Article pubs.acs.org/est
Quantification of Element Fluxes in Wastewaters: A Nationwide Survey in Switzerland Bas Vriens,† Andreas Voegelin,† Stephan J. Hug,† Ralf Kaegi,† Lenny H. E. Winkel,†,‡ Andreas M. Buser,§ and Michael Berg*,† †
Eawag, Swiss Federal Institute of Aquatic Science and Technology, 8600 Dübendorf, Switzerland Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, 8092 Zurich, Switzerland § Swiss Federal Office for the Environment (FOEN), 3063 Ittigen, Switzerland ‡
S Supporting Information *
ABSTRACT: The number and quantities of trace elements used in industry, (high-tech) consumer products, and medicine are rapidly increasing, but the resulting emissions and waste streams are largely unknown. We assessed the concentrations of 69 elements in digested sewage sludge and effluent samples from 64 municipal wastewater treatment plants as well as in major rivers in Switzerland. This data set, representative of an entire industrialized country, presents a reference point for current element concentrations, average per-capita fluxes, loads discharged to surface waters, and economic waste-stream values. The spatial distribution of many individual elements could be attributed either to predominant geogenic or to anthropogenic inputs. Per-capita element fluxes ranged from 1 mg day−1 (e.g., Zn, Sc, Y, Nb, and Gd) and >1 g day−1 (e.g., for P, Fe, and S). Effluent loads of some elements contributed significantly to riverine budgets (e.g., 24% for Zn, 50% for P, and 83% for Gd), indicating large anthropogenic inputs via the wastewater stream. At various locations, precious metal concentrations in sludge were similar to those in profitable mining ores, with total flux values of up to 6.8 USD per capita per year or 15 USD per metric ton of dry sludge.
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INTRODUCTION Trace elements are of great importance for a variety of applications in electronics, optics, catalysts, ceramics, and pharmaceuticals. The global use of trace elements in industry and (high-tech) consumer products has increased tremendously due to economic and population growth over the last decades.1−3 This increase applies not only to classical trace metal pollutants such as Ni, Cu, Zn, Pb, or Cd but also to rareearth elements (REE) and platinum-group elements (PGE) as well as Ag, Au, and other generally less-studied elements, such as Nb, Ta, Ga, Ge, In, Tl, and Te. Many of the aforementioned elements are critical for applications in emerging technologies in communications, energy, and mobility.1−3 The anthropogenic use of some REE,8,9 platinum-group elements,10,11 and other trace elements, such as In12 has recently been quantitatively described. The current societal use of certain trace elements (e.g., Hg, Pb, Zn, or Os) is so large that it may affect their global distribution.13,14 The increasing use of trace elements may cause increases in emissions to the environment, which can occur during the mining of raw (ore) materials4 and during manufacturing and application5 as well as during the recycling or degradation of e-waste.6,7 Despite the awareness of the increased use of trace elements and the potential environmental impact thereof, the quantita© 2017 American Chemical Society
tive understanding of the scale of trace element emissions and associated environmental impacts and health risks is currently quite limited. Many trace elements are viewed as emerging pollutants,5,7 but a profound understanding of the biogeochemical behavior and toxicology15 of most of these trace elements is missing, which obstructs the forecasting of potential ecotoxicological consequences of their emissions. Prevention of human-induced adverse impacts on the environment requires a thorough understanding of the environmental behavior of trace elements and a quantification of their fluxes. The presence of pollutants in wastewater is an indicator of societal use. Good examples are pharmaceuticals and personal care products,16 flame retardants and surfactants,17,18 and engineered nanoparticles.19,20 Similarly, the growing use of trace elements may lead to increasing loads in wastewater treatment plants (WWTP) because the trace elements used in society often find their way into municipal and industrial wastewater streams. Unfortunately, only a few studies have investigated the concentration ranges of trace elements in Received: Revised: Accepted: Published: 10943
April 4, 2017 June 30, 2017 July 3, 2017 July 3, 2017 DOI: 10.1021/acs.est.7b01731 Environ. Sci. Technol. 2017, 51, 10943−10953
Article
Environmental Science & Technology
Figure 1. Map of Switzerland showing the locations of the 64 studied WWTP, their corresponding catchment areas, and the national monitoring stations (NADUF) of the four major rivers leaving Switzerland. The WWTP depicted by black dots and dashed underlines were investigated three times. The size of the WWTP dots correspond to the number of connected population equivalents. A selection of WWTP properties is listed in Table S1 according to the indicated WWTP numbers.
numbers of connected people and industries in the catchment areas, and treatment types employed by the WWTP. Communal and industrial WWTP from both rural and urban areas were included, representing a total of 3.94 million connected population equivalents (54% of the country; Table S1). Average daily wastewater inflow and sludge production rates of the sampled WWTP are given in Table S1. Digested sludge samples were collected after anaerobic digestion and dewatering through centrifugation, pressing, or additional drying. The average age of the investigated sludges was 30.6 days. The effluent waters consisted of composite samples of 1to 7-day periods to account for diurnal variation in wastewater composition. The seasonal variation of element concentrations was monitored by collecting and investigating additional sludge and effluent samples from ten selected WWTP (Figure 1) two more times in June and July 2016. All sampling material was precleaned by acid washing in 1% ultrapure HNO3 (ROTIPURAN ≥65%, Carl Roth GmbH, Germany) and rinsing with ultrapure water (NANOpure, Thermo Scientific). The sludge samples were collected in precleaned 200 mL polypropylene boxes (Semadeni). The effluent water samples were collected in precleaned 250 mL PFA flasks equipped with a PTFE cap (Semadeni). The WTTP effluent samples were immediately acidified by adding 2.5 mL concentrated ultrapure HNO3 and subsequently filtered through 0.45 μm Teflon filters (Faust AG, Switzerland). The sludge samples and effluent samples were stored in the dark at −20 and 4 °C, respectively, until analysis. Water samples from the four major rivers that drain Switzerland (i.e., Rhine, Rhône, Inn, and Ticino) were obtained from the Swiss National River Monitoring and Survey Program (NADUF)31 between March and August 2016, a period roughly corresponding to the period in which the WWTP samples were collected. Flow-proportional (discharge-
wastewater and their behavior during wastewater treatment,21−23 and these studies usually considered only one or a few selected WWTP. Furthermore, previous studies have focused mainly on the fluxes and behavior of “classical” trace elements such as metals24 or precious metals, the latter receiving attention because of their potential recovery value from sewage sludge.25−28 Comparable quantitative information on concentration ranges and fluxes for most other trace elements is missing; the few existing surveys (e.g., from Sweden29 and the United States)30 are over a decade old. Quantitative information on the present concentrations of trace elements in wastewater streams is thus critically needed. Here, we quantified the concentrations of 69 major and trace elements in 64 communal and industrial WWTP and in the major rivers in Switzerland (194 samples in total). This data set, representative of an industrialized country in its entirety, allows us to present estimates of (i) the current ranges of trace element concentrations in wastewater, (ii) the per-capita element fluxes, and (iii) the wastewater discharge value estimates for selected precious trace elements. We use this data set to calculate the contributions of WWTP effluent fluxes to riverine fluxes. Switzerland is uniquely suited for this because virtually the entire Swiss population (∼98%)73 is connected to wastewater treatment systems, all major rivers have their origin in the country, and these rivers are continuously monitored at Switzerland’s border.
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MATERIALS AND METHODS Sample Collection. Samples of digested sewage sludge and treated wastewater (effluent) were collected from 64 WWTP in Switzerland (Figure 1) between February and March 2016 (one sludge sample and one effluent sample per WWTP). The sampled WWTP were selected to guarantee a broad nationwide representation on the basis of geographical distribution, 10944
DOI: 10.1021/acs.est.7b01731 Environ. Sci. Technol. 2017, 51, 10943−10953
Article
Environmental Science & Technology
Figure 2. Concentration ranges of elements measured in sludge samples (dry-weight basis) from 64 WWTPs in Switzerland. The thicker gray bars mark the median, the boxes indicate the 0.25 and 0.75 percentiles, and the whiskers indicate the lowest and highest values within 1.5 times the interquartile range. Outliers are indicated by circles or by arrows above the graph when outliers exceeded the shown log scale. For statistical analysis, only values above the method detection limit (MDL), indicated by triangles in the figure (values in Table S4) were considered; the fraction of measurements > MDL is indicated by red digits. A summary of the concentration ranges of all the studied elements is given in Table S5.
reaction with the organic matter in the sludge samples and resulting release of CO2. After acid digestion, the digests were diluted with ultrapure water to 50 mL in 50 mL Greiner tubes. All vials and tubes were acid washed in 1% ultrapure HNO3 and rinsed with ultrapure water before use. For quality control, selected samples were acid-digested in triplicate, and three certified reference materials were aciddigested with each sample series: European Community Bureau Reference Standard BCR 145R (sewage sludge of mixed origin), NIST 2781 (domestic sewage sludge), and NIST 2782 (industrial sewage sludge). The recovery of the certified elements using the most suitable of the investigated aciddigestion methods ranged between 45 and 139% for the individual elements (see complete overview in Table S2). Elemental Analyses of Sludge and Water Samples. The concentrations of 69 elements were quantified in the digested sludge samples and in the acidified and filtered WWTP effluents and river water samples using a quadrupole dynamic reaction cell inductively coupled plasma mass spectrometry (ICP-MS) (Agilent 7500cx). A pair of ICP-MS methods were used: ICP-MS method A included 48 target elements, with 1 mg L−1 Nb and Au as internal standards, and ICP-MS method B included 21 target elements, with 10 mg L−1 Sc and 1 mg L−1 Lu as internal standards. Multiple isotopes of each element were recorded to monitor interferences, but a single isotope was used for element quantification. The selected isotope was typically the one with the highest abundance, the lowest detection limit, the best agreement with the certified reference materials, or a combination thereof. The digests from the three acid-digestion methods were analyzed with both ICP-MS methods, but each element was quantified only in the matrix resulting from the most appropriate acid-digestion method as indicated in Table S2. Details of both ICP-MS methods (e.g., the measured target masses, used calibration standards, and matrix composition resulting from acid-digestion) are provided in Table S3. The digests from methods 1 and 2 were analyzed without further dilution (matrices were 10% HNO3 for method 1 and 10% HNO3 and 0.6% HF for method 2), while the digests from method 3 were diluted 1:10 with ultrapure water
weighted) biweekly collective river water samples were collected from the national monitoring stations at Weil am Rhein (Rhine), Chancy (Rhône), Riazzino (Ticino), and Martinsbruck (Inn), which are located at the respective outflows at the Swiss border (Figure 1). These river water samples were acidified with 1% concentrated ultrapure HNO3, filtered through 0.45 μm Teflon filters (Faust AG) and subsequently stored in the dark at 4 °C until further use. Acid Digestion of Sludge Samples. Because the sludge samples had a variable water content (3−76%), aliquots (∼10 g each) of the sludge samples were freeze-dried under vacuum to constant dry weight at −20 °C for 48 h (LyoAlpha 10−55, Telstar, Madrid, Spain) and subsequently homogenized using a ceramic mortar. The total element fractions were quantitatively recovered using different acid-digestion methods from which at least one method guaranteed element stability for long periods of time. A total of three digestion techniques were applied: (i) acid digestion with hydrogen peroxide (H2O2) and nitric acid (HNO3), (ii) acid digestion with H2O2, HNO3, and hydrofluoric acid (HF), and (iii) acid-digestion with H2O2 and aqua regia (1:3 v/v mixture of HNO3 and hydrochloric acid (HCl)). All chemicals were of ultrapure grade: ultrapure HF (47−51%) and ultrapure H2O2 (≥30%) from Sigma-Aldrich and ultrapure HNO3 (≥65%) and HCl (≥32%) from Carl Roth GmbH, Germany. For acid-digestion methods 1 and 2, 50 mg of dried sludge was mixed with 1 mL of concentrated H2O2 and 5 mL of concentrated HNO3; for acid-digestion method 2, 0.3 mL of concentrated HF was added to this mixture. These mixtures were subsequently digested at 230 °C and 130 bar for 45 min in 15 mL Teflon−tetrafluormetoxil (PTFE−TFM) tubes using an MLS UltraClave 4 (Milestone, Shelton, CT). For acid digestion method 3, 100 mg of dried sludge was mixed with 1 mL of concentrated H2O2, 2 mL of concentrated HNO3, and 6 mL of concentrated HCl and subsequently digested at 210 °C and 130 bar for 45 min in 50 mL TFM tubes using an Ethos 1 Microwave digestion oven (MLS GmbH, Heerbrugg, Switzerland). In all acid-digestion methods, the peroxide and acids were added slowly to prevent loss of sample due to the vigorous 10945
DOI: 10.1021/acs.est.7b01731 Environ. Sci. Technol. 2017, 51, 10943−10953
Article
Environmental Science & Technology
Figure 3. Concentration ranges of elements measured in effluent water samples from WWTP in Switzerland (N = 64). Figure and explanations analogous to Figure 2; the fraction of measurements > MDL is indicated by blue digits. A summary of the concentration ranges of all the studied elements is given in Table S5.
concentration ranges of the most abundant elements (e.g., Fe, P, and Ca, 10−100 g kg−1 range) and those of the rarest trace elements (e.g., Pt, Au, and Pd, 50% of the sludge concentrations were
