Mechanistic Insights into Synergistic VFA and NH3 Recovery Using a Hybrid BPMED-HFMC System

Volatile fatty acids (VFAs) and ammonia are valuable intermediate compounds generated during anaerobic digestion and are attractive targets for circular carbon and nitrogen management. Selective separation from ion-rich waste streams remains challenging because of the complexity of the waste-based mixture. Here, we develop a hybrid bipolar membrane electrodialysis-hollow fiber membrane contactor (BPMED-HFMC) system that enables simultaneous recovery and electrochemically controlled fractionation of VFAs while corecovering ammonia without the need for costly external acid or base inputs. In BPMED, electromigration separates charged VFA carboxylates and ammonium. The bipolar membrane (BPM) then generates acid and base in situ, which converts the concentrated VFA carboxylates and ammonium into volatile neutral forms. In parallel, HFMC units then recover VFAs and ammonia as distinct product streams via membrane-mediated gas-liquid transport. We demonstrate that the anion exchange membrane selectivity is in the order acetate > propionate > butyrate > valerate, which correlated strongly with the hydrodynamic radius of the VFAs (R2 = 0.96-0.99). During treatment of synthetic anaerobic digestate in the BPMED-HFMC system, acetate selectivity over chloride increased to ∼10 and resulting in a VFA product stream >600 mM (∼71% acetate) at 60 A m-2. The specific energy consumption was 0.19-0.21 kWh mol-1 (VFA + NH3) across current densities of 20-60 A m-2, ∼50% lower than an HFMC-only control, with electricity and chemical costs of ∼1.6 cents mol-1.