Oil/water separation using nanofiltration membrane technology

Traditionally, ultrafiltration has been used to separate relatively clean water from oil/water emulsions. This water was once suitable for discharge to all sewer lines but generally inadequate for re-use. Recent changes in sewer discharge limits have created a need to generate cleaner water. A different type of membrane filtration, nanofiltration, may offer an effective means of recycling water from oil solutions or generating clean enough water for discharge. Two different oily solutions were tested: machining coolant and oily washwater. Results of ultrafiltration and nanofiltration batch concentration tests on machining coolant indicate that a feasible process can be designed with nanofiltration membrane technology. Various models were used to analyze flux behavior including the resistance model, the mass transfer model, and the osmotic pressure model. Except at very high per cent recovery levels, lower levels of COD and conductivity were observed in the nanofilter permeate as compared to the ultrafilter permeate; the quality of nanofiltration permeate resembled that of ultrafiltration permeate near the end of the concentration cycle. Nanofiltration flux was slightly lower mostly due to the higher membrane resistance of the nanofilter. A first order economic analysis shows that the nanofiltration process is as cost-effective as the ultrafiltration process on this particular machining coolant. The same comparative tests on oily washwater were carried out. Results indicate that washwater contained much more dissolved solids than the machining coolant, thus creating too much osmotic pressure for nanofiltration permeate to be produced effectively at conventional ultrafiltration operating parameters. Even though higher nanofiltration flux could be obtained at much higher operating pressures and the permeate was significantly cleaner than ultrafiltration permeate, the cost of the nanofiltration process on oily washwater would be significantly higher than the cost of the ultrafiltration process