During the last years, the efficiency of pollution removal and sludge treatment processes are major concerns because of the more restrictive legislation related to nutrient discharge. A key point in the field of activated sludge wastewater treatment plants (WWTPs) is the secondary settler efficiency. In this paper, the COST 624 Simulation Benchmark (model and data) is used to develop a fuzzy controller of the sludge height in the secondary settler. The presented control strategy is based on simple on-line data (influent, removal and recycle flows) and daily analytical values of the sludge volume index (SVI) allowing the fuzzy algorithm to reduce sludge height variations and thus to increase the settling process efficiency. The developed controller has then been adapted and applied to the Cassa de la Selva activated sludge WWTP (Spain) model, with satisfying results. (c) 2006 Elsevier Ltd. All rights reserved.