Electrochemical Generation of Hydroxide for Precipitative Water Softening

Centralized treatment facilities frequently add lime to raise the pH of hard water and induce softening by the precipitation of CaCO3 and Mg(OH)2. Electrochemical softening produces OH- on site by the reduction of oxygen and/or water at the cathode, but previous works either used hard water as the catholyte to induce precipitation directly on the cathode, leading to decreased performance, or recirculated a high-pH catholyte between the cathode and a crystallizer similar to 30 times over 8 h to achieve softening. Using softened groundwater as both the anolyte and catholyte, the current study generated a high-pH catholyte (similar to 11.3) within 30 min, inducing precipitative softening over similar to 1 h when applied to hard groundwaters with approximately one recirculation of groundwater. Application of a low-pH anolyte (similar to 2.8) to the precipitative softener effluent reduced the pH to similar to 9, suitable for distribution, thereby avoiding the cost of CO2 for recarbonation. The electrochemical system was as effective as lime softening for hardness removal by precipitative softening, but reduced the mass of waste solids by 67-84%, thereby reducing waste disposal costs. Based on a specific energy usage of 1.4 kW h/m3, the initial estimate of the operational cost for electrochemical softening, dominated by electricity costs, was competitive with lime softening, dominated by the costs of chemical reagents and waste disposal. The additional benefits of electrochemical softening include avoidance of handling lime solutions and reductions in CO2 emissions associated with lime production and the shipping of lime and waste solids.