Effluent pH correlates with electrochemical nitrogen recovery efficiency at pilot scale operation

Abstract

A bipolar electrodialysis (BP-ED) pilot plant including 3.15 m2 of cation exchange membrane and bipolar membrane each was operated for ammonia recovery. The pilot treated source-separated diluted urine (1 gNH4+/L). Previously found set operation parameters for lab-scale such as current density and nitrogen load did not directly influence the stack performance. However, the effluent pH was directly related to the removal efficiency of the system. 80% nitrogen removal was achieved at a set effluent pH of 4. Operating under an effluent pH control strategy was more effective to control NH4+ removal than controlling current density or nitrogen loading, as it accounts for fluctuation in wastewater availability and composition. The pilot plant removed up to 88% of the NH4+ from urine and recovered around 700 g/day (from 1 m3 of urine). This was a significant improvement compared to the pilot plant previous performance on digestate. The energy consumption was around 13 Wh/gN. The overall current efficiency was ∼40% with most losses caused by parasitic ionic shortcut currents occurring at the hydraulic manifolds of the BP-ED stack. Therefore, the energy demand can be further decreased by preventing these ionic shortcuts in the new cell designs.