Abstract :
Performance of two types of reverse osmosis membranes in silica bearing waters were investigated. The experiments were carried out using the Osmonics SEPA CF membrane flat sheet unit. The membranes investigated were polyamide (MS19) and thin film (DS11); their rejection, compaction, concentration polarization, fouling propensity and ease of cleaning were examined. In addition, the effect of silica concentration and polyvalent ions were investigated. The average silica rejections on DS11 and MS19 membrane were found to be 96.2% and 93.8%, respectively. The thin film (DS11) membrane was found to be easier to foul by silica than polyamide (MS19) membrane, but more easily cleaned. Both membranes underwent compaction; however MS19 was more prone to compaction (15¯30%) than DS11 (0¯15%). The concentration polarization ranged between 2 and 3 for the runs. Increases in Ca, Mg and total hardness for runs with given silica feed concentration increased fouling penalties. Analysis of the deposit by EDS indicated that Ca and Mg did not partake in chemical structure of the deposit; it appears that they catalyzed polymerization and were minutely adsorbed by the silica deposit. The results indicated that keeping low feed silica concentration might not, under all conditions, be effective in minimizing fouling. For Ca to Mg ratio of 1.5, the optimum initial silica feed concentration was found to be 100 ppm, not the lowest concentration of 50 ppm. The emphasis should be put to understand the mechanism of silica fouling (i.e. crystallization or particulate fouling) in order to mitigate this problem. Further experiments are needed to investigate the effects of other parameters on silica fouling in dynamic systems and to understand the mechanism of silica fouling.
