A process for controlling the aeration rate during the aerobic phase of a wastewater treatment process is disclosed, which comprises: (a) measuring at moment t1 the ammonium concentration [NH4+]1 and nitrogen oxide concentration [NOx]1 in a mixture of wastewater and microbial sludge; (b) determining a nitrogen oxide target concentration [NOx]F1 at the end of the aerobic phase based on at least the current ammonium concentration [NH4+]1 and the current NOx concentration [NOx]1; (c) determining a setpoint [NOx]SP1 based on interpolation between [NOx]1 and [NOx]F1; (d) adjusting the aeration rate to minimise error between [NOx]1 and [NOx]SP1; and (e) repeating steps (a) to (d) at further moments t1. The invention further concerns a process for the treatment of wastewater, using the process.