Minimizing salinity accumulation via regulating draw solute concentration in a bioelectrochemically assisted osmotic...

A suitable draw solute (DS) concentration in bioelectrochemically assisted osmotic membrane bioreactor (BEA-OMBR) can convert the "negative effect" of salinity accumulation into a "beneficial effect" by using the reverse-fluxed DS as a buffer agent or a carbon source supplement. Herein, the effect of DS concentration from acid buffer solution (i.e., ammonium chloride, NH₄Cl), alkaline buffer solution (i.e., sodium bicarbonate, NaHCO₃), and organic solution (i.e., sodium acetate, NaOAc) on salinity accumulation was systematically investigated. Salinity accumulation with NaHCO₃ DS mainly derived from reversal fluxed sodium ion (Na⁺, major contributor with DS concentration ≤0.25 M) and bicarbonate ion (main contributor with DS concentration ≥0.50 M): Na⁺ accumulation could be mitigated by Na⁺ transport dominant by electrically driven migration (i.e., 21.3-62.1% of reverse-fluxed Na⁺), and bicarbonate accumulation could be reduced by buffer system. A medium-low concentration of 0.25 M NH₄Cl DS had a better performance on current density of 165.0 ± 23.0 A m^-3 and COD removal efficiency of 91.5 ± 3.4% by taking advantage that 77.7 ± 1.3% of reverse-fluxed ammonium could be removed by biological treatment and ammonium transport. A high NaOAc DS concentration (i.e., ≥0.05 M) exhibited a higher current density of 145.3-146.0 A m^-3 but a lower COD removal efficiency due to the limited carbon source utilization capacity of anaerobic bacteria. Both concentration diffusion (20.9-28.3%) and electrically driven migration (29.5-39.4%) promoted reverse-fluxed Na⁺ transport to catholyte and thus mitigated Na⁺ accumulation in the feed/anolyte. These findings have provided an optimal DS concentration for BEA-OMBR operation and thus encourage its further development.