NOM Accumulation at NF Membrane Surface: Impact of Chemistry and Shear

Abstract

The effects of solution chemistry, surface shear, and composition of natural organic matter (NOM) were investigated for their impact on accumulation of foulant material at the surface of charged polymeric nanofiltration membranes. The source of NOM was the Suwannee River. A bench-scale, batch recycle system was used with 20 hollow fiber, nanofiltration membranes. Membrane flux decline and foulant accumulation increased at low pH and high ionic strength as a result of neutralization of charge, electric double layer compression, and the apparent shift in conformation of charged NOM macromolecules. The rate of NOM accumulation decreased with operating time, suggestive of an eventual steady state between adsorption and desorption. The effect of NOM composition on membrane fouling could not be discerned by a standard technique to isolate hydrophobic and hydrophilic NOM fractions, quite possibly because of the fractionation methodology's failure to recover a small but important fouling fraction or because of NOM interactions that are lost when individual fractions are separately tested. However, a greater percentage of the hydrophilic than hydrophobic fraction permeated the membrane, in agreement with prior observations by others. Increasing the cross flow velocity from 85 to 255 cm/s reduced the extent of flux decline, presumably due to hydrodynamic disruption of cake layer formation.