The biological wastewater treatment step plays a critical role in providing the capacity necessary for achieving biochemical oxygen demand (BOD) and nutrient removal targets. Biological process optimization can improve treatment plant reliability and help meet current and future regulatory requirements while at the same time minimizing energy consumption. This paper shows how nitrification and denitrification rates can be monitored online in a biological treatment step and used to improve process and energy performance. Nitrification and denitrification rates were monitored at two SBR wastewater treatment facilities in Sweden and Wisconsin, USA, using online probes measuring ammonia-N (NH4), nitrate-N (NO3), dissolved oxygen (DO), total suspended solids (TSS), temperature and water level. Besides being used as a health check of the process, the rates can be used to calculate real time required sludge age for nitrification. When using this knowledge of the process requirements at one of the two facilities, the sludge age was reduced by 50% without affecting the treatment performance of the plant. This demonstrates how online nitrification and denitrification rates can be used both for evaluating the health of a treatment process and optimizing energy consumption.