Correlation of Flux Performance to Mass Deposition and Salt Rejection in Nanofiltration (NF) Fouling By Organic Macromolecules

Most reverse osmosis (RO)/ nanofiltration (NF) membrane fouling studies focus on permeate flux performance, which can hardly reveal information on foulant deposition, such as the cake packing density. This study investigates the cake mass deposition under various fouling conditions (i.e., initial flux, feed pH and ionic strength) and the correlation between cake deposition, flux performance and solute rejection for NF fouling by organic macromolecules (i.e., bovine serum albumin (BSA), lysozyme and their mixture). Results show that greater flux decline/cake resistance correspond to more cake mass deposition at severe fouling conditions, which are at high initial flux, high ionic strength (IS) for single foulant case and low IS for dissimilar charged protein mixture case, and that solution pH is at the protein isoelectric point (IEP) for single foulant case and pH is between the two IEPs of the proteins for protein mixture. However, beyond expectation, cake layer of high specific resistance (cake resistance normalized by cake mass) does not always correspond to severe fouling conditions. One exception is the minimal cake deposition under unfavorable fouling condition (such as under low initial flux). More interestingly, the specific cake layer resistance decreases with increased ionic strength (1 mM to 100 mM NaCl) disregarding the foulant composition, probably due to the formation of a more porous and heterogeneous foulant cake layer under high ionic strength. This observation is also consistent with the salt rejection results. Salt rejection upon fouling can be improved at initial period or moderate fouling conditions where the dense cake layer may be formed. Thick or loosely formed cake results in more cake enhanced polarization which deteriorates rejection.