Recovery of nitrate and fluoride salts from stainless steel pickling wastewater with flow-electrode capacitive deionization

Flow-electrode Capacitive Deionization (FCDI) is an innovative method for practical salt removal and recycling applications. Here, we report that FCDI facilitates the recovery of nitrate and fluoride salts from brines produced during the wastewater treatment process in a stainless steel pickling line. Laboratory-scale experiments with synthetic wastewaters were used to evaluate the influence of (a) the membrane thickness, (b) feed flow rates, and (c) applied voltage on the outlet concentrations and the average salt transfer rate. In continuous single-pass experiments, the flow rates of diluate and concentrate have the greatest influence on the resulting outlet concentrations in the FCDI process as they directly influence the residence time. The operating voltage of the FCDI process can be varied to increase the ratio of fluoride over nitrate for recycling. • Proposition of a novel process for the treatment of nitrate and fluoride salts from stainless steel pickling wastewater. • Introduction of Flow-electrode Capacitive Deionization (FCDI) for salt concentration with low energy demand. • Investigation of membrane thickness, feed flow rate and FCDI cell voltage. • Increased cell voltages lead to higher concentration ratios of fluoride over nitrate for recycling.