Assessing the effect of SiO₂ and TiO₂ nanoparticles on granule stability and microbial community shift in aerobic granular sludge process

The widely used SiO₂ and TiO₂ nanoparticles (NPs) can accumulate in industrial wastewaters, thereby posing challenge to biological wastewater treatment processes. In this work, the performance of aerobic granular sludge (AGS) reactors fed with wastewater containing 50 mg L^-1 SiO₂ and TiO₂ NPs were investigated. The results showed that the granules could resist the NPs in wastewater (no disintegration of granules was observed). SiO₂ NPs had a negative effect on the settleability of granules, with the SVI₃₀ increased by 64.5% and protein secretion decreased by 29.9%. To the contrary, the settleability of granules was improved in the presence of TiO₂ NPs due to the increase of the protein secretion. Possibly because of the compact and layered structure of granules, in the presence of both types of NPs, no obvious reduction of the overall removal efficiency of organics was found, and nanoparticle-resistant strains were enriched. The overall nitrification and denitrification efficiencies were hardly affected by SiO₂ NPs while significantly inhibited by TiO₂ NPs. Some functional genera, such as Hyphomicrobium and Acidovorax, showed growth inhibition with TiO₂ NPs, which might be responsible for the reduction of nitrification and denitrification efficiencies.