Can nanoplastic particles pass through drinking water treatment plants?

, Pulido-Reyes Gerardo, Mitrano Denise M., Kaegi Ralf, Von Gunten Urs.

Microplastics have been detected in both potable water sources and bottled water, leading to questions about the efficacy of current water treatment practices. It is hypothesized that all water resources will experience an increasing influx of particulate plastic down to the nanometer dimensions as degradation products of larger plastic items. Ozonation, sand and activated carbon filtration are key treatment processes in drinking water treatment plants, yet studies addressing the impacts on and the retention efficiency of nanoplastics by these treatment steps are still lacking. Therefore, we evaluated the impact of ozonation on the physicochemical properties of metal doped nanoplastic particles (polystyrene/polyacrylonitrile, 215±1 nm diameter) and the effectiveness of sand/activated carbon filtration for nanoplastic removal. For this purpose, nanoplastics were treated with ozone in Lake Zurich water (source for drinking water production in Zurich). The nanoplastics remained colloidally stable with unchanged diameter. However, the particle surface charge decreased with exposure to increasing ozone doses. Pristine and aged sand and activated carbon, used in the treatment plant and thus coated with a biofilm, were evaluated as filtration media. Results from colloid column transport studies showed a higher degree of particle retention in used sand in comparison with pristine sand, with 92% and 42% of nanoplastic retention, respectively. However, an opposite trend was found for columns packed with activated carbon where the highest level of particle retention was obtained with pristine materials in contrast with columns packed with used activated carbon. Using the data from used substrates, a filter length of 0.9 and 2.7m would be required for reaching a 99% of nanoplastic removal for sand and activated carbon filters, respectively. In order to ensure that these estimated values can be transferred to real drinking water treatments plants, pilot plant experiments are currently being conducted in collaboration with the Water-Works of Zurich.

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