Optimizing electrochemical recovery of ammonia and citric acid from residual streams via response surface methodology (RSM)

While citric acid presents a feasible alternative to strong acids such as sulphuric acid in stripping-scrubbing technology for recovering ammonia from effluents rich in total ammoniacal nitrogen (TAN), its relatively high cost increases operational expenses. This study proposes integrating bipolar membrane electrodialysis (BPMED) with a citric acid scrubber, to regenerate citric acid while extracting dissolved ammonia from ammonium effluents. The influence of BPMED's membrane stack configuration and scrubber effluent characteristics (pH, concentration, and temperature) on recovery efficiency (RE), current efficiency (CE), and energy consumption (E N ) was investigated. The membrane configuration combining the feed and acid compartments demonstrated the lowest E N , and up-to 60 % RE. Subsequently, this configuration was optimised using response surface methodology (RSM), where regression models for RE, CE, and E N were developed. pH (3–7) and concentrations (2–10 g-N/L) were found to have the most considerable influence on BPMED performance, while temperature (20–40 °C) had minimal impact. By maximizing RE and CE, while minimising E N , the optimum conditions were identified as pH 6.05, concentration 6.67 g-N/L, and temperature 30 °C. Predicted RE (53.6 %), CE (48.2 %), and E N (7.0 kWh/kg-N recovered) were in good agreement with experimental data, validating the models' accuracy within the investigated parameter's ranges. The E N achieved under optimal conditions was among the lowest reported to date, demonstrating RSM's effectiveness in optimizing BPMED for ammonium and citric acid recovery. This study highlights the potential of integrating a citric acid scrubber, and BPMED as a promising approach for extracting ammonium while minimising citric acid costs. • BPMED optimised to recover dissolved ammonia and citric acid from scrubber effluent • BP-C configuration reduced energy by 37 % and NH₃ diffusion losses by 27 % vs BP-C-A • Increasing scrubber effluent concentration and pH improved energy and current efficiency • At optimum operating conditions, BP-C achieved 5.1 g-N/L at 5.4 kWh/kg-N removed