Economics of ion exchange versus vibratory shear enhanced processing for minimization of reverse osmosis concentrate volume

Achieving a supply-demand water balance in Arizona depends on full utilization of the regional allotment of Central Arizona Project (CAP) water. The total dissolved solids (TDS) concentration at the CAP turnout to Cortaro Marana Irrigation District (CMID) in Tucson, Arizona (∼790 mg/L), is more than three times that of native groundwater. As a consequence, 200,000 t/year of salt accumulates, creating a major long-term issue. Reverse osmosis (RO) was evaluated for salt management. Pilot-scale studies indicated that scaling limits water recovery to 75-80% during conventional RO desalination of CAP water. To increase water recovery (1) ion exchange (IX) pretreatment of the RO influent and (2) posttreatment of RO brine using vibratory shear enhanced processing (VSEP) were evaluated. When scale-forming cations, barium and calcium, are removed via IX pretreatment, the expected maximum water recovery is >95% based on solubility product calculations. Alternatively, the water lost as brine can be reduced to 2-4% via post-RO VSEP treatment. Estimated costs for a hypothetical 0.657 m³/s (15 mgd) treatment plant were compared in three different scenarios. Use of VSEP with RO results in savings of more than $5 million per year relative to RO treatment alone. The total annualized cost of VSEP operation was insensitive to the operational variables analyzed (VSEP recovery and time between VSEP membrane cleanings) in the vicinity of the optimal operating point. The maximum long-term recovery using a combination of IX and RO treatment has not yet been established experimentally. A theoretical recovery of 99% via IX and RO was optimal, resulting in ∼$6 million per year savings relative to RO treatment alone.