In the field of the wastewater treatment, membrane bioreactor technology continues to gain more and more interest thanks to high purification performance, low environmental impact, high possibility of treated water reuse. The more crucial factor in membrane bioreactor application is fouling tendency. In this paper, three different submerged membranes were used for municipal wastewater treatment. The relationship between critical flux and mixed liquor suspended solid concentration was investigated. Information about the fouling tendency of the membranes was obtained by studying the flux vs. transmembrane pressure behavior.