The effects of intermittent permeate flow and crossflow on membrane coalescence of oil-in-water emulsions

Membrane coalescence is a chemical-free technique for breaking stable oil-in-water emulsions. In this work the effect of oil droplet residence time within the membrane porous matrix was investigated and a novel process is proposed based on intermittent permeate flow (flow or no flow) at 0.01 Hz. Polytetrafluoroethylene membranes with 0.45 and 1.20 µm nominal average pore size were used to coalesce emulsions with an average 1.5 µm droplet diameter. Experiments were carried out in a crossflow filtration cell and crossflow velocities were varied from 0.07 to 0.28 ms–1. Fluxes were found to increase with crossflow although oil rejection was zero, implying no concentration polarization. Results also demonstrated an improvement in coalescence performance during intermittent operation at low crossflow velocities. Under some conditions intermittent permeate flow operation produced higher permeate fluxes than without intermittent operation and is a potential novel process for membrane coalescence.

Intermittent operation at 0.01 Hz (50 s open and 50 s closed), contrary to expected, did not significantly improve separation efficiency. Improvements of about 5% were observed only for the 0.45 µm membranes at low (non zero) crossflow velocities. However it was found that both crossflow velocity and intermittence were able to reduce the amount of flux decline. The fluxes were increased by crossflow eventhough there was no oil rejection. This suggests interfacial film formation, which would respond to crossflow. Further investigations on different intermittence frequencies and wave patterns are required in order to optimize the process and its effects on separation efficiency. Membrane coalescence under crossflow and intermittent permeate operation promises to be a potentially novel process for the treatment of oily wastewater.