Investigating the fate of nanomaterials in waste water treatment plants; removal, release and subsequent impacts

Manufactured nanomaterials (MNMs) are used in a wide range of industrial applications and consumer products, including textiles, household products and cosmetics. During use, nano-enabled consumer products can release MNMs, which may enter wastewater treatment plants (WWTPs). The NanoWASTE project addressed existing knowledge gaps on the behaviour, fate and impacts of MNMs in WWTPs. We used titanium dioxide (TiO2) and silver (Ag) MNMs to study (i) the fate of TiO2 and Ag MNMs in WWTPs using laboratory tests and in real samples from Norwegian WWTPs, (ii) the environmental effects of "pristine" MNMs and those "aged" in lab-scale WWTP systems, and (iii) impacts on bacteria that are important for wastewater treatment processes. The concentrations of Ag, Ti and 20 additional elements (including P, S, Al, Cr, Fe, Co, Ni, Cu, Zn, As, Cd, Pb) were determined in two Trondheim WWTPs that apply only primary treatment. Measurements were conducted on the influent and effluent wastewater, allowing MNM removal rates, daily influx patterns, and concentrations in different size fractions to be determined. We observed average concentrations of 170 µg/L (Ti) and 0.4 µg/L (Ag) in the influent, and removal efficiencies typically >70%, comparable to the removal of solids in WWTPs. Most Ti and Ag was associated with larger particles (>0.7 µm), with only a small proportion present in the smaller colloidal fraction or as ions. Electron microscopy showed individual TiO2 MNMs and aggregates were commonly present in raw wastewater influent, treated effluent and sludge. Ag MNMs were only found in sludge, probably due to low concentrations in wastewater and treated effluent. Ti profiles correlated to wastewater flow rates and the concentration of suspended solids, indicating that households are the predominant Ti source. Ag exhibited a more irregular pattern, likely influenced by short-term discharges from industrial point sources. Modelling highlighted different influx patterns for Ti and Ag but correlated well with measured data. The concentrations and removal efficiencies of other elements varied but were related to the size fraction. Elements strongly associated with the particulate fraction (e.g. Al, V and Ba) were removed more efficiently than those associated with dissolved fractions. Nanoparticles of several elements (e.g. Cu, Zn) in association with S or Fe (O) were detected. MNMs may be transformed ("aged") during wastewater treatment processes, resulting in effects that differ from their pristine counterparts. TiO2 and Ag MNMs were both stable in artificial wastewater for a duration resembling the typical retention time in a WWTP with primary treatment. The aged MNMs seemed to be more stable when transferred into seawater than the "pristine" MNMs. Neither aged nor pristine TiO2 MNMs were toxic to the marine crustacean Tisbe battagliai. In contrast, Ag MNMs had negative effects on T. battagliai development, with the effects being more pronounced for aged than pristine MNMs. However, effects occurred only at concentrations that were above those detected in real WWTP samples. In a laboratory-scale WWTP simulating biological wastewater treatment, >80% of the total Ag and TiO2 detected in the effluent was associated with solids, suggesting high removal rates. Effluent from the lab-scale WWTP with aged particles caused 20-45% mortality of T. battagliai, while no effect was found for the freshwater water flea Daphnia magna. The effluent also had negative effects on fish cells, causing oxidative stress, compromised epithelial integrity and alteration in defence pathways. The observed effects were organism-dependent with benthic organisms directly ingesting sedimented particles bound to solids being particularly affected. Species-dependent effects were also observed with algae, where the effluent decreased the growth of the marine algae Skeletonema pseudocostatum but stimulated growth for the freshwater algae Raphidocelis subcapitata (also causing oxidative stress). The project also investigated the potential for Ag and TiO2 MNMs to impair the operational efficiency of WWTPs. Nitrate removal from wastewater is done by microorganisms through a process called denitrification. Pure cultures of bacteria isolated from activated sludge and indigenous bacterial communities sampled in actively operating WWTPs were exposed to Ag and TiO2 MNMs. While TiO2 MNMs did not affect denitrification efficiency in any of the tested bacterial strains, Ag MNMs induced abnormal N2O accumulation in Thauera linaloolentis and impaired the oxic respiration in Paracoccus denitrificans. However, the denitrification efficiency of activated sludge and denitrifying biofilms from two WWTPs in Oslo were not affected by similar concentrations of TiO2 and Ag MNMs (separately or in combined exposure). This can be explained by the functional redundancy of microbial communities in WWTPs and a potentially reduced bioavailability of MNMs in wastewater.

The NanoWASTE project will investigate the fate of manufactured nanomaterials (MNMs) in waste water treatment plants (WWTP), impacts of MNMs on WWTP processes and the subsequent environmental impacts of MNMs in waste effluent. Traceable doped MNM test materials will be used to investigate the direct impact of MNMs on physical, chemical and biological WWTP processes and the ecotoxicity of 'aged' MNMs present in waste water effluent and sewage sludge. Overall, the project will provide a greater insight into the potential hazards of MNMs to both the WWTP and the environment. Understanding how MNMs behave, change and are removed in a WWTP will help develop relevant preventive measures to limit the release of MNMs during their production, use (e.g. in consumer products) and application in end-products or as components in other products or processes. The project will challenge technology developers to consider 'end of life' as a key aspect of their approach at all life stages; production (industrial waste), use (discharge into waste systems) and waste/recycling. This project will include mapping of MNM sources leading into the WWTP and the development of models for predicting MNM fate in WWTP processes. Furthermore, the project will specifically develop new methodologies for characterising MNMs in WWTPs and how they are transformed (aged) during the process. Finally, the project will investigate the ecotoxicity of both pristine and 'aged' MNMs from WWTP water and sludge effluents in relevant environmental compartments. Importantly, the data generated in this project will be presented to stakeholders together with suggestions for protecting WWTPs from MNM impacts and proposals for reducing MNM release from WWTPs. We believe that the knowledge generated about MNMs in WWTPs can serve as a basis for development of future regulations regarding MNMs in consumer products and their disposal at 'end of life'.