A brackish water source containing high magnesium concentration (333 mg/L as CaCO3) for reverse osmosis (RO) was studied for silica scaling. Threshold limit for RO recovery and required silica removal were firstly determined by a removal–saturation–recovery curve. Three different ratios of lime/soda ash combination were used to test the efficiency of silica removal as well as the impact of calcium and magnesium. Higher pH was more effective for silica removal due to electrostatic attraction since H3SiO4 and H2SiO−24 were dominant when pH >9.9. The precipitation of Mg(OH)2(s) has assisted more for silica removal than that of CaCO3(s) and the ratio of 0.044 mg SiO2/mg Mg(OH)2(s) and 0.027 mg SiO2/mg CaCO3(s) were statistically determined. Moreover, the presence of high magnesium causes Mg(OH)2(s) to precipitate at lower pH (9.41) instead of formation of forsterite (Mg2SiO4(s)), which typically occurs at pH >12.3. Therefore, no forsterite was observed and was verified by X-ray diffraction (XRD) analysis. Consequently, adsorption is more dominant than chemical reaction in this study. Silica removal was also enhanced by coagulation. With the addition of coagulant (PACl), highest silica removal was achieved at pH of 10, but was decreased when the pH was over 10, due to the amphoteric properties of aluminum hydroxide to reduce the electrostatic attraction to silica at higher pH. Since both softening and coagulation were effective for silica removal, seven decision-making criteria were developed to compare the pros and cons of these two processes.

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