Control of silica scaling phenomena in reverse osmosis systems
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Industrial development in the Border region of Mexico and Texas depends on the availability and quality of water; several industries require high quality process water. Reverse osmosis systems achieve that quality, but high concentrations of silica in the local water supplies limit the recovery. Silica is problematic in membrane systems due to its complicated chemistry. Silica may induce both scaling and fouling, causing decline in the water production rate, reduced water product quality and permanent damage to membranes. The objective of this research was to develop pretreatment strategies for RO systems subject to silica fouling and scaling. To accomplish this objective required a thorough investigation of processes to remove silica prior to membrane treatment with softening and adsorption/precipitation. This investigation included chemical equilibrium modeling of possible treatment schemes, and subsequent bench-scale batch experiments of the most promising treatments identified by that modeling. A bench-scale RO system was then operated with and without pretreatment to test vii the efficacy and potential benefits of the best treatments identified in the batch experiments. Lime softening proved to be an effective method to reduce silica concentration. Results show that addition of magnesium chloride (35 mg/L of Mg+2) and 165-180 mg/L lime plus soda ash for calcium control (160-190 mg Na2CO3/L) dramatically decreased the silica concentration from 28 mg/L as Si to approximately 5 mg/L as Si. According to Mineql simulations, the precipitated compound is chrysotile (Mg3Si2O5(OH)4). RO experiments with both synthetic Rio Grande water (i.e., water made to mimic the chemical composition) and Rio Grande water were performed with and without influent water pretreatments. Operational conditions were 1550 kPa, and cross-flow velocity of 3.6 cm/s at 25 ºC. Untreated Rio Grande water presented a specific flux decline of 53 %. Pretreated water presented a lower flux decline at the same conditions, 7 %. SEM and XPS analysis showed less amount of silica fouling over the membranes. Pretreatment of silica-bearing waters before entering an RO unit dramatically improved the flux behavior, which is the main parameter describing membrane system performance.