Response to shock load of titanium dioxide nanoparticles on aerobic granular sludge and algal-bacterial granular sludge processes

Titanium dioxide nanoparticles (TiO₂ NPs) are extensively used in various fields and can consequently be detected in wastewater, making it necessary to study their potential impacts on biological wastewater treatment processes. In this study, the shock-load impacts of TiO₂ NPs were investigated at concentrations ranging between 1 and 200 mg L⁻¹ on nutrient removal, extracellular polymeric substances (EPSs), microbial activity in aerobic granular sludge (AGS), and algal-bacterial granular sludge (AB-AGS) bioreactors. The results indicated that low concentration (≤10 mg L⁻¹) TiO₂ NPs had no effect on microbial activity or the removal of chemical oxygen demand (COD), nitrogen, and phosphorus, due to the increased production of extracellular polymeric substances (EPSs) in the sludge. In contrast, the performance of both AGS and AB-AGS bioreactors gradually deteriorated as the concentration of TiO₂ NPs in the influent increased to 50, 100, and 200 mg L⁻¹. Specifically, the ammonia‑nitrogen removal rate in AGS decreased from 99.9 % to 88.6 %, while in AB-AGS it dropped to 91.3 % at 200 mg L⁻¹ TiO₂ NPs. Furthermore, the nitrate‑nitrogen levels remained stable in AB-AGS, while NO₃-N was detected in the effluent of AGS at 100 and 200 mg L⁻¹. Microbial activities change similarly as smaller decrease in the specific ammonia uptake rate (SAUR) and specific nitrate uptake rate (SNUR) was found in AB-AGS compared to those in AGS. Overall, the algal-bacterial sludge exhibited higher resilience against TiO₂ NPs, which was attributed to a) higher EPS volume, b) smaller decrease in LB-EPS, and c) the favorable protein to polysaccharide (PN/PS) ratio. This in turn, along with the symbiotic relationship between the algae and bacteria, mitigates the toxic effects of nanoparticles.