Disclosed is a method for biological treatment of wastewater in a non-woven membrane bioreactor (MBR) system. The mixed liquor contained in the bioreactor is filtered with a non-woven fabric membrane having a pore size in the range of 0.1 μm to 5.0 μm. Membrane fouling is controlled and flux is improved in the non-woven MBR system by adding an effective amount of a water soluble sludge filterability improvement chemical to the mixed liquor.