A novel electrocoagulation-membrane stripping hybrid system for simultaneous ammonia recovery and contaminant removal

Abstract

This study developed an innovative and cost-effective electrochemical membrane filtration system, which utilized electrocoagulation (EC) with electrically conductive membranes (ECMs) as cathode and membrane stripping (MS) for simultaneous ammonia recovery and contaminant removal. The novel system takes advantages of the two complementary processes (EC and MS) when sequentially performed. Electrocoagulation produced positively charged coagulants which removed contaminants. ECMs served as cathode of EC in-situ generated OH–, which converted NH4+ ions to volatile NH3. MS recovered NH3 and prevented from membrane fouling or wetting due to the protection of ECMs. The mechanism involves electrocoagulation, electrolysis and electrostatic interaction. Operating conditions (e.g., current density, electrolyte concentration and pollutants concentration) have significant impacts on process performance. Increasing the current density from 0 to 40 A/m2 exerted a positive effect on humic acid (HA) removal, ECM fouling mitigation and ammonia recovery. However, excessive current density (60 A/m2) shortened the contact time between NH4+ and OH–, and thus decreased ammonia recovery. High electrolyte concentration enhanced ammonia recovery, which can be attributed to increased pH and longer contact time between NH4+ and OH–. On the contrary, low electrolyte concentration was beneficial for ECM fouling mitigation through enhancing electrostatic repulsion between membrane electrode and HA contaminants. The improvement of HA concentration not only aggravated ECM fouling, but also had adverse effect on ammonia recovery. NH4Cl concentration had negligible effect on HA removal and membrane fouling, but the generation of OH– became a rate-limiting factor for ammonia recovery at high NH4Cl concentration. Our results demonstrated that the novel system exhibited a competitive performance with other alternatives, with HA removal efficiency of 99.2% and ammonia recovery efficiency of 68.6% at a current density of 40 A/m2, electrolyte concentration of 100 mmol/L, HA concentration of 30 mg/L and NH4Cl concentration of 300 mg/L, which can be considered as an appealing technology for ammonia resource recovery.