REACTIVE-SILICA REMOVAL FROM REVERSE OSMOSIS CONCENTRATE IN LAB-SCALE UNIT PROCESS WETLANDS

Brackish groundwater desalination can provide many arid, landlocked regions with a reliable water supply. However, given the challenges associated with inland brine disposal, this process only becomes economically feasible at high water recovery rates. Silica and divalent cations in groundwater commonly cause membrane scaling which severely limits water recovery. Diatoms are single-celled photosynthetic microorganisms that are encased in a silica matrix and thus can sequester silica under ambient conditions. Unit process open water wetlands, a nature-based system originally designed for removal of nutrients and organic contaminants, could provide a passive means for removal of silica through the cultivation of a diatom-rich biomat. To assess the potential of open-water unit process wetlands for removing reactive silica, a lab scale treatment system was constructed to mimic the conditions expected in the field. When operated with a 2-day hydraulic residence time, approximately 45% of the reactive silica was removed from a synthetic brackish groundwater reverse osmosis concentrate containing about 90mg/L reactive silica. Diel trends were observed, with lower reactive silica concentrations in the effluent during the day. Silica removal is likely due to a combination of diatom growth and co-precipitation with divalent cations like magnesium at elevated pH values (e.g. a pH swing from 8.5 to 10 is observed over the course of a day). To enhance rates of silica removal, periodic harvesting of diatoms will be examined along with analysis of the role of nutrients and flow rates on diatom growth rates. This process optimization will progress towards the end use goal of feeding the wetland effluent through a second pass RO unit to achieve further water recovery, pushing towards zero liquid discharge.