Abstract
Due to their widespread application in consumer products, titanium (Ti) and silver (Ag) nanomaterials are increasingly released to urban wastewater treatment plants (WWTPs) from households and industrial facilities. In the current study, we assessed the occurrence of Ti and Ag in WWTP influents and their elimination in full-scale WWTPs employing preliminary and primary treatment steps. Influent Ti and Ag were characterised as (nano)particulate, colloidal and dissolved fractions using a size-based fractionation method employing sequential filtration. A seven-day sampling campaign was conducted in two full-scale WWTPs in Trondheim, Norway. Eight-hour composite influent samples were collected to assess diurnal variations in Ti and Ag occurrence, covering morning, afternoon/evening and night discharges. Daily influent and effluent samples were used to determine removal during wastewater treatment.
Measured influent Ti concentrations (up to 211 µg L-1) were significantly higher than Ag (<0.15–2.2 µg L-1). Most of the influent Ti and Ag occurred in particulate form or was associated with suspended solids (>0.7 µm), while 5% Ag and <1% Ti was found to occur in colloidal or ionic form. Different diurnal concentration profiles were observed for the two elements. Ti profiles were comparable to flow and pollutant patterns in municipal wastewater (morning and/or evening peaks, night minima), indicating predominant emissions from households. Irregular profiles were exhibited by Ag, possibly determined by short-term discharges from one or more point sources (e.g. industry). Removal efficiencies of ≥ 70% were observed for both elements with the exception of one WWTP, where Ti removal was shown to be on average < 40%. These differences may be attributed to the different removal of suspended solids (where most Ti and Ag were found to occur) in the two WWTPs. The present findings will be complemented with identification and characterisation of Ti and Ag NMs using transmission electron microscopy and X-ray photoelectron spectroscopy, and prediction of occurrence dynamics by means of influent generation algorithms. Overall, this study permitted (i) identification and tentative quantification of (nano)metallic Ti and Ag discharge from urban/industrial catchments, and (ii) determination of emissions from WWTPs to environmental compartments.
Measured influent Ti concentrations (up to 211 µg L-1) were significantly higher than Ag (<0.15–2.2 µg L-1). Most of the influent Ti and Ag occurred in particulate form or was associated with suspended solids (>0.7 µm), while 5% Ag and <1% Ti was found to occur in colloidal or ionic form. Different diurnal concentration profiles were observed for the two elements. Ti profiles were comparable to flow and pollutant patterns in municipal wastewater (morning and/or evening peaks, night minima), indicating predominant emissions from households. Irregular profiles were exhibited by Ag, possibly determined by short-term discharges from one or more point sources (e.g. industry). Removal efficiencies of ≥ 70% were observed for both elements with the exception of one WWTP, where Ti removal was shown to be on average < 40%. These differences may be attributed to the different removal of suspended solids (where most Ti and Ag were found to occur) in the two WWTPs. The present findings will be complemented with identification and characterisation of Ti and Ag NMs using transmission electron microscopy and X-ray photoelectron spectroscopy, and prediction of occurrence dynamics by means of influent generation algorithms. Overall, this study permitted (i) identification and tentative quantification of (nano)metallic Ti and Ag discharge from urban/industrial catchments, and (ii) determination of emissions from WWTPs to environmental compartments.