The use of dynamic optimisation methods to provide optimal operating strategies for small size wastewater treatment plants is investigated. The optimisation objective is to determine the aeration profile (air-on and air-off periods) which minimise the concentration of total nitrogen that is discharged over a specified time horizon. Long term effects of the optimisation are considered; it is shown that appreciable reductions of total nitrogen residual concentration of about 30% can be achieved with respect to usual operating modes, along with large reductions of aeration time.