Cross-flow ultrafiltration for surface water treatment in North Thailand

Experimental studies on cross-flow ultrafiltration were carried out to eval uate the applicability to treat surface water in North Thailand. Hollow-fib er UF modules (polyacrylonitrile, MWCO 13,000 daltons, I.D. 0.8 mm, and mod ule length 1126 mm) were used in the experiments. While the permeation flux was controlled at 1.0 m/d, the cross-flow velocity was either fixed at 0.1 m/s or varied as the membrane fouling proceeded. The comparative study on fixed and variable cross-flow velocities revealed that the variable velocit y operation was superior to achieve a longer operational period before chem ical cleaning. Intensive backwashing was performed when the filtration resi stance rose over 10(13) m(-1), which revealed that more than 80% of the res istance was caused by cake-layer that can be removed by the intensive backw ashing. However, the operational period was only 54% of the one for the new membranes, which suggested that the percentage reduction of the filtration resistance should not be used as an index of the efficiency of chemical or physical membrane cleaning. Despite shortened operational periods, the int ensive backwashing was proposed to be an easy and rapid method to recover t he permeability of fouled membranes without chemical use. The cross-flow UF system used in this study was found to be easy to operate even in small wa ter systems in developing countries.