Abstract
Mercury (Hg) pollution threatens ecosystems and human health. Wastewater treatment plants (WWTPs) play a key role in limiting Hg discharges from wastewaters to rivers and lakes, but large-scale studies to estimate Hg loads and discharge at national levels are scarce. We assessed the concentration, flux, speciation, and removal of Hg in municipal wastewater throughout Switzerland by investigating 64 WWTPs in a pre-study and a subset of 28 WWTPs in the main study. We also studied the behavior and pathways of Hg along the various treatment steps in a state-of-the-art WWTP. The resulting dataset, representative of industrialized countries, provides an overview of (i) current Hg concentration ranges, (ii) average per capita loads, and (iii) wastewater Hg inputs into surface waters. The results allowed estimation of a total Hg (THg) load in Swiss wastewater of 130 ± 30 kg THg/year (15.7 mg/capita/y), of which 96 ± 4% is retained in sewage sludge. About 4.7 ± 0.5 kg THg/year (0.57 mg/capita/y) is discharged with the treated wastewater into surface waters. This corresponds to only 1.5–3% of the THg load carried by the major Swiss rivers, indicating that >95% of riverine Hg originates from other sources. Extrapolation to the population of Europe would yield a total amount of 11,700 kg THg/year in raw wastewater, with some 480 kg THg/year discharged to surface waters. Monomethyl mercury on average accounted for 0.23% of THg, and its fraction remained constant along the different treatment steps.
Highlights
Mercury is classified as a priority hazardous substance by many countries, including Switzerland, the European Union, and the United States (EPA, 1972, 2002; Ritscher et al, 2018)
In addition to the samples collected from wastewater treatment plants (WWTPs) in 2017, the freeze-dried sewage sludge samples collected in 2016 from 64 Swiss WWTPs were analyzed for total Hg (THg) contents and speciation for comparison (Vriens et al, 2017)
Raw wastewater The THg concentrations in the inflows of the 28 WWTPs are shown in Fig. 2a
Summary
Mercury is classified as a priority hazardous substance by many countries, including Switzerland, the European Union, and the United States (EPA, 1972, 2002; Ritscher et al, 2018). Among the key convention objectives are the identification, quantification, control, and reduction of Hg emissions to land and water and the attainment of a better understanding of Hg environmental distribution and transformation processes (EU, 2017; UNEP, 2013b). In this context, wastewater treatment plants (WWTPs) play a key role in limiting Hg discharge to aquatic ecosystems. Most WWTPs receive mixed wastewaters of domestic and industrial origins, which include discharges from agriculture, hospitals, dentists, research institutions, households, and the chemical, cement, and metal industries, as well as landfill leachates and surface drainage (Balogh and Liang, 1995; Cantinho et al, 2016; Gbondo-Tugbawa et al, 2010; Koch, 2002; So€rme and Lagerkvist, 2002; UNEP, 2013a; Wang et al, 2004). Hg is usually effectively retained in sewage sludge, with retention efficiencies often exceeding 95% (Fricke et al, 2015; Liu et al, 2018; Perusini, 2016; Stoichev et al, 2009)
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