Performance of RO membranes in silica bearing waters

S ilica fouling of reverse osmosis membranes w ere studied using a fiat sheet Osmonics RO cell u sing MS 19 and DS 11 membranes. The analysis o f silica fouling was based on flux decline, d issolved silica concentration and membrane p recipitate analysis. From the analysis of the e xperimental data, several conclusions can be d rawn. Results indicate that thin-film (DSll) m embrane was easier to foul than polyamide ( MS19) membrane. At 80% water recovery, t here was 65% flux decline for DS11 while 35% d ecline was observed for MS19. However, it s eems that DS 11 membrane was more effective in silica removal than MS19 as the average silica r ejection on DSll and MS19 membranes were, r espectively, 96.2% and 93.8%. W ith higher calcium, magnesium and total h ardness, membranes suffered more fouling as c alcium and magnesium salts are able to catalyze t he polymerization of silica with magnesium b eing more effective than calcium. More fouling w ould incur in presence of these salts as they facilitate colloidal silica formation and hence result in particulate deposition in addition to direct p recipitation of monomeric silica. Membrane p recipitate analysis by EDS showed that only a s mall amount of Ca and Mg was present on the m embrane surface, with Mg being even less than Ca. Results indicate that Ca and Mg did not cop recipitate with silica in dynamic system. It seemed that for unsaturated feed solutions a g lasslike scale formed on the surface by direct p recipitation of monomeric silica due to concent ration polarization at the surface. However, for m etastable supersaturated solutions, there would b e a fair degree of colloidal particle, which were p roduced in the bulk or the boundary layer, d eposition. The effect of silica feed concent ration on fouling should be further investigated.