(589b) Removal and Recovery of Ammonium and Phosphate from Wastewater Using a Redox Flow Deionization Cell

Wastewater from domestic and industrial sources often contains significant amounts of ammonium (N) and phosphate (P). A high concentration of N and P can cause problems to the water body, leading to a series of issues like eutrophication and algal growth. On the other hand, N and P are valuable resources in many applications, with 90% of them being used as fertilizers for food production. With the world population increasing, these two nutrients are in huge demand to ensure food security. The synthesis of ammonia is energy intensive and causes one of the largest carbon emissions on a global scale. Phosphorous is a limited resource, with the reserves projected to deplete in 50–100 years at current usage rates. To make this worse, the phosphorous demand is expected to rise by 50–100% by 2050. Thus, removal and recovery of these nutrients from the wastewater would be both environmentally and economically beneficial.

Herein, we report the electrochemistry-based redox flow deionization cell (RFDC) to be utilized as a new technology for removal of recovery of N and P from wastewater. The study shows significant cell voltage and concentration effects on the cell performance. In both N and P removal experiments, an increase in the cell voltage leads to faster average salt removal rate (ASRR) and removal efficiency (RE) at cost of a higher electrical energy consumption (EEC), which can be attributed to excess energy consumption resultant of higher overpotentials to promote the redox kinetics. A cell voltage at about 1 V was found an appropriate value for balancing ASRR and EEC considerations. 4.64 mmol_NH4Cl/m²‧min, 99.1% and 105.3 J/mmol_NH4Cl were achieved in the N removal experiment with an initial 10 mM NH₄Cl, and 2.81 mmol_Na3PO4/m²‧min, 80% and 273.7 J/mmol_Na3PO4 were achieved in the P removal experiment with an initial 7 mM Na₃PO₄ at 1 V cell voltage, demonstrating a fast, energy efficiency process for N and P removal. An increase in the initial N and P concentrations results in faster ASRR and lower EEC, which can be explained with a lower internal resistance that benefits with a lower Ohmic potential drop to drive the redox kinetics and a lower Ohmic energy loss and thus higher energy utilization efficiency. The effects of coexisting ions (Na⁺ and Cl^-) on N and P removal and recovery were also investigated. The results show a relatively high N removal selectivity against Na⁺ and a relatively low P removal selectivity against Cl^-, which can be attributed to a difference in the ion mass, size, and concentration parameters. The coexisting ions cause an increase in the EEC due to their simultaneous removal, which would be addressed by research of ion selective exchange membrane to suppress migration of coexisting ions. Nevertheless, regardless the presence of coexisting ions N and P can be removed all the way to the ppm level in the ions removal stream, well below the regulation limits, and in the meantime get recovered in the ions concentrated stream. Besides, the stability test confirms an excellent durability of the RFDC. All the findings suggest RFDC is effective and efficient for N and P removal and recovery from wastewater. With technological advantages of low cost, high efficiency, and continuous operation, this new technology has a great potential to be applied in domestic and industrial wastewater treatments to fulfill environmental and economic goals.