Sustainable waste water deammonification by vacuum membrane distillation without pH adjustment: Role of water chemistry

© 2017 Elsevier B.V. Traditional aerobic processes used for deammonification in waste water treatment plants consume the majority of electrical energy. Ammonia can instead be recovered by membrane distillation but research to date proposes chemically intensive pH adjustment to release free ammonia from bulk solutions. Here we propose the novel application of membrane distillation to deammonify waste water without any pH adjustment. To demonstrate the concept, it was first shown that simple synthetic ammonia solutions strictly follow the classic ammonia-ammonium relationship with pH which shows little release as free ammonia until at least pH 9. However vacuum membrane distillation removed essentially all ammonium in solution as low as pH 6.1 due to carbonate that is often found in waste waters. Via a novel ‘pH polarisation and buffering’ mechanism, the functional effect was established due to CO2 released from the liquid-vapour membrane boundary layer causing a pH rise that released ammonia to the permeate. Via this mechanism, high depletions, up to 99.6%, occurred consistently on solutions containing carbonate and volatile acids at low pH, including two ‘as received’ industrial waste waters such as anaerobic digester effluent. Future work can consider effects including membrane fouling and cleaning for longer term waste water deammonification by ammonia capture.