Real-time control of nitrogen removal using three ORP bending-points: signification, control strategy and results

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.