Cross-flow ultrafiltration for surface water treatment in North Thailand

Experimental studies on cross-flow ultrafiltration were carried out to evaluate the applicability to treat surface water in North Thailand. Hollow-fiber UF modules (polyacrylonitrile, MWCO 13,000 daltons, I.D. 0.8 mm, and module 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 velocity operation was superior to achieve a longer operational period before chemical cleaning. Intensive backwashing was performed when the filtration resistance rose over 10¹³ m^–1, which revealed that more than 80% of the resistance was caused by cake-layer that can be removed by the intensive backwashing. 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 intensive backwashing was proposed to be an easy and rapid method to recover the 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 water systems in developing countries.