Waste Management 48 (2016) 404–408 Contents lists available at ScienceDirect Waste Management journal homepage: www.elsevier.com/locate/wasman Metal concentrations in lime stabilised, thermally dried and anaerobically digested sewage sludges M.G. Healy a, O. Fenton b, P.J. Forrestal b, M. Danaher c, R.B. Brennan a, L. Morrison d,⇑ a Civil Engineering, National University of Ireland, Galway, Ireland Teagasc Johnstown Castle Environment Research Centre, Co. Wexford, Ireland c Food Safety Department, Teagasc Food Research Centre, Ashtown, Dublin 15, Ireland d Earth and Ocean Sciences, School of Natural Sciences and Ryan Institute, National University of Ireland, Galway, Ireland b a r t i c l e i n f o Article history: Received 12 May 2015 Revised 20 October 2015 Accepted 14 November 2015 Available online 25 November 2015 Keywords: Treated sludge Biosolids Metals Land application a b s t r a c t Cognisant of the negative debate and public sentiment about the land application of treated sewage sludges (‘biosolids’), it is important to characterise such wastes beyond current regulated parameters. Concerns may be warranted, as many priority metal pollutants may be present in biosolids. This study represents the first time that extensive use was made of a handheld X-ray fluorescence (XRF) analyser to characterise metals in sludges, having undergone treatment by thermal drying, lime stabilisation, or anaerobic digestion, in 16 wastewater treatment plants (WWTPs) in Ireland. The concentrations of metals, expressed as mg kg 1 dry solids (DS), which are currently regulated in the European Union, ranged from 11 (cadmium, anaerobically digested (AD) biosolids) to 1273 mg kg 1 (zinc, AD biosolids), and with the exception of lead in one WWTP (which had a concentration of 3696 mg kg 1), all metals were within EU regulatory limits. Two potentially hazardous metals, antimony (Sb) and tin (Sn), for which no legislation currently exists, were much higher than their baseline concentrations in soils (17–20 mg Sb kg 1 and 23–55 mg Sn kg 1), meaning that potentially large amounts of these elements may be applied to the soil without regulation. This study recommends that the regulations governing the values for metal concentrations in sludges for reuse in agriculture are extended to include Sb and Sn. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction More than 10 million tonnes of sewage sludges were produced in the European Union (EU) in 2010 (Eurostat, 2014). Legislation such as the Landfill Directive, 1999/31/EC (EC, 1999), the Urban Wastewater Treatment Directive 91/271/EEC (EC, 1991), the Waste Framework Directive (2008/98/EC; EC, 2008) and the Renewable Energy Directive (2009/28/EC; EC, 2009), means that rather than incinerating it or sending it to landfill, there is an increased emphasis on its reuse as a ‘product’. Consequently, it is used in the production of energy (Gikas, 2014), bio-plastics (Yan et al., 2008), construction materials (Jiang et al., 2011) and, when appropriate treatment is applied, as an agricultural fertiliser (Koutroubas et al., 2014). There are considerable public acceptance issues around the reuse of treated municipal sludge (‘biosolids’) as fertiliser (LeBlanc et al., 2008) and, depending on the part of the world, legislation regarding its reuse as such, differs (Milieu et al., 2013a,b,c). ⇑ Corresponding author. E-mail address: [email protected] (L. Morrison). http://dx.doi.org/10.1016/j.wasman.2015.11.028 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved. Moreover, in some countries such as Belgium (Brussels and Flanders), Switzerland and Romania, the use of biosolids in agriculture is prohibited (Milieu et al., 2013a,b,c). While concerns over the presence of persistent organic pollutants and emerging contaminants, such as pharmaceuticals, have been expressed (Clarke and Cummins, 2014), the presence of toxic metals in sludge, due to the mixing of industrial wastewater with sewage, means that the application of metal-contaminated sludge may cause the contamination of soil and water (Cornu et al., 2001) and accumulation of metals in the food chain (Kidd et al., 2007; Latare et al., 2014). In an attempt to address these concerns, guidance values concerning the maximum allowable concentration of certain metals in biosolids (Table 1) are in place in countries where the reuse of biosolids on land is permitted. The level of exceedance in wastewater treatment plants (WWTPs) is therefore of interest. The application of biosolids to agricultural land is governed by various legislation (e.g. in Europe by EU Directive 86/278/EEC (EEC, 1986); in the US by 40 CFR Part 503 (US EPA, 1993)). These require that sewage sludge undergoes biological, chemical or heat treatment, long-term storage, or any other process to reduce the potential for health hazards associated with its use. In the EU, land 405 M.G. Healy et al. / Waste Management 48 (2016) 404–408 Table 1 Limit values for metal concentrations in sludge for use in agriculture. Selenium (Se) mg kg Brazil 100 1 Molybdenum (Mo) Arsenic (As) Copper (Cu) Nickel (Ni) Lead (Pb) Zinc (Zn) Cadmium (Cd) Chromium (Cr) Mercury (Hg) Reference 41 1500 40 300 2800 39 1000 17 75 800– 1500 1000– 1750 100–200 2000– 3000 2500– 4000 5–20 300 300– 1000 750– 1200 100 LeBlanc et al. (2008) LeBlanc et al. (2008) EEC (1986) dry weight (=ppm) 50 China EU Japan Jordan Russian Fed. USA 50 100 100 75 75 300–400 5–15 20–40 – 16–25 5 500 2 41 1500 300 300 2800 40 900 17 10 750 200 250 1750 15 500 7.5 41–75 1500– 4300 420 300– 840 2800– 7500 39–85 application of biosolids is typically based on its nutrient and metal content, although individual member states often have more stringent limits than governing directives (LeBlanc et al., 2008; EC, 2010; Milieu et al., 2013a,b,c). Guidelines govern the maximum rate of nutrients and metals (e.g. Fehily Timoney and Company, 1999), although as the metal content is normally low relative to the nutrient content of biosolids, application rates are frequently determined by the nutrient content of the biosolids and not their metal content (Lucid et al., 2013). As soil acidification may increase the solubility of metals (Antoniadis et al., 2008), there is a potential risk of metal accumulation in the soil (Álvarez et al., 2002; Mamindy-Pajany et al., 2014), in plants (Latare et al., 2014), or of transport to groundwater, particularly if added in excess (McBride et al., 1999). In countries such as the USA, where in the majority of states biosolids are applied to land based on the nitrogen (N) requirement of the crop being grown and not on a soilbased test (McDonald and Wall, 2011), excessive metal accumulation in soil and plants (Wen et al., 2014), or losses in surface and subsurface waters (Oun et al., 2014), may potentially occur. Laboratory and field studies have demonstrated that the addition of biosolids to land as a fertiliser replacement has several beneficial effects (Monera et al., 2002; Latare et al., 2014). They provide nutrients and micronutrients (e.g. zinc (Zn), copper (Cu), cobalt (Co)) required for plant and crop growth, and can be used as an aid in the development of a soil’s physical and chemical characteristics. Latare et al. (2014) found that applications of biosolids to land at rates ranging from 10 to 40 tonnes ha 1 increased the grain yield of rice by up to 40% and increased the available nutrient content of the soil in comparison to equivalent doses of fertilizers. However, the metal content of both the plants (cadmium (Cd)) and soil (Zn) also increased in comparison to the regular fertiliser. Similar results have been found by other researchers (McBride et al., 1999; Stietiya and Wang, 2011). Due to the increasing awareness regarding potential risks to the environment and human health, the application of sewage sludge, following treatment, to land as a fertilizer in agricultural systems has come under increased scrutiny. This is mainly a perception issue by the food production sector, which is driven by the belief that best practices for sludge treatment are not being followed (EPA, 2014b). As metals are likely to remain in the soil indefinitely, the characterisation of biosolids prior to land application is important. The aim of this study was to: (1) examine if the metal content of biosolids from high population equivalent (PE) WWTPs in Ireland exceeded permitted limit values and (2) establish a baseline for unregulated metals – potential pollutants of which little is known and from which other global studies may be compared. 17–57 LeBlanc et al. (2008) LeBlanc et al. (2008) LeBlanc et al. (2008) US EPA (1993) To our knowledge, this is the first time that extensive use was made of a handheld X-ray fluorescence (XRF) analyser to carry out analysis on biosolids. 1.1. Study context in Ireland In Ireland there were 541 urban areas, with PEs ranging up to 2.3 million, that received either preliminary, primary, secondary, or secondary treatment and nutrient reduction in 2012 (EPA, 2014a). In 2012, approximately 94% of the national wastewater load received at least secondary treatment, and the WWTPs produced sewage sludge with a total load of 72,429 tonnes (dry solids, DS), of which 94.3% was diverted to agriculture, 5.7% was diverted to composting and other uses, and <0.01% was sent to landfill (EPA, 2014a). Of the treatment processes currently in use in Ireland (anaerobic and aerobic digestion, composting, thermal drying), lime stabilisation remains the most popular, due to the relatively small amount of costs involved (EPA, 2014b). 2. Materials and methods 2.1. Sample collection and preparation Biosolids were collected from 16 WWTPs or agglomerations, with PEs ranging up to approximately 2.3 million (Table 2). Selection of the WWTPs was predicated on willingness to participate in this monitoring study and geographical location (a good geographical spread was desirable). None of the plants selected had a history of persistent failures in meeting water discharge standards (EPA, 2014a). Of the WWTPs examined, most received landfill leachate in low quantities (no greater than 2% of the total BOD loading on the WWTP), while others received industrial, commercial and domestic/septic tank sludge comprising up to 30% of the total influent BOD loading on the WWTP (Table 2). Eight discrete samples (n = 8) of 100 g were collected in clean LDPE containers (Fisher, UK) from each WWTP and stored at 20 °C prior to analysis. The biosolids samples were freeze dried (Freezone 12, Labconco, Kansas City, USA) at 50 °C and pulverised in an agate ball mill (FritschTM Pulverisette 6 Panetary Mono Mill) with a rotational speed of 500 rpm for 5 min (repeated three times) using an 80 ml agate vial and balls (Ø 10 mm). 2.2. Elemental determination A handheld X-ray fluorescence (XRF) analyser (DELTA Series 4000, Olympus INNOV-X, Woburn, MA, USA) in the laboratory 406 M.G. Healy et al. / Waste Management 48 (2016) 404–408 Table 2 Site agglomerations and type of treatment conducted in each location. a Site no. WWTP/ agglomeration size (PEs) Leachate as % of influent BOD load Industrial/commercial and domestic/septic tank sludgea as % of influent BOD load Type of treatment 1 2,362,329 <0.01 <0.01 2 284,696 0.3 24 3 179,000 Unknown 30 4 130,000 Unknown 0.008 5 101,000 2.0 Unknown 6 86,408 0.2 2.1 7 76,456 0 0 8 46,428 0.1 25 9 42,000 <0.01 15 10 31,788 0.25 Unknown 11 30,000 0.081 0 12 27,731 0 2.8 13 27,000 0.2 0 14 25,000 0.7 0 15 22,440 0 0 16 6500 Unknown Unknown Thermal drying, anaerobic digestion Thermal drying Anaerobic digestion Thermal drying Lime stabilisation Anaerobic digestion Anaerobic digestion Lime stabilisation Thermal drying Lime stabilisation Thermal drying Anaerobic digestion Thermal drying Thermal drying Lime stabilisation Thermal drying Most recent available figures in all WWTPs (2013). (mounted in an integrated bench-top workstation and interfaced with a PC) in soil environmental mode was employed to determine metal (Cd, chromium (Cr), copper (Cu), iron (Fe), mercury (Hg), molybdenum (Mo), nickel (Ni), lead (Pb), antimony (Sb), selenium (Se), tin (Sn), and Zn) concentrations. This portable XRF system consists of a powerful X-ray tube (4 W, Au anode) and a 30 cm2 Silicon Drift Detector (SDD). An internal instrument standardisation was performed using an alloy chip (aligns the Fe and Mo peaks on the spectrum to compensate for temperature drift) and sewage sludge certified reference materials (Trace Metals – Sewage Sludge 2 CRM029, Sewage Sludge 3 CRM031 and Sewage Sludge 4 CRM055, Sigma–Aldrich RTC, Inc., USA) were used for calibration/ verification of the P-XRF to matrix match the ‘unknown sewage sludge samples’ as closely as possible in order to eliminate matrix effect from the P-XRF analysis. Calibration using the Certified Reference Materials (CRMs) was achieved by plotting the XRF data against certified data and inserting a linear trend line to determine the linearity of the calibration (which is used to calculate the factor and offset required to correct the data within the instrument). An aliquot of the homogenised biosolids (approximately 5 g) was packed into polyethylene XRF sample cups and covered with a 4 lm Prolene sample support window (ChemplexÒ Industries Inc., USA). Metal concentrations were detected simultaneously and the operating parameters included a measurement time of 180 s at beam currents of up to 200 lA (maximum voltage of 40 kv and energy resolution of 150 eV). The software uses a compton normalisation algorithm to determine mg kg 1 concentrations of elements by correlation of the X-ray tube parameters and the intensity and energy seen by the detector. 2.3. Quality control Quality control included the use of instrumental blanks (SiO2), analysis of duplicate samples, and the performance of the method and stability of the instrument was evaluated by using CRMs of sewage sludge (Trace Metals – Sewage Sludge 2 CRM029, Sewage Sludge 3 CRM031 and Sewage Sludge 4 CRM055, Sigma–Aldrich RTC, Inc., USA), sediments (LKSD-4, lake sediment and PACS-1 marine harbour sediment, National Resources Canada) and soils (SRM 2709a San Joaquin Soil and SRM 2710a Montana Soil I, National Institute of Standards and Technology (NIST), USA). The results of the analysis of the CRMs were in good agreement with their respective certified and reference ranges (Tables S1 and S2). Further confirmation of the validity of the P-XRF technique was provided by the analysis of 15% of the sewage sludge samples (taken systematically, representing elemental concentrations across the entire range, as determined by P-XRF) using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) (Agilent 7700) after digestion with aqua-regia (Trace SELECTÒ, Sigma Aldrich) in a graphite heating block. For the elements that were above the limit of detection (LOD) of the P-XRF technique (Fe, Cu, Zn, Pb, Se, Mo, Ni, Sn and Cr) in this portion of the sewage sludge samples, a comparison was made between the results obtained from the P-XRF and the concentrations determined by ICP-MS. Correlation coefficients (Pearson Product Moment Correlation for normal distributions and Spearman’s Rank Order Correlation for non-normal data) between the P-XRF and ICP-MS results were also determined (SigmaPlot 12, Systat Software Inc, San Jose, CA). 3. Results and discussion 3.1. Validation of the P-XRF technique Correlation coefficients between P-XRF and ICP-MS results indicated the suitability and satisfactory use of the P-XRF technique for the quantification of these elements in sewage sludges (Fe: r = 0.99, P < 0.001; Cu: r = 0.95, P < 0.0001; Zn: r = 0.98, P < 0.0001; Se: r = 0.95, P < 0.0001; Mo: r = 0.79, P < 0.0001; Sn: r = 0.63, P < 0.01; Ni: r = 0.85, P < 0.001; Cr: r = 0.82, P < 0.01; Pb: r = 0.99, P < 0.0001). Results of the ICP-MS analysis also confirmed that the levels of Sb and Hg were below the LOD of the P-XRF technique for this portion of comparative samples. 3.2. Overview of metal concentrations in sewage sludge The mean concentrations of the metals in the sewage sludge following treatment in the 16 WWTPs are given in Table 3. The concentrations of the metals, which are regulated in the EU, and all expressed as mg kg 1 DS, ranged from 11 (Cd, anaerobically digested (AD) biosolids) to 1273 mg kg 1 (Zn, AD biosolids), and were well under EU regulatory limits. Of the parameters not regulated in the EU, but regulated elsewhere (Table 1), As, Se, Mo and Cr (Table 3) were well below the upper limits of 75, 100, 75 and 1000 mg kg 1, respectively. Of the elements considered bioessential micro-nutrients measured in this study (Se, Fe, Cu and Zn), all were within either EU or international limits (Table 1) (no limits govern Fe). The biosolids from one WWTP, in which anaerobic digestion was carried out, had an average Pb concentration of 3696 mg kg 1, well in excess of the threshold value of 1200 mg kg 1. The average concentrations (across all treatments) of Cu, Pb and Zn were also well above the median values of internationally published results (Table 4). Lead is amongst the most hazardous metals, which are potentially harmful to human health (Johnson and Bretsch, 2002). Other metals measured in this study, which are also 407 M.G. Healy et al. / Waste Management 48 (2016) 404–408 Table 3 Mean (±standard deviation, SD) metal concentration (mg kg 1 dry weight) in sludge following anaerobic digestion, lime stabilisation, or thermal drying. n refers to the number of treatments. Metal Anaerobic digestion (n = 5) Lime stabilisation (n = 4) Thermal drying (n = 8) Mean Mean SD Mean SD 452 2.5 25 1 388 464 15 54 10 869 <LOD 205 7 30 3 400 SD Regulated parameters in EU Cu 640 411 491 Ni 25 5 13 Pb 791 1625 33 Cd 11 1 13 Zn 1273 749 526 a Hg <LOD <LOD Non-regulated parameters in EU Asb <LOD <LOD Se 3 2 3 Sr 162 61 183 Mo 5 2 4 Ag 11 2 11 Sn 55 57 23 Sb 20 5 17 Cr 51 43 25 Fe 32,135 41,717 9654 a b 1 75 1 3 4 3 15 7264 <LOD 2 114 5 8 23 17 16 33,087 EU regularity upper limits EEC (1986) compounds such as tributyltin (McBride, 2003). The concentrations of Sn measured in this study ranged from 23 to 55 mg kg 1 (Table 3), which was of the same order as other studies (26 mg kg 1 – Eriksson, 2001). Normal ranges of Sn in nonpolluting Irish soils are around 1.68 mg kg 1 (Fay et al., 2007). Both parameters, Sb and Sn, however, are not considered to be of risk to animals or humans (US EPA, 1995). 3.3. Environmental policy and management implications 1750 400 1200 40 4000 25 Land application of biosolids is, in the main, determined by the nutrient content of biosolids and not by the metal content (Lucid et al., 2013). Therefore, the metal content, even if present in relatively high concentrations in the biosolids, may not have any significant impact on soil quality in the short term. However, accumulation of metals in soil following repeated applications of biosolids, may be problematic – particularly for those elements that are not regulated and are harmful to human health. Guidelines should aim to govern the maximum allowable concentrations of these elements in biosolids, as well as the land to which they are applied. Handheld XRF analysis is a useful, quick and relatively inexpensive method for determining the metal content of biosolids, and should be used frequently to characterise it. 1 36 1 3 5 4 12 43,373 Limit of detection (LOD) = 10 ppm. LOD = 100 ppm. 4. Conclusions potentially harmful, are: Cr, Cd, Sn and Sb. Of these parameters, to date no international standards exist for Sb or Sn in biosolids for reuse in agriculture. In the present study, the average concentration of Sb ranged from 17 to 20 mg kg 1 (Table 3), which was substantially higher than recorded elsewhere, e.g. <0.01–0.06 mg kg 1 (LeBlanc et al., 2008), 3.4 mg kg 1 (Eriksson, 2001). As the average concentration of Sb in non-polluted soils is around 0.53 mg kg 1 (Fay et al., 2007) and elevated concentrations in the soil inhibit the early growth of crop plants (Fjällborg and Dave, 2004; Baek et al., 2014), the possibility exists that potentially large applications of this parameter are being land applied without regulation. Tin, in inorganic form, is non-toxic, but a significant portion of sewage sludges may be in a highly toxic, organic form and include The metals from 16 WWTPs in Ireland were below the maximum allowable concentrations of metals for use in agriculture in the EU. In addition, they were also within the median levels for biosolids globally. While current EU and international regulations govern certain priority metal pollutants and bio-essential elements, other metals that are potentially harmful to human health, such as Se and Sn, are omitted from the regulations. This means that a number of toxic metals, which are much higher than their baseline concentrations in soils, are being applied without regulation. It is recommended that the regulations governing the values for metal concentrations in biosolids for reuse in agriculture are extended to cover Sn and Sb. A handheld XRF analyser is a costeffective and rapid method for the analysis of biosolids, and may be easily applied in WWTPs. Its frequent use would mean that Table 4 Measured values for metal concentrations in sludge for use in agriculture (adapted from LeBlanc et al. (2008)) compared with average concentrations (across all treatments) measured in the current study. Selenium (Se) mg kg Brazil Bogota, Columbia Denver, USA Los Angeles, USA Milwaukee, USA Ottawa, Canada British Columbia, Canada Finland Germany Italy Slovenia Turkey Sapporo, Japan Suzu, Japan Moscow, Russ Fed. Current study a b 1 Molybdenum (Mo) Arsenic (As)a Copper (Cu) Nickel (Ni) Lead (Pb) Zinc (Zn) Cadmium (Cd) Chromium (Cr) Mercury (Hg)b 15 19 3 6 8 1 5 255 163 670 1060 266 460 888 42 43 16 51 32 16 26 80 88 39 39 57 51 56 689 1014 714 1180 534 593 588 11 76 2 10 4 1 3 144 73 84 289 50 51 2 8 1 2 0.3 1 3 244 380 261 200 70 140 18 61 22 90 34 29 20 18–1280 0.4 1 0.2 2 3–3820 1 2 2 1 1 <1 2 0–300 520 18 9 62 76 150 34 10 5 0.8– 1070 252 332 956 577 600 300 300 0.9–1200 30 32 16 35 62 35 32 1.4–306 886 12 35 <LOD dry weight (=ppm) 27 24 15 15 4 113 4 8 20 18 11 2 7 8 0–24 3 LOD = 100 ppm. Limit of detection (LOD) = 10 ppm. 5 <LOD 0.2 1 0–11 408 M.G. Healy et al. / Waste Management 48 (2016) 404–408 plant managers may determine, with relative ease, the suitability of biosolids for reuse in agriculture. 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