Dynamic modelling and simulation is increasingly being employed as an aid in the design and operation of wastewater treatment plants (WWTPs). In this paper a simulation model is used to investigate the control of an activated sludge process. Two different control strategies to optimise nitrogen removal in a predenitrification process are presented, which are based on the control of the sludge blanket height (SBH) or sludge age (sludge retention time, SRT) and mixed liquor suspended solids (MLSS) in the reactor, by adjusting the sludge wastage and sludge recycle flow rates, respectively. It is shown how the recycle and wastage flow rate variables can be employed to continually maintain plant operation in the presence of disturbances. The performance of the control strategies has been assessed by the COST 624 benchmark plant and a pilot plant. The results have shown the robustness of the implementation and the efficiency of the proposed control strategies, which have been operated automatically in a safe, stable and optimum operating point, improving effluent quality and reducing energy costs. Consideration is also given as to how plant operation could be optimised in the long-term in order to meet the yearly total nitrogen standard. (c) 2005 Society of Chemical Industry.