Threshold flux for shear-enhanced nanofiltration: Experimental observation in dairy wastewater treatment

High and stable permeate flux can be obtained with shear-enhanced nanofiltration (NF) in model dairy wastewater treatment. In order to sustain such flux in long-term operation, critical fouling conditions should be studied and an optimum flux needs to be selected. Based on resistance-in-series and particle deposition models, a threshold flux was proposed to make a distinction between low and high fouling rates. At and below threshold flux, adsorption and reversible fouling are almost stable, and the fouling rate is low and nearly constant, irrespective of permeate flux; while above it, flux declines markedly with increasing flux mainly due to incremental reversible fouling. Flux remained quite stable with slight fouling (<5%) in 6 h operation at threshold flux, up to 6.1 × 10−5 ms−1 at shear rate of 1.81 × 105 s−1 for diluted milk. Flux anomaly occurred in operations above threshold flux and fouling was more important (permeability loss > 20% for undiluted milk). This threshold flux is determined by pressure stepping experiments as the point above which flux ceases to increase linearly with transmembrane pressure (TMP). Each flux step only needs to last a few minutes to ensure reversible fouling stabilization at high shear rate. In shear-enhanced filtration, these threshold fluxes are, of course, larger than critical ones, and more compatible with industrial expectations. It is concluded that the separation performance of membranes should be evaluated at or below threshold flux to eliminate the effect of fouling, and the effect of feed characteristic on membrane fouling should be investigated above threshold flux, while operating at threshold flux is recommended for industrial long-term operation.