Donnan Dialysis Desalination with a Thermally Recoverable Solute

couples Donnan dialysis (DD) with thermally-recoverable solutes and utilizes low-grade heat as energy input. In the proposed process, saline feed streams and receiver solutions of concentrated NH4HCO3(aq) flow stepwise across cation- and anion-exchange membranes. The large transmembrane concentration differences of NH4+ and HCO3??? set up electrochemical potential gradients to drive the uphill transport of Na+ and Cl??? ions, respectively, from the saline feed into the receiver stream. Warming the two outlet streams using low-temperature thermal sources volatilizes NH3 and CO2, thus removing NH4HCO3 to yield desalinated product water and concentrated brine. The separated NH3(g) and CO2(g) are then recycled to reconstitute the receiver solution. The concept was first experimentally validated by desalinating brackish water simulated with 100 mM NaCl solutions to freshwater salinities (<17 mM). DD desalination was then demonstrated for larger ranges of feed and receiver concentrations of 100???1000 mM, and the experimental salt removals showed good agreement with theoretical Donnan equilibria (within 5%). The experimental results revealed that the unavoidable permeation of receiver solute co-ions due to imperfect membrane permselectivities is the main factor that prevents the theoretical thermodynamic potential from being reached. Nonetheless, current commercial ion-exchange membranes are sufficient to suppress the undesired co-ion leakage, yielding salt removals adequate for practical desalination. Module-scale analysis quantitatively showed that countercurrent DD operation can obtain higher desalination performance compared to co-current flows, achieving salt removals and water recovery yields as high as 95.5 and 87.5%, respectively. The utilization of low-grade thermal sources, such as waste heat and low-temperature geothermal reservoirs, as the primary energy input to drive the innovative approach opens up opportunities to lower the carbon intensity of desalination.

Automated summary

Couples Donnan dialysis (DD) with thermally-recoverable solutes and utilizes low-grade heat as energy input. The experimental results showed good salt removal performance, and module-scale analysis demonstrated that countercurrent DD operation can achieve higher desalination performance.