on-line using sensors and automatic control, or off-line using lab results and manual implementation of computed control action. Both methods were studied using computer simulation. Feedback control reduced the error in DSA by 60% for a step change in setpoint, and 80% for a step change in influent biochemical oxygen demand (BOD), as compared to no control (constant waste flow rate). For a realistic forcing function consisting of 6 months of influent flow and BOD data, the variability of the DSA was reduced by 22% by continuous PID control, and 14% by off-line control. Improved DSA control resulted in increased variability in system biomass.