Silica fouling in direct contact membrane distillation

Recent studies of membrane distillation have shown varying effects on flux when sparingly soluble salts of calcium precipitate. Another common sparingly soluble mineral often found in brackish water feeds is silica. Because of its normal solubility silica is stable to higher concentrations at higher temperature than at room temperature and this should allow further water recovery and volume reduction of brackish water RO brines by membrane distillation. We report here on a series of experiments conducted with hollow fiber and flat sheet DCMD modules with silica solutions with saturation indices in the range 1.5 to 2.2 at temperatures of 60-75 C. We found that unlike previous work on calcium salt scalants in cross-flow hollow fiber modules, in the same type of modules silica fouling consistently caused significant flux decline after 2-7 h effective induction time, with declines of up to 70%. In flat sheet modules, the induction time for 400 mg/L silica solutions at 75 C was 4-7 h independent of initial flux ranging from 15 to 50 kg/m²-h, implying very different supersaturations at the membrane surface. Scanning electron microscopy studies of silica fouled membranes support a proposed model that the fouling occurs by deposition of colloidal silica in the pore mouths leading to partial wetting followed by aggravated temperature and concentration polarization leading to a cascade of precipitation that then blocks the pore to vapor transport.