BACKGROUND: Membrane bioreactors are used in an increasing number of wastewater treatment facilities because of their compactness and efficiency in solid-liquid separation, but energy requirements are still higher than other competitive technologies such as conventional activated sludge. RESULTS: In this paper the development of an air-scour control algorithm based on short-term and long-term membrane permeability evolution is presented. An open loop calibration and partial validation was carried out in a semi-industrial scale pilot plant where manual changes in air-scour flow had been previously carried out. The control system was successfully tested in closed loop in an industrial scale pilot plant, defining a maximum daily air-scour decrease or increase of 6% of the air-scour recommended by membranes suppliers. An average air-scour saving of 13% and a maximum of 20%, calculated in terms of air flow saved, were achieved during the validation test without any apparent long-term effect on membrane permeability. CONCLUSION: It was proven that the air-scour control system is an important tool capable of reducing operational costs in MBR systems. (C) 2011 Society of Chemical Industry