Simultaneous removal of nitrate nitrogen and orthophosphate by electroreduction and electrochemical precipitation

Electrochemical methods can effectively remove nitrate nitrogen (NO₃-N) and orthophosphate phosphorus (PO₄-P) from wastewater. This work proposed a process for the simultaneous removal of NO₃-N and PO₄-P by combining electroreduction with electrochemically-induced calcium phosphate precipitation, and its performance and mechanisms were studied. For the treatment of 100 mg L^-1 NO₃-N and 5 mg L^-1 PO₄-P, NO₃-N removal of 60-90% (per cathode area: 0.25-0.38 mg h^-1 cm^-2) and 80-90% (per cathode area: 0.33-0.38 mg h^-1 cm^-2) could be acquired within 3 h in single-chamber cell (SCC) and dual-chamber cell (DCC), while P removal was 80-98% (per cathode area: 0.10-0.12 mg h^-1 cm^-2) in SCC after 30 min and 98% (per cathode area: 0.37 mg h^-1 cm^-2) in DCC within 10 min. The faster P removal in DCC was due to the higher pH and more abundant Ca²⁺ in the cathode chamber of DCC, which was caused by the cation exchange membrane (CEM). Interestingly, NO₃-N reduction enhanced P removal because more OH^- can be produced by nitrate reduction than hydrogen evolution for an equal-charge reaction. For 10 mg L^-1 PO₄-P in SCC, when the initial NO₃-N was 0, 20, 100, and 500 mg L^-1, the P removal efficiencies after 1 h treatment were < 10%, 45-55%, 86-99%, and above 98% respectively. An increase in Ca²⁺ concentration also promoted P removal. However, Ca and P inhibited nitrate reduction in SCC at the relatively low initial Ca/P, as CaP on the cathode limited the charge or mass transfer process. The removal efficiency of NO₃-N in SCC after 3 h reaction can reduce by about 17%, 40%, and 34% for Co₃O₄/Ti, Co/Ti, and TiO₂/Ti. The degree of inhibition of P on NO₃-N removal was related to the content and composition of CaP deposited on the cathode. On the cathode, the lower the deposited Ca and P, and the higher the deposited Ca/P molar ratio, the weaker the inhibition of P on NO₃-N removal. Especially, P had little or even no inhibition on nitrate reduction when treated in DCC instead of SCC or under high initial Ca/P. It is speculated that under these conditions, a high local pH and local high concentration Ca²⁺ layer near the cathode led to a decrease in CaP deposition and an increase in Ca/P molar ratio on the cathode. High initial concentrations of NO₃-N might also be beneficial in reducing the inhibition of P on nitrate reduction, as few CaP with high Ca/P molar ratios were deposited on the cathode. The evaluation of the real wastewater treatment was also conducted.