The dynamics of polarisation in unstirred and stirred ultrafiltration

Polarisation of a retentive UF membrane has been studied for three types of solute, a colloid (silica sol), a protein (albumin) and a branched chain polymer (Dextran), with and without stirring. For unstirred conditions the data have been analysed by modified constantpressure filtration theory, which gives specific resistances, a, of the solutes consistent with their sizes and conformation. The simple Carman—Kozeny relationship approximates a for the colloid and the protein. For all three solutes a increased with pressure and concentration. Times to steady-state flux for stirred conditions ranged from 10 to 50 seconds, with longest times for the lowest concentrations and the largest solute. The amount of solute in the polarised layer was estimated from the measured cake (gel) resistances and the known specific resistances. Layer thicknesses ranged up to to ? 5 μm for the protein, ? 6μm for the Dextran and ? 20 μm for the silica sol. Slight deviations of flux—time profiles from the filtration model are explained by membrane—solute interactions, such as irreversible pore plugging and reversible pore obstruction.