The Influence of Complex Ions in High Recovery and Brine Concentrator Saturation Calculations

As RO systems are pushed to increasingly higher recoveries, the concentration of salts in solution increases exponentially. This drastic increase in salt concentration demands more complex scaling calculations for proper system operation. One major factor in high salinity RO concentrates is the influence of complex ions on ionic equilibria. Complex ions play a major role in long-range electrostatic interactions between ions. These interactions can drastically change ion equilibria and concentrate pH, both of which in turn affect the saturations of sparingly soluble salts. Traditional projection software was never intended to correctly calculate the influence of complex ions on high salinity concentrates. This paper presents a chemical model that uses complex ion pairing to project scaling chemistry at very high recoveries. This model was tested and validated during a high-recovery RO pilot study. The modelís projection of scale formation beginning around 94% was confirmed with flux decline noted at 94-95% recovery and stable performance at lower recoveries. Membrane element autopsies also confirmed model predictions of the constituents that would govern scaling.