The paper presents a dynamic optimisation study of a small size single basin wastewater treatment plant. The objective is to determine optimal sequences of aeration/non-aeration times so that for a typical diurnal pattern of disturbances, the effluent constraints are respected, the plant remains in the cyclic steady-state, and the energy consumption is minimised. Based on the results of optimal state trajectories, two simple feedback rules are proposed. The simulation results obtained with these rules show very satisfactory control performances.