Seasonal occurrence and fate of nanoparticles in two biological wastewater treatment plants in Southern California.

Nano-sized particles in wastewater are generally considered colloids, but their production and size distribution are not well understood. Organic nano-sized particles are more abundant than engineered nanomaterials in wastewater, where they may cause membrane fouling, harbor pathogens, and transport contaminants to the environment. To our knowledge, this study is the first to examine the seasonal behavior, removal, and the quantity and size of suspended particles (both unfiltered and filtered through a 450 nm filter) at multiple points within different processes along two water resource recovery facilities (WRRFs, formerly wastewater treatment plants). In Southern California where wastewater is often reused or reclaimed, a better understanding of nano-sized particles generation and removal may help reduce cost. We found that both types of the biological secondary treatments investigated (conventional activated sludge process and trickling filter) were more efficient in removing suspended particles larger than 450 nm than they were smaller ones. However, the results show that current treatment processes are not designed to remove nano-sized particles efficiently. We also investigated the factors that correlate with their occurrence and found that 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. Although no conclusive seasonal correlations were found, dissolved COD management may control nano-sized particle production. PRACTITIONER POINTS: Conventional secondary treatments (activated sludge and trickling filter) could efficiently remove particles but not as efficiently for nano-sized particles (40.1-52.7% removal). At one facility, particles of all sizes were found to correlate with dissolved carbon and EPS, meaning they were biogenic. Monitoring dissolved carbon or EPS precursors may help control membrane fouling post-secondary treatment, and this warrants more studies.