The simultaneous treatment of nitrogen, carbon and phosphorus in a SBR is made possible by alternating the aerobic, anoxic and anaerobic phases, which requires that the order and duration of the different reaction stages be carefully optimised. The carbon and nitrogen treatment of wastewater effluent was monitored using an instrumented pilot. Monitoring the pH, ORP and OUR has made it possible to distinguish clearly points with different characteristics: termination times of nitrification, carbon oxidation, as well as endogenic and exogenic denitrification. An automated procedure based on the hierarchical detection of these characteristic points and on the combined use of two sensors has led to the optimisation of the duration of the different reaction phases. In this way, a real saving in oxygenation has been achieved, an increase in reactor productivity (in terms of C, N, P removal) and a good adaptation of load variation. (c) 2006 Elsevier Ltd. All rights reserved.