Simultaneous Nitrogen and Phosphorus Recovery from Anaerobic Digestion Effluent via Enzymatic Bicarbonate Conversion...

Anaerobic digestion effluent (ADE) is an attractive feedstock for resource recovery due to its high concentrations of ammonia (NH3) and phosphate (PO43-). However, the presence of elevated bicarbonate (HCO3-) imparts strong buffering capacity, which inhibits PO43- precipitation and increases chemical demand for NH3 recovery. To overcome HCO3- interference, this study proposes an enzyme-based pretreatment that converts HCO3- into PO43- 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 HCO3- concentration (from 6013.8 ± 339.8 mg·L-1 to 2045.5 ± 283.5 mg·L-1) within 12 h, while simultaneously increasing PO43- concentration 10.79-fold (from 590.8 ± 148.3 mg·L-1 to 6363.3 ± 383.5 mg·L-1). The enriched PO43- 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 PO43- precipitation promoted NH4+ conversion to volatile NH3, enabling up to 82.4 ± 6.1% NH3 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.