Application of mathematical tools to improve the design and operation of activated sludge plants. Case study: the new WWTP of Galindo-Bilbao. Part II: Operational strategies and automatic controllers.

This paper presents a new sensitivity analysis methodology for Activated Sludge WWTPs. It is based on both (a) the calculation of the range of "manipulated input variables" that satisfy the restrictions imposed on the "output variables" and (b) on the computation of isolines of the output variables inside the feasible operating space. This analysis allows a more precise description of the operating constraints, facilitates the understanding of the steady-state behaviour of the process and detects possible areas where the process is very sensitive to small disturbances. The feasible operating space for two Activated Sludge WWTP processes for CN removal (RDN, DRDN), using SRT and DO level as "input variables" as well as effluent quality and exploitation costs as the main "output variables" is studied. The proposed methodology facilitates the selection of the appropriate operational strategy and the design of automatic controllers. Some examples of the application of this methodology for the design of automatic controllers in a real WWTP are briefly presented.