Application of dynamic current density for increased concentration factors and reduced energy consumption for concentrating ammonium by electrodialysis

Ammonium (NH₄⁺) can be recovered from water for fertiliser production or even energy production purposes. Because NH₄⁺ recovery is more effective at increased concentrations, electrodialysis (ED) can be used to concentrate NH₄⁺ from side streams, such as sludge reject water, and simultaneously achieve high NH₄⁺ removal efficiencies. However, the effect of osmosis and back-diffusion increases when the NH₄⁺ concentration gradient between the diluate and the concentrate stream increases, resulting in a limitation of the concentration factor and an increase in energy consumption for NH₄⁺ removal. In this study, we showed that operation at dynamic current density (DCD) reduced the effect of osmosis and back-diffusion, due to a 75% decrease of the operational run time, compared to operation at a fixed current density (FCD). The concentration factor increased from 4.5 for an FCD to 6.7 for DCD, while the energy consumption of 90% NH₄⁺ removal from synthetic sludge reject water at DCD remained stable at 5.4 MJ·kg-N⁻¹.