Transport of nanoplastics in water treatment systems. = Transport of nanoplastics in water treatment systems

Microplastics and nanoplastics are contaminants of emerging concern due to their pervasive presence in environmental compartments, and in particular in water bodies. The increasing interest of the scientific and public authorities on the matter has caused in the last decade an exponential growth of scientific research on this topic. Wastewater treatment plants (WWTPs) are currently considered one of the main sources of micro- and nanoplastics due to the fact that they treat sewages, which usually transport high loads of primary plastic particles. On the other hand, water treatment plants (WTPs) are asked to guarantee sufficient standards of quality, which can be reduced by the presence of micro- or nanoplastics in the water supply source. The aim of this thesis is to assess the capability of conventional WTPs, and in particular of the filtration stage, to remove plastic particles from the Zürich Lake water. To this purpose, the lake water was spiked with palladium doped NP particles before being introduced in a pilot scale WTP. The rare metal, which is not naturally present in the lake water, was used as a marker to track the particle mobility and quantify their concentration in water. The NP removal efficiency was tested in two granular filtering media frequently present in the treatment chain of WWTPs and WTPs, i.e. a sand and an activated carbon bed filter. This study originated from the collaboration of Eawag, which conducted the experimental part of the study, and the Groundwater Engineering Research Group of Politecnico di Torino, which was responsible for the data interpretation and modelling. At first, the particle behaviour within the filtering media was characterized at the laboratory scale. Eawag conducted several column filtration tests exploring different operating conditions. The experimental data were elaborated and analysed to understand the interaction mechanism between nanoplastics and the filter material. The filtration mechanism and the related kinetic parameters were estimated using the numerical model MNMs, developed at Politecnico di Torino, to fit the column experimental breakthrough curves. These parameters were then applied to predict the particle behaviour within the pilot plant and to support the design of the larger scale experiments. Finally, the modelled and experimental data were compared to refine the pilot model and enhance its reliability and prediction capability. From this study emerged that the filtration mechanism that better describes the particle behaviour in a sand or in an activated carbon filter is “blocking”. As a consequence, the filtration efficiency is expected to approach zero for both filtering materials once all the active sites available for deposition are occupied by particles and the porous medium saturation is achieved. According to the model, given the same filtration time and filter length, a higher retention capacity was predicted for the sand bed, thanks to the lower filtration rate and smaller grains size compared to activated carbon. From an operational point of view, this means that a periodic backwashing of the filters is necessary to achieve and keep acceptable NP retention capacities in WTPs. In conclusion, knowing the behaviour of nanoplastics in granular media filters used in WTPs and WWTPs is a potential asset to firstly count, and eventually reduce, the amount of plastic particles released in water bodies and secondly to increase the drinking water quality.