Comparison of Methods To Enhance Separation Characteristics in Nanofiltration

Abstract

This manuscript explores the possibilities of obtaining a more efficient separation of dissolved organic compounds from an aqueous matrix using nonconventional process designs in nanofiltration. It was attempted to transform the typical sigmoid rejection curve (representing the rejection of a solute as a function of molecular size) into a step function with a sharp rejection increase at the molecular weight cutoff. This was first done using a membrane stack that contained up to three membranes in the same module without any spacers between them. Experimental results with four commercially available nanofiltration membranes showed that the rejection of various test solutes was slightly higher, but the sharpness of the rejection curve was not improved. The results were compared with an in-series filtration using up to three membrane passages. The rejection curves were steeper in this case, but the overall molecular weight cutoff shifted toward very low molecular weights, which was not intended, and the overall permeate recovery of this system was too low. However, good results were obtained by recycling each retentate stream to the feed of the previous filtration step. Mass balances showed that the steepness of the rejection curves was greatly improved.