This paper examines on-off systems and automatic monitoring and control of a biological aerated filter to identify the end point of nitrification and denitrification processes, and chemical oxygen demand (COD), ammonia-nitrogen removal (NH3-N) and aeration savings. Oxidation-reduction potential (ORP), pH and dissolved oxygen (DO) were measured on-line and chemical parameters were measured in the wastewater. The ‘nitrate knee’ in the ORP profile was characterised by a breakpoint at average 160 min, representing the complete removal of NO3-N, i.e. the end of the denitrification period, as well as the end of the nitrification period was clearly shown in the pH profile (ammonia valley) at average 210 min for all C/N ratios. The NH3-N removal efficiencies were 92.30, 97.57 and 98.02% whilst the COD removals of 95.06, 96.38 and 97.56% were achieved for the C/N ratios of 10, 4 and 1 respectively. Therefore, the on-off control was operated at average 230 min for aeration time and 130 min for the anoxic period. Thus significant improvements can be achieved with respect to the continuous aeration strategy, and average operational costs reduced by 36.11%. The study showed that an on-off controller can easily be implemented in wastewater process-control, and monitoring systems improve effluent quality and reduce energy consumption.