Simultaneous Nitrogen and Phosphorus Recovery from Anaerobic Digestion Effluent via Enzymatic Bicarbonate Conversion Coupled with a Membrane Contactor

Anaerobic digestion effluent (ADE) is an attractive feedstock for resource recovery due to its high concentrations of ammonia (NH 3 ) and phosphate (PO 4 3– ). However, the presence of elevated bicarbonate (HCO 3 – ) imparts strong buffering capacity, which inhibits PO 4 3– precipitation and increases chemical demand for NH 3 recovery. To overcome HCO 3 – interference, this study proposes an enzyme-based pretreatment that converts HCO 3 – into PO 4 3– using phosphoenolpyruvate carboxylase (PEPC). PEPC was immobilized onto carboxylated polystyrene beads via EDC/NHS coupling (PB@PEPC) to enhance stability and reusability. When applied to real ADE, 25 PB@PEPC beads achieved a 2.94-fold reduction in HCO 3 – concentration (from 6013.8 ± 339.8 mg·L –1 to 2045.5 ± 283.5 mg·L –1 ) within 12 h, while simultaneously increasing PO 4 3– concentration 10.79-fold (from 590.8 ± 148.3 mg·L –1 to 6363.3 ± 383.5 mg·L –1 ). The enriched PO 4 3– was recovered as calcium phosphate with efficiencies exceeding 95% using Ca(OH) 2 at a Ca/P molar ratio of 2.5. The elevated pH during PO 4 3– precipitation promoted NH 4 + conversion to volatile NH 3, enabling up to 82.4 ± 6.1% NH 3 recovery using a membrane contactor without additional alkaline reagents. PB@PEPC retained 86.1 ± 4.2% of its initial activity after five reuse cycles. Overall, this integrated process offers a sustainable pathway for simultaneous nutrient recovery from complex wastewater.