A seasonal observation on the distribution of engineered nanoparticles in municipal wastewater treatment systems exemplified by TiO2 and ZnO

The present research attempted to assess the seasonal variation of engineered nanoparticles (ENPs) in a major municipal wastewater treatment system. A monthly survey over a 12-month period was conducted to monitor the concentration of TiO2 and ZnO nanoparticles throughout the treatment process. Results showed inflow concentrations in the range of 21.6 ± 5.0–391.0 ± 43.0 μg/L and 20.0 ± 12.0–212.0 ± 53.0 μg/L for TiO2 and ZnO, respectively. Seasonal pattern of the inflow ENPs concentration showed elevated value in the summer and winter periods for both TiO2 and ZnO. Based on the concentration profile, the hydraulic flow rate, and the concentration of mixed liquid suspended solid (MLSS), the daily mass loading (DML) or mass flow rate of nanoparticles and the mass ratio of engineered nanoparticle to MLSS were calculated. DML provided a real-time estimate of temporal distribution of ENPs in the treatment processes. Results indicated a daily mass loading of 50.1 ± 12.7 and 44.7 ± 14.1 kg/day (yearly average) for TiO2 and ZnO, respectively. The amount of ENPs captured by sludge particulates were, yearly average, of 7.1 kg-ZnO/d and 39.8 kg-TiO2/d, and 8.9 kg-ZnO/d and 25.1 kg-TiO2/d, by the primary and the secondary sludge particulates, respectively. ENPs to MLSS mass ratio also showed a seasonal patter similar to the inflow ENPs concentration, where summer and winter periods showed elevated values. Additionally, loss of ENPs throughout the treatment plant that was not accounted for, also can be estimated from the daily mass loading rate and the mass ratio of ENPs to MLSS. Based on the seasonal distribution of ENPs in wastewater treatment systems, especially the daily mass loading rate, it is possible to estimate the uses of nanoparticle-related commercial and personal care products in the urban areas and enable decision-making on the strategy of sludge disposal management.