Electrodialysis of softened produced water from shale gas development

Electrodialysis (ED) is one of several technologies for treating hypersaline solutions such as produced water (PW). A sample of PW collected from an experimental well (total dissolved solids 161,000 mg/L) was softened using sodium hydroxide and sodium carbonate to precipitate scaling cations. After pH adjustment, the softened PW contained mainly sodium chloride with some bicarbonate and traces of other elements. Both the softened PW and a simulated softened PW were converted into diluates and concentrates using electrodialysis in two stages. The stages were defined by a fixed recovery of the diluate. Mass balances for trace ions in the softened produced water revealed a significant effect of sequestration and release by the ion exchange membranes. Two anionic species, bicarbonate and silicate, accumulated in the diluate due to low transport through the anion exchange membranes. The final diluate had a sufficiently low TDS that could be further treated by reverse osmosis. The metrics of the electrodialysis stages (current efficiencies of sodium, chloride, and bicarbonate ions, transport number of water, specific energies, salt transport energies, and specific costs) and the fate of the trace elements are reported.