Optimization of constant-current operation in membrane capacitive deionization (MCDI) using variable discharging operations

•Electrode discharging conditions significantly affect MCDI desalination performance.•High-current discharge improves productivity by 70% c.f. low-current discharge.•Low- c.f. high-current discharge results in 40% lower volumetric energy consumption.•Stopped flow discharge enables water recovery of up to 85%. Membrane capacitive deionization (MCDI) is an emerging electric field-driven technology for brackish water desalination involving the removal of charged ions from saline source waters. While the desalination performance of MCDI under different operational modes has been widely investigated, most studies have concentrated on different charging conditions without considering discharging conditions. In this study, we investigate the effects of different discharging conditions on the desalination performance of MCDI electrode. Our study demonstrates that low-current discharge (1.0 mA/cm2) can increase salt removal by 20% and decrease volumetric energy consumption by 40% by improving electrode regeneration and increasing energy recovery, respectively, while high-current discharge (3.0 mA/cm2) can improve productivity by 70% at the expense of electrode regeneration and energy recovery. Whether discharging electrodes at the low current or high current is optimal depends on a trade-off between productivity and energy consumption. We also reveal that stopped flow discharge (85%) can achieve higher water recovery than continuous flow discharge (35–59%). However, stopped flow discharge caused a 20–30% decrease in concentration reduction and a 25–50% increase in molar energy consumption, possibly due to the higher ion concentration in the macropores at the end of discharging step. These results reveal that an optimal discharging operation should be obtained from achieving a balance among productivity, water recovery and energy consumption by varying discharging current and flow rate. [Display omitted]