Electrodialytic crystallization to enable zero liquid discharge

Abstract

The management of hypersaline brines (that is, wastewater of high salinity) is a technical challenge that has received increasing attention due to their growing volume, environmental impacts and increasingly stringent regulations. Here we present electrodialytic crystallization (EDC) as a new process to achieve brine crystallization without evaporation. In an EDC process, the brine stream recirculating between an electrodialysis cell and a crystallizer remains oversaturated via continuous electromigration of ions from the feed stream across the ion exchange membranes. We first used Na2SO4 as the model salt to demonstrate the feasibility of EDC and to perform a systematic investigation of how crystallization kinetics and crystal size distribution depend on current density and crystallization mode. We then elucidated the criterion of crystallizability and showed how it depends on salt species, membrane properties and operating conditions. Lastly, we analysed the energy consumption of an EDC-reverse osmosis treatment train for achieving zero liquid discharge of a Na2SO4 brine. Overall, this study provides a proof of concept for EDC as an electric-field driven and non-evaporative crystallization process, and lays the foundation for its future technical development and optimization. Achieving zero liquid discharge is an essential step towards the sustainability of hypersaline brine treatment. A potential method to achieve zero liquid discharge on the basis of electrodialysis crystallization is now demonstrated.