Mechanistic insights into Cu(II) and nutrient removal in algal-bacterial granular sludge during treatment of raw aquaculture wastewater

Copper (Cu) from aquaculture is poorly assimilated and contaminates wastewater. Aquaculture effluents face dual challenges of low C/N ratios (2-6) and Cu contamination. This study investigates algal-bacterial granular sludge (ABGS) for simultaneous removal of organics, nitrogen, phosphorus, and Cu(II) under low C/N (~5). Two reactors (0.5 mg/L Cu(II) vs. control) operated for 150 days achieved >90% COD, 61.0-77.8% TN, and 71.0-82.3% Cu(II) removal. Cu(II) exposure improved sludge settling by suppressing excessive growth. Sequential extraction showed >85% Cu(II) in exchangeable and reducible fractions via ion exchange and precipitation. Cu(II) enriched tolerant denitrifiers (Thauera, Dechloromonas, Arenimonas, Zoogloea) and shifted algal communities (Makinoella, Sellaphora). ABGS provides a low-energy solution for aquaculture wastewater treatment under carbon limitation.