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.
The necessity to achieve nitrogen and phosphorus removal in wastewater, according to the European Directive (EEC 1991), leads to the conception of new methods to control the aeration of low-loaded activated sludge plants. The behaviour of N.NOx concentrations and of ORP during a complete nitrification-denitrification cycle is described by a typical profile with 3 bending-points: α, β and χ. The goal of this study is to get more insights into the biological and chemical signification of the bending-points. This leads to the conception of new real-time control systems able to be adaptive to the influent load variations and free from ORP drift problems. The results obtained on pilot-scale plant using a three bending-points based control strategy show a real advantage through a decrease of the global aeration duration.
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