The impact of titanium dioxide nanoparticles on biological nitrogen removal from wastewater and bacterial community shifts in activated sludge

The potential impact of titanium dioxide nanoparticles (TiO₂ NPs) on nitrogen removal from wastewater in activated sludge was investigated using a sequencing batch reactor. The addition of 2-50 mg L^-1 of TiO₂ NPs did not adversely affect nitrogen removal. However, when the activated sludge was exposed to 100-200 mg L^-1 of TiO₂ NPs, the effluent total nitrogen removal efficiencies were 36.5 % and 20.3 %, respectively, which are markedly lower than the values observed in the control test (80 %). Further studies showed that the decrease in biological nitrogen removal induced by higher concentrations of TiO₂ NPs was due to an inhibitory effect on the de-nitrification process. Denaturing gradient gel electrophoresis profiles showed that 200 mg L^-1 of TiO₂ NPs significantly reduced microbial diversity in the activated sludge. The effect of light on the antibacterial activity of TiO₂ NPs was also investigated, and the results showed that the levels of TiO₂-dependent inhibition of biological nitrogen removal were similar under both dark and light conditions. Additional studies revealed that different TiO₂ concentrations had a significant effect on dehydrogenase activity, and this effect was most likely the result of decreased microbial activity.