Bipolar Membrane Electrodialysis for Ammonia Recovery from Synthetic Urine: Experiments, Modeling, and Performance Analysis

Recovering nitrogen from source-separated urine is an important part of the sustainable nitrogen management. A novel bipolar membrane electrodialysis with membrane contactor (BMED–MC) process is demonstrated here for efficient recovery of ammonia from synthetic source-separated urine (∼3772 mg N L^–1). In a BMED–MC process, electrically driven water dissociation in a bipolar membrane simultaneously increases the pH of the urine stream and produces an acid stream for ammonia stripping. With the increased pH of urine, ammonia transports across the gas-permeable membrane in the membrane contactor and is recovered by the acid stream as ammonium sulfate that can be directly used as fertilizer. Our results obtained using batch experiments demonstrate that the BMED–MC process can achieve 90% recovery. The average ammonia flux and the specific energy consumption can be regulated by varying the current density. At a current density of 20 mA cm^–2, the energy required to achieve a 67.5% ammonia recovery in a 7 h batch mode is 92.8 MJ kg^–1 N for a bench-scale system with one membrane stack and can approach 25.8 MJ kg^–1 N for large-scale systems with multiple membrane stacks, with an average ammonia flux of 2.2 mol m^–2 h^–1. Modeling results show that a continuous BMED–MC process can achieve a 90% ammonia recovery with a lower energy consumption (i.e., 12.5 MJ kg^–1 N). BMED–MC shows significant potential for ammonia recovery from source-separated urine as it is relatively energy-efficient and requires no external acid solution.