NF/RO Fouling By Organic Macromolecules and Their Mixture – Exploration of Dominant Force in Fouling

Nanofiltration (NF) / reverse osmosis (RO) fouling by organic macromolecules and their mixture was investigated under crossflow and constant pressure conditions. Bovine Serum albumin (BSA) and lysozyme (LYS) were used in single foulant case, while BSA/LYS mixture and LYS/alginate (ALG) mixture were selected to represent a feed containing two oppositely charged foulants. By studying the effect of feed solution pH and ionic strength on single foulant fouling, most severe flux drop occurred when pH was at foulant isoelectric point (IEP), and when the ionic strength was high. This reveals that the electrostatic repulsion between (similarly charged) foulant molecules plays an important role in single foulant fouling. In contrast, the foulant mixture cases showed completely different results: flux decline was more severe than the single foulant cases when the feed pH was between the two IEPs of foulants, but was weakly dependent on pH within this IEP range; and increased ionic strength led to less fouling. The possible explanation was that the electrostatic attraction between the two molecules dominated fouling. To further prove the existance of this strong attractive force, the masses of LYS and ALG in the cake layer were quantified separately. It is interesting to observe a relatively constant mass ratio (LYS:ALG=1:4) disregarding the foulant composition (foulant ratio varied from 3:7 to 7:3) in the feed water. This study reveals that membrane fouling by mixed macromolecules may behave very differently from that by typical single foulant system, especially when the inter-foulant-species interaction dominates over the intra-species interaction in the mixed foulant system.