In the present study a hydrophilic SPG membrane was used for microbubble generation to aerate a pilot-scale biofilm reactor treating synthetic municipal wastewater. The external fouling of SPG membrane was observed and identified during continuous operation of the bioreactor. Then the SPG membrane was cleaned offline by ultrasonic, thermal and acid treatment to remove fouling layer, organic and inorganic foulants on the membrane surface. The structural changes of SPG membrane were investigated after fouling and offline cleaning, using scanning electron microscopy (SEM), mercury and gas adsorption porosimetry, X-ray Diffraction (XRD), and Infrared (IR) absorption. SEM tests clearly demonstrated that these offline cleaning methods were effective to clear membrane external fouling. There was no obvious change in membrane pore size distribution after SPG membrane fouling and offline cleaning, but the membrane porosity and pore surface area increased slightly. XRD and IR analysis also showed that rare inorganic or organic foulants deposited on the pore surface inside the membrane. Therefore, the external fouling was identified as the main mechanism of SPG membrane fouling used in aerobic wastewater treatment process. The microbubble generation and oxygen mass transfer deteriorated due to external fouling of SPG membrane, which recovered after membrane offline cleaning. (C) 2012 Elsevier B.V. All rights reserved.