Effect Of Nano-titanium Dioxide On Sludge Dewatering Performance And Mechanism Analysis

The widespread application of titanium dioxide nanoparticles（TiO₂ NPs）inevitably leads to its release into the environment and a large amount of accumulation in sewage sludge.The negative impact of TiO₂ NPs on activated system has already raised increasing concerns.However,the effect of TiO₂ NPs on the dewatering performance of sludge and its mechanism is poorly understood.This work therefore aims to deeply understand how the entering TiO₂NPs affect sludge dewatering through comparing the dewatering performance among sludge withdrawn from four laboratory-scale sequencing batch reactors receiving different concentrations of TiO₂ NPs.The research results will further deepen the understanding of the impact of TiO₂ NPs on the activated sludge system,and have guiding significance for the future operation of the WWTPs.It was found that although short-term exposure to environmentally relevant level of TiO₂NPs（i.e.,1mg/L）had no significant effect on sludge dewaterability,long-term exposure to1mg/L TiO₂ NPs increased specific resistance to filtration（SRF）of sludge by 13.8%.When TiO₂ NPs increased to 50 mg/L,short-term and long-term exposure increased SRF by 14.8%and 43.8%,respectively.After CPAM conditioning,the dewatering performance of sludge exposed to TiO₂ NPs for long-term was still worse than that of unexposed sludge.After exposure to TiO₂ NPs,the bound water content of the sludge increased,the particle size of the sludge decreased,the specific surface area of the sludge flocs increased,the relative hydrophobicity decreased,the absolute value of the zeta potential increased,and the microstructure of sludge was destroyed and became looser.The mechanism analysis revealed that the entering TiO₂ NPs triggered substantial amount of reactive oxygen species（ROS）,which was the key mechanism of TiO₂ NPs’cytotoxicity to sludge.The ROS produced after exposure to TiO₂ NPs led to a decrease in ATP production.Under low energy conditions,Patescibacteria,which was dominated by ultra-small bacteria,had a competitive advantage and proliferates in large quantities,resulting in a reduction in the size of sludge particles.Excessive production of EPS by sludge cells under the toxic stress of TiO₂ NPs was the main reason for the increase of EPS content in sludge.The absorption of water by EPS increased the content of bound water in sludge.The increase in soluble microbial by-products in EPS and the multiplication in the number of Proteobacteria reduced the relative hydrophobicity of sludge flocs.The decrease ofα-helix/（β-sheet+random coil）in secondary structure increased the specific surface area of the sludge flocs.Under the comprehensive effect of these factors,the dewatering performance of sludge eventually deteriorated.Finally,ultrasonic treatment was used to improve the dewatering performance of sludge after exposure to TiO₂ NPs,and achieved good results.Ultrasonic treatment with 2.5W/m L intensity can reduce the bound water content in the sludge and transfer the inner layer of EPS to the outer layer,thereby effectively improving the dewatering performance of the sludge after being exposed to the environmental concentration of TiO₂ NPs.