Brackish water with a silica concentration of 30 mg/L as SiO2 was studied for silica removal using in-line coagulation/ultrafiltration (UF) processes to prevent scaling problems in a reverse osmosis (RO) membrane treatment plant. Other than pH and coagulant dosage, mixing intensity and velocity gradient are also a critical factor with respect to silica removal. Different velocity gradient (G), pH and alum dose were experimented and contour plots were used to determine the optimum silica removal. The optimum silica removal of 65% was achieved with G = 2000 s(-1), pH = 7.1 and alum dosage = 30 mg/L as Al2O3, corresponding to a maximum of 87% recovery for the RO membrane plant. Silica was removed by co-precipitation of aluminum hydroxide, supported by the results of turbidity and silica measurement. Although the optimum G value to reduce the membrane fouling potential was 1000 s(-1), G value of 2000 s(-1) can serve the purposes of silica removal with lower fouling potential for this UF membrane. Streaming potential by electrokinetic analyzer was used to analyze the UF membrane before and after filtration, and a slight reduction of negative charge was observed at medium pH, resulting from the adsorption of positively charged coagulated particles on the negatively charged UF membrane surface. energy dispersive X-ray spectroscopy also showed a slight aluminum and silica signal on the used UF membrane surface. (C) 2009 Elsevier B.V. All rights reserved.