Reverse osmosis desalting of inland brackish water of high gypsum scaling propensity: kinetics and mitigation of membrane mineral scaling

The potential for mineral scaling that may limit the generation of new potable water resources by reverse osmosis (RO), from inland brackish water of high gypsum scaling propensity, was experimentally explored via flux decline measurements and real-time RO membrane surface imaging. Antagonistic gypsum and calcium carbonate scaling kinetics were demonstrated for high-sulfate brackish water desalting. RO scaling studies with brackish water from the California San Joaquin Valley (approximately 10 000 mg/L total dissolved solids) revealed that membrane gypsum scaling was increasingly retarded with rising bicarbonate concentrations. Crystal growth rate, fractional membrane scale coverage, and flux decline decreased by up to about 63, 78, and 73%, respectively, as the bicarbonate concentration increased, at the membrane surface, from < 0.01 to 7.81 mM, for a gypsum saturation index of 2. Inhibition of gypsum crystal growth was attributed to bicarbonate adsorption onto the crystal surfaces, and CaCO3 scaling was undetected even up to a calcite saturation index of approximately 16. Given the suppression of gypsum scaling by bicarbonate, it is essential to considerthis effect in the conventional practice of pH adjustment to suppress CaCO3 scaling. The present results suggest that antagonistic and synergistic mineral crystallization kinetics effects are important for optimizing scale-control strategies (e.g., acid and antiscalants addition to the RO feed).