Response mechanisms of microalgal-bacterial granular sludge to different loading strategies of copper oxide nanoparticles

This study investigated the effects of copper oxide nanoparticles (CuO NPs) at concentrations of 1–50 mg/L on the microalgal-bacterial granular sludge (MBGS) wastewater treatment process, comparing two nanoparticle addition methods: shock-load (SL) and stepwise (SW) increases. The results demonstrated that the chemical oxygen demand (COD) and ammonia‑nitrogen (NH₃-N) removal were more sensitive in the reactor with shock-load (R_SL), phosphate concentrations were higher in the effluent from the reactor with stepwise increases (R_SW). Accumulation of nitrate‑nitrogen was observed only in the SW method at CuO NP concentrations ≥20 mg/L, likely due to the nanoparticles becoming entrapped within the granule interiors, as evidenced by scanning electron microscopy coupled with energy-dispersive X-ray (SEM-EDX) mapping. Although total EPS levels were highest in both reactors at 20 mg/L CuO NP dosage, the levels were significantly higher in the RSW sludge (191.1 mg/g MLVSS in R_SL vs. 218.3 mg/g MLVSS in R_SW), primarily due to the long-term exposure leading to increased loosely-bound EPS (LB-EPS) content (75.7 mg/g MLVSS in R_SL vs. 102.9 mg/g MLVSS in R_SW). Furthermore, CuO NPs interacted with EPS, binding to NH, CN, CO, and C-O-C groups in protein (PN) and polysaccharide (PS) structures.