Biofouling affinity of membrane surfaces under quiescent conditions

Biofouling affinity of four thin film membranes was evaluated under quiescent conditions by exposing the membrane to the effluent from a membrane bioreactor. Coupons of the thin film membranes were left for 5 days in the bioreactor effluent which was filtered through a 0.2 micron filter. The surface morphologies of the membranes before and after the exposure were evaluated by atomic force microscopy (AFM). All membranes showed significant amounts of deposits after the exposure. The easily disturbed nature of the deposits on the polyamide membrane indicated that these deposits were either entrapped within the surface crevices or loosely attached on the membrane. The extent of surface coverage and uniform appearance of the deposits on the polysulfone membrane were indicative of molecular adhesion by functional groups on the membrane surface. Based on the wetting angle measurements, all clean membranes exhibited hydrophilic characteristics. After exposure to the bioreactor effluent, the membranes exhibited complete wetting characteristics. The use of AFM image analysis technique provided insight for the morphological changes and substrate accumulation patterns on membrane surfaces.

Significant differences were observed before and after exposure of the membranes to the filtered bioreactor effluent. The polyamide membranes had soft deposits which were easily disturbed by the AFM tip. The loose nature of the deposits on the polyamide membrane indicates that these deposits were entrapped within the surface crevices of the membrane. The polysulfone membrane had clusters of firmly deposited particles with very different characteristics from the deposits observed on the polyamide membranes. The extent of surface coverage and uniform appearance of the deposits indicate that these deposits were held by functional groups on the membrane surface. After exposure to the bioreactor effluent, the roughness of all of the membranes increased with the exception of NF90. The polysulfone membrane had the highest increase in roughness after exposure to bioreactor effluent. After exposure to the bioreactor effluent, all membranes became highly hydrophilic with complete wetting characteristics. The use of AFM image analysis technique provided insight for the morphological changes and substrate accumulation patterns on membrane surfaces.