Aggregates size of TiO2 NPs was related with the adsorption of CODs in wastewater. TiO2 NPs inhibited the oxygen uptake in the activated sludge process. The Inhibition caused by TiO2 NPs depended of substrates type and TSS in wastewater. TiO2 NPs altered microorganisms by membrane cell disruption and internalization. The effects of TiO2 NPs in biological WWTP must be evaluated by using raw wastewater.