AWWA MTC95004

Separation in nanofiltration (NF) processes can occur due to a variety of phenomena. In all membrane processes, sieving occurs when the constituent of interest is much larger than the pore size of the membrane. Convection occurs when small, uncharged molecules are able to pass through the membrane with the bulk flow. Diffusion occurs under a concentration gradient across the membrane, but solubility of the permeating species in the membrane matrix will also impact transport. In NF systems where convection and/or diffusion occur, recovery will have a direct impact on the rejection of solutes. As the recovery is increased, the concentration of solutes in bulk solution on the feed side of the membrane increases, resulting in an increase in convective and diffusive transport. The mass transfer resistance of a membrane can change due to physical alterations of the polymeric matrix, the formation of organic and inorganic cake layers and concentration polarization effects. These mass transfer resistances can decrease the flux of water and solutes. This paper investigates the impact of mass transfer resistance and recovery on nanofiltration separation. Both natural waters and specific organic solutes were investigated.