Influence of wastewater type in the effects caused by titanium dioxide nanoparticles in the removal of macronutrients by activated sludge.

The imminent arrival of nanoparticles (NPs) to the wastewater treatment plants (WWTP) brings concern about their effects, which can be related to the wastewater composition. In this work, the effects of titanium dioxide (TiO2) NPs in the removal of carbon, nitrogen, and phosphorus by activated sludge bioreactors during the treatment of synthetic, raw, and filtered wastewaters were evaluated. Floc size, compaction of sludge, and morphological interactions between sludge and NPs were also determined. The main effect of TiO2 NPs was the inhibition of up to 22% in the removal of ammonia nitrogen for all types of wastewaters. This effect is strong dependent on combined factors of TiO2 NPs concentration and content of organic matter and ammonia in wastewater. The removal of dissolved organic carbon was affected by TiO2 NPs in lower level (up to 6%) than nitrogen removal for all types of wastewaters. Conversely to adverse effects, the removals of orthophosphate in the presence of TiO2 NPs were improved by 34%, 16%, and 55% for synthetic, raw, and filtered wastewater, respectively. Compaction of the sludge was also enhanced as the concentrations of NPs increased without alterations in the floc size for all types of wastewaters. Based on TEM and STEM imaging, the main interaction between TiO2 NPs and the activated sludge flocs was the adsorption of NPs on cell membrane. This means that NPs can be attached to cell membrane during aerobic wastewater treatment, and potentially disrupt this membrane. The effects of TiO2 NPs on macronutrient removal clearly depended on wastewater characteristics; hence, the use of realistic media is highly encouraged for ecotoxicological experiments involving NPs.