Streaming potential and SEM-EDX study of UF membranes fouled by colloidal iron

Destabilization-flocculation is often used as a pretreatment before ultrafiltration of drinking water and secondary wastewater effluent. The fouling of ultrafiltration membranes by colloidal iron hydroxide-oxide has been studied by measuring the pore streaming potential of a series polysulfone type UF membranes with 10 KDa–50 KDa NMWCO. SEM–EDX was carried out on the fouled membranes and the iron distribution in and on the fouled membrane was determined. Fouling of the membrane as measured by flux reduction was usually accompanied by a positive change in zeta potential and iso-electric point (IEP) of the membrane. Severity of fouling seems to correlate with the presence of some pores of size larger than part of the iron colloid particle size distribution. SEM-EDX confirmed presence of iron within those membranes (20 KDa) suffering the most fouling. An initially large change in zeta potential was seen even after relatively small amounts of solution were filtered through the membrane. A control experiment showed this is not due to iron adsorption equilibrium, but should probably be attributed to fouling.

• • • • • coated. The higher the negative zeta potential of the clean membrane, the higher ∆ζ. Based on results with 50 KDa MWCO membrane, a linear relationship exists between fouling degree and ∆ζ. However, an initially large change in zeta potential was seen even after relatively small amount of solution was filtered through the membrane. Therefore, change in zeta potential, ∆ζ, can be used as an indicator for commencement of fouling even for small flux reductions. The reduction of ∆ζ with reduction of pH of the streaming potential measurement is probably due to release of both soluble and colloidal iron. While the amount of deposit correlated with MWCO, the extent of flux reduction did not. The extent of colloidal iron fouling correlated with the fouled membrane matrix composition (in particular the O%). The extent of fouling is sensitive to dose-pH combination. The lower the pH the higher the fouling (even in dose ratio of 1:10, acidic to basic pH) due to pore clogging (iron solubility).