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

Ammonium (NH) can be recovered from water for fertiliser production or even energy production purposes. Because NH recovery is more effective at increased concentrations, electrodialysis (ED) can be used to concentrate NH from side streams, such as sludge reject water, and simultaneously achieve high NH removal efficiencies. However, the effect of osmosis and back-diffusion increases when the NH 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 NH 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% NH removal from synthetic sludge reject water at DCD remained stable at 5.4 MJ·kg-N.