Fouling effects of polysaccharides and humic acid in nanofiltration

Fouling in nanofiltration was studied using model substances similar to tho se in the pulp and paper mill waters, such as vanillin, humic acid, locust bean gum, and karaya gum. The parameters studied were the concentration of foulants, pH, cross-flow velocity and membrane hydrophobicity. The substanc es were nanofiltered alone and in their binary mixtures. The nanofiltration membranes (NTR-7450 and Desal-5 DK) differed by their cut-off values and t heir material properties, such as hydrophobicity. Various causes of fouling were observed to exist with the model substances studied. The electrostatic attraction markedly influenced the flux of charg ed substances, but had an insignificant effect on the flux of uncharged sub stances. Hydrophobic interactions overcame the electrostatic repulsion and caused more fouling of the more hydrophobic membrane. The prediction of fil tration properties of the binary mixtures of model substances from the filt ration results of individual substances is extremely difficult. Both the in teractions between two model substances as well as the interactions between the model substances and the membrane determined the flux and fouling in t he filtration of the binary mixtures of the model substances. Humic acid im proved both the permeate flux and the pure water flux at neutral pH, but sl ightly fouled the membranes under acidic conditions. Humic acid might increase both the membrane hydrophilicity and its negative charge when being adsorbed by hydrophobic attraction. (C)2000 Elsevier Sci ence B.V. All rights reserved.