Better control of wastewater treatment requires the development of pertinent sensors. For Alternating Aerobic-Anoxic Activated Sludge, the relatively simple sensors measuring ORP, DO and pH appeared to be useful for managing the aeration. Bending-points on the ORP and pH curves are linked to the major biological activities playing a role in nitrification and denitrification. In this work, it is shown that the appearance or the absence of these bending-points and the pH variation slope allow the operating state of the biological system to be identified. Case studies of over- and under-loaded conditions and over- and under-aerated systems are examined. The diagnosis procedure can be used to regulate the aeration flow rate, to optimize the ORP thresholds and the timers, and to evaluate the organic load and the potentials of the bacterial populations involved in nitrogen elimination.