Assessment of pervaporative concentration of dairy solutions vs ultrafiltration, nanofiltration and reverse osmosis

Abstract

Concentrating raw milk or semi-finished dairy products is an important step in dairy industry to produce milk powder or products enriched in certain components, or to reduce product volume and energy consumption during transportation. This study deals with pervaporative concentration of dairy solutions vs ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO). While the highest flux was observed with the UF, pervaporation (PV) exhibited almost complete retention of dairy solids. A decline in flux due to concentration polarization and membrane fouling was observed in all the membrane processes at different extents. The UF, NF and RO experienced severe flux declines at a dairy solid content above 16 wt%, while the flux decline in pervaporation was much less significant, even at a feed solid content as high as 40 wt%. Unlike UF, NF and RO where the external mass transfer resistance due to membrane fouling and concentration polarization dominated the overall mass transfer, pervaporative mass transport in dehydrating the dairy solutions was mainly determined by the membrane itself. The UF membrane suffered from irreversible fouling and the fouled membrane could not be fully cleaned even with a 0.1 M NaOH solution, whereas the fluxes in NF and RO could be restored significantly by rinsing the membrane surfaces with a dilute NaOH solution (0.1 mM). In pervaporation, the permeation flux was fully restored by a simple water rinse due to nonporous structure of the membrane and its hydrophilic surface.