Critical flux measurement for model colloids

The conventional operating membrane of a laboratory membrane filtration pro cess is to apply controlled transmembrane pressures to the retentate side o f the membrane, with the permeate side open ended. Often the minimum transm embrane pressure available is sufficient to cause membrane fouling in a giv en system. A membrane rig has been built which monitors transmembrane press ure in increments of 0.001 bar and by pumping permeate at a specified rate controls the flux to be constant. The technique used allows sensitive detec tion of trace fouling. Under a variety of low flux conditions fouling was n ot observed and it was found to be useful to produce an experimentally rela ted definition of two types of critical flux. In the first definition a 'st rong form' of critical flux exists if the flux of a suspension is identical to the flux of clean water at the same transmembrane pressure. In the seco nd definition a 'weak form' of the critical flux exists if the relationship between transmembrane pressure and flux is linear, but the slope of the li ne differs from that for clean water. This paper describes how the use of t his operating mode led to the successful experimental measurements of criti cal fluxes for two colloidal silica suspensions, BSA solution and a baker's yeast suspension with a 50k MWCO membrane. These measurements could not be made successfully in constant-pressure mode. The paper also reports experi mental evidence in support of a 'strong form' of the critical flux for the filtration of X30 silica suspension. Finally, we report the effect of membr ane pore size on critical flux measurements for the three types of feed flu ids. (C) 1999 Elsevier Science B.V. All rights reserved.