Naturally occurring nanoparticles in two biological wastewater treatment plants in Southern California

Abstract

Naturally occurring nanoparticles (NONPs) in wastewater are generally considered colloids, but their production and size distribution are not well understood. NONPs are more abundant than engineered nanoparticles (ENPs) in wastewater, where they may cause membrane fouling, harbour pathogens, and transport contaminants to the environment. In this study we aimed to understand the seasonal behaviour as well as the quantity and size of suspended particles (both unfiltered and filtered through a 450 nm filter) along two water resource recovery facilities (WRRFs, formerly wastewater treatment plants) located in Southern California. We also investigated the factors that correlate with their occurrence. We found that both of the biological secondary treatments investigated, conventional activated sludge process (ASP) and trickling filter (TF), were more efficient in removing suspended particles larger than 450 nm than they were smaller ones. The results show that current treatment processes are not designed to remove nano-sized particles efficiently. Additionally, there was a significant and direct correlation between influent dissolved chemical oxygen demand (COD) and the abundance of suspended particles both larger and smaller than 450 nm, suggesting that the suspended particles increased with dissolved COD in the WRRFs and thus were biogenically generated during the wastewater treatment.