Abstract
Many small- or medium-sized communities in Northern Europe employ only primary wastewater treatment plants (WWTPs) and effluent discharges can be a relevant source of pollution. The current study combines monitoring and modelling approaches to investigate concentrations, influent patterns, size distribution and removal of 20 elements for the two primary WWTPs (Ladehammeren, LARA; Høvringen, HØRA) serving Trondheim, the third largest city in Norway. Element concentrations were determined in raw influent wastewater, effluents and biosolids, and diurnal inflow patterns were assessed. The elemental distribution in particulate, colloidal and dissolved fractions of untreated wastewater was characterized using filtration separation and electron microscopy. An influent generator model and multivariate statistical analyses were used to determine release patterns and to predict the (co-)occurrence of selected elements. Raw influent wastewater concentrations for most elements were similar in the two WWTPs, with only Ca, Mn, Fe, Co and Ba being significantly higher (p < 0.05) in HØRA (which receives more household and hospital discharges). Removal efficiencies varied between elements, but in most cases reflected their association with particulates. Nanosized particles of several elements were detected, with Cu/Zn being most common. Measured concentrations of most elements followed typical diurnal wastewater discharge patterns and enrichment factors calculated for biosolids confirmed the importance of anthropogenic sources for P, Cu, Zn, Cd, As, Cr, Ni, Pb, V, Co and Fe. Elemental concentrations generally correlated well with total suspended solid (TSS) concentrations at HØRA, while this was less pronounced in LARA (possibly due to higher industrial contributions). In one of its first applications for WWTP influent pattern examination, principal component analysis was found to be instrumental for source identification of target elements, showing significant differences between LARA and HØRA influents. The combined experimental, statistical and modelling approaches used herein allowed for improved understanding of element sources, patterns of discharge and fate in primary WWTPs.
Highlights
Municipal wastewater treatment plants (WWTPs) receive highly heterogeneous household and industrial discharges in terms of inorganic element composition
Influent concentrations of most elements were relatively similar in both WWTP (e.g., Na, Mg, K) or slightly higher in HØRA, compared to LARA (e.g. Al, Cu, Ni, Zn, Gd)
Priority elements Al, V, Co, Ni, Cu, Zn, Cd, Ba and Pb were detected in every raw influent wastewater sample, while As and Cr were below the detection limit in 33% and 15% of the samples in SLUDGE
Summary
Municipal wastewater treatment plants (WWTPs) receive highly heterogeneous household and industrial discharges in terms of inorganic element composition These include potentially toxic elements and metals that, if not adequately removed, can be released to recipient surface waters, sediments and soils, posing a threat to organisms (Deycard et al, 2014; Park et al, 2020). In several small- and medium-sized communities in Northern regions (e.g., Norway, Iceland, Canada) and South-East Europe, WWTPs with only preliminary and primary treatment are in use (Government of Canada, 2017; European Environmental Agency, 2017; Berge and Sæther, 2018) Such limited treatment can result in a reduced removal of conventional pollutants (suspended solids, organics, nutrients) and emerging pollutants compared to WWTPs with biological and advanced treatment steps (Vogelsang et al, 2006; Polesel et al, 2018). These studies revealed (i) the limited contribution of primary treatment to the overall removal of elements; (ii) considerable variability in removal efficiencies for the same element
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