In this paper, dynamic optimisation of small size wastewater treatment plants is studied. The problem is stated as a hybrid dynamic optimisation problem which is solved using a gradient-based method. The aeration policy which minimises the energy consumption and satisfies discharge requirements under specified constraints (process and physical constraints) is then determined. The comparison between usual rule-based control policies and optimised aeration strategies showed that the optimised aeration profiles lead to reductions of energy consumption of at least 30%.