Electrochemical tap water softening: A zero chemical input approach

Electrochemical water softening was proposed as a sustainable alternative for ion exchange softening, avoiding the input of salt to drinking water and the production of a concentrated brine. Here we demonstrated two novel modes of operation combining an electrochemical cell with a fluidized bed crystallizer. The first approach relied on an electrochemical cell consisting of an anode and cathode separated by a cation or anion exchange membrane. The feed water was first directed into a crystallizer where it was blended with alkaline cathode effluent. The effluent of the crystallizer, softened water, was in part recirculated to the cathode to generate alkalinity, in part to the anode compartment, where the pH was again decreased. Average removal efficiencies for calcium and magnesium of 75-86% and 7-21% respectively, could be sustainably reached, at a specific energy consumption of 7.0-10.1 kWh kg^-1 CaCO₃ (0.86-1.39 kWh m^-3 water). This configuration allowed reagent-free water softening, albeit with an effluent with a pH between 3.0 and 3.6. In a second mode of operation, the process influent to soften was also directed to the crystallizer and recirculated over the cathode, which was separated from the anode using an anion exchange membrane. In this mode of operation, the cathode effluent was sent through the crystallizing unit, and the anode compartment was operated in closed-loop. Average calcium and magnesium removal efficiencies of 73-78% and 40-44% were obtained at specific energy consumptions of 5.8-7.5 kWh kg^-1 CaCO₃ (0.77-0.88 kWh m^-3 water). Although the softened water had an elevated pH (∼9.4), the advantage of this configuration is concomitant removal of anions and the formation of acids/disinfectant in the anode compartment. Both methods of operation thus showed reagent-free water softening at a relatively low specific energy consumption. These novel methods of softening could be used in remote locations where access to chemicals or discharge of ion exchange brines proves to be difficult, or in case addition of chemicals for softening is unwanted. Further research is needed to further decrease the specific energy consumption during long-term operation.