Changes in the oxidation-reduction potential (ORP), pH and dissolved oxygen (DO), along with organic load and nutrient removal in a municipal wastewater treatment plant (WWTP) have been monitored throughout one year. The "nitrate knee" and the "nitrate break point" in ORP profiles, the "nitrate apex" and the "ammonia valley" in pH profiles and the "DO elbow" in DO profiles have been identified. Furthermore, these bending points have been correlated with the oxygen uptake rate (OUR), the temperature in the vessel and the aeration and non-aeration time profiles by using Principal Component Analysis (PCA). The data have been previously split up into wet and dry weather cycles by means of a K-means clustering algorithm. Finally, two new parameters have been defined: the "ORP Arrow", which is closely related to the inhibition of the denitrification process, and the "Oxygen Rise Average Slope" (ORAS), which shows the oxygen transfer rate. (C) 2012 Elsevier Ltd. All rights reserved.