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European Biosolids and Organic Resources Conference 15-16 November, Edinburgh, Scotland

INVESTIGATIONS INTO POTENTIAL RISKS ASSOCIATED WITH THE USE OF TREATED MUNICIPAL SLUDGE ON AGRICULTURAL LAND

Healy, M.G.1*, Fenton, O.2, Cummins, E.3, Clarke, R.,3 Peyton, D.P.1,2, Fleming, G.T.A.4, Wall, D.2, Morrison, L.5, and Cormican, M.6

1 Civil Engineering, National University of Ireland, Galway, Co. Galway, Ireland, 2Teagasc, Environment Research Centre, Johnstown Castle, Co. Wexford, Ireland, 3School of Biosystems and

Food Engineering, Agriculture and Food Science Centre, University College Dublin, Belfield, Dublin 4, Ireland, 4Microbiology, National University of Ireland, Galway, Co. Galway, Ireland.

5Earth and Ocean Sciences and Ryan Institute, National University of Ireland, Galway, Co. Galway, Ireland, 6 School of Medicine, National University of Ireland, Galway, Co. Galway, Ireland.

*Corresponding Author Email: [email protected]; Ph: +353 91 495364

Abstract

Recycling to land is currently considered the most economical way for sewage sludge management. However, there is considerable concern over the presence of metals, nutrients, pathogens, pharmaceutical and personal care products (PPCPs), which may cause environmental and human health problems. The main aims of this research were to (1) quantify the range of concentrations of metals and the antimicrobials triclosan and triclocarban (two of the most abundant PPCPs in the world), in biosolids from a range of wastewater treatment plants in Ireland (2) undertake a field-scale experiment to assess losses of nutrients, metals, triclosan and triclocarban, and microbial matter following successive rainfall events on grassland onto which biosolids had been applied (3) to measure the uptake of metals by ryegrass for a period of time after the application of biosolids (4) conduct a risk assessment of potential hazards of human health concern based on the experimental data.

Keywords

Biosolids; land application; metals; public health; triclosan; triclocarban.

Introduction

In 2010, more than 10 million tonnes of municipal sludge was produced in the European Union (EU) (Eurostat, 2014). The amount of sludge production has generally increased, which is reflective of changes in European legislation, such as the Urban Waste Water Treatment Directive (91/271/EC; Council of European Communities 1991) regarding the treatment of wastewater. Recently, legislation such as the Waste Framework Directive (2008/98/EC; Council of European Communities 1998) has imposed measures to reduce potential environmental impact arising from the generation and management of waste. A major driver for these changes has been the concept of a ‘circular economy’ (EC 2015), which fosters the concept that products, materials and services are maintained within the economy for as long as possible, in order to attain the notional goal of a ‘zero waste’ society (Grace et al. 2016). This has prompted those within the waste management community to consider municipal sewage sludge as a resource. Consequently, many uses have been found for it, such as the production of energy, construction materials and other potentially useful compounds (Healy et al. 2015). To date, municipal sewage sludge is most commonly disposed of by application to land, with some countries, such as Ireland, reusing up to 80% of it in agriculture (Eurostat, 2016).

www.european-biosolids.com Organised by Aqua Enviro

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Before land application is permitted, sewage sludge must be stabilised by chemical, thermal or biological means, after which it is commonly referred to as ‘biosolids’. The land application of biosolids is strictly controlled within the EU, and its application to land is typically controlled by the metal and nutrient content of the biosolids and the land to which it is being applied (Lucid et al. 2013). Legislation regarding its reuse on land differs (Mulieu et al. 2013; Lucid et al. 2014), and in some EU countries, such as Belgium, Switzerland and Romania, its reuse on land is prohibited (Milieu et al. 2013). This is due to concerns regarding the potential environmental and human health implications arising from the land application of biosolids (Peyton et al. 2016). Issues that have been raised are the potential presence of emerging contaminants, such as pharmaceuticals and personal care products (PPCPs) in the sewage sludge (Clarke et al. 2016a), the risk of contamination of surface and groundwater following the application of biosolids to land (Fu et al. 2016; Clarke et al. 2016b), and the risk of the accumulation of metals in the soil and incorporation into the human food chain (Latare et al. 2014) – all of which may adversely impact human health. Against these risks, however, there are several benefits associated with the reuse of sewage sludge in agriculture. Following land application of biosolids, biomass yield of crops (Liu et al. 2015) and soil test phosphorus (Shu et al. 2016) have been reported to increase, and biosolids also have been reported to have a positive impact on organic matter content and water holding capacity of soil (Cele and Maboeta, 2016).

To address some of the concerns surrounding the application of biosolids to land, the aims of this study were to (1) characterise the municipal sewage sludge from wastewater treatment plants (WWTPs) in the Republic of Ireland for the presence of metals and PPCPs (2) undertake a field-scale experiment to assess losses of nutrients (nitrogen (N) and phosphorus (P)), metals, PPCPs and microbial matter (total and faecal coliforms) following successive rainfall events on grassland onto which biosolids had been applied, and to compare the water quality of the surface runoff with another commonly spread organic fertiliser, dairy cattle slurry (DCS) (3) measure the uptake of metals by ryegrass for a period of time after the application of biosolids (4) conduct a risk assessment of potential hazards of human health concern based on the experimental data.

Materials and Methods

Metal and PPCP concentrations in sludge

Municipal sewage sludge, having undergone treatment by lime stabilisation (n = 4 WWTPs), thermal drying (n = 8 WWTPs), or anaerobic digestion (n = 5 WWTPs), were collected from 16 WWTPs in Ireland (one WWTP carried out both thermal drying and anaerobic digestion of sludge). The WWTPs had population equivalents (PEs) ranging from approximately 6,500 to 2.3 million, and were selected based on willingness to participate on this study, PE and geographical location. Eight discrete samples were collected from each WWTP, freeze dried at -50oC and pulverised in an agate ball mill with a rotational speed of 500 rpm for 5 min. The metal content was determined using a handheld X-ray fluorescence (XRF) analyser (DELTA Series 400, Olympus INNOV-X, Woburn, MA, USA). The elements measured were: cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), mercury (Hg), molybdenum (Mo), nickel (Ni), lead (Pb), antimony (Sb), selenium (Se), tin (Sn), and zinc (Zn). Quality control was ensured by the use of sewage sludge certified reference materials (Healy et al. 2016a).

The PPCPs examined in this study were triclosan (TCS) and triclocarban (TCC), which were selected based on a quantitative risk ranking model for human exposure to ECs, following land application of biosolids, conducted by Clarke et al. (2016a). The methodology employed to measure TCS and TCC was after USEPA Method 1694 (USEPA, 2007).

Surface runoff studies




The study site was a 0.6-ha grassland (ryegrass) plot located at Teagasc, Johnstown Castle Environment Research Centre, Co. Wexford, Republic of Ireland. The site had an undulating topography and an average surface slope of 6.7%. Hydraulically isolated micro-plots, each measuring 0.4 m in width and 0.9 m in length, and with collection troughs positioned at the end of each micro-plot (to intercept the surface runoff), were constructed in the field. Prior to the start of the experiment, the soil was fully characterised in each micro-plot (Peyton et al. 2016). The biosolids used in the experiment originated from the same WWTP, and had undergone either lime stabilisation, thermal drying, or anaerobic digestion. The biosolids were fully characterised for nutrient and metal content, and were applied to the plots in accordance with current guidelines (S.I. 31 of 2014; Fehily Timoney & Co., 1999) – 40 kg P/ha, based on the P content of the soil. As the biosolids had different dry matter (DM), the application rate per plot varied between 2.6 tonnes DM/ha for the TD biosolids to 29 tonnes DM/ha for the LS biosolids. To compare the runoff of contaminants with another common wastewater that is applied to land, dairy cattle slurry was also applied at the same rate to the micro-plots. Five treatments, each replicated in n=5 micro-plots, were used in this study: thermally dried (TD), lime stabilised (LS) and anaerobically digested (AD) biosolids, DCS, and a grassland only study control. Rainfall was applied to each micro-plot over three successive rainfall events, each lasting up to 1 hr, at 24 hr (rainfall simulation (RS) 1), 48 hr (RS2) (the minimum time that should elapse between land application of any waste and the first rainfall event; SI 31 of 2014) and 360 hr (RS3) after land application of the biosolids/DCS. During each rainfall event, surface runoff samples were collected, and later analysed for P (including dissolved reactive phosphorus (DRP), particulate phosphorus (PP), and dissolved unreactive phosphorus (DUP)), N (including ammonium-N (NH4-N), nitrate-N (NO3-N), and organic N), metals, total and faecal coliforms, and TCS and TCC. All testing was conducted in accordance with the standard methods.

Metal concentration in ryegrass

Three grass samples, each comprising a composite of six to eight blades or shoots of ryegrass, were cut at the soil surface in each of the micro-plots immediately prior to the second (at 48 hr after application) and third (at 360 hr after application) rainfall simulations, and finally at a time varying between 55 and 130 days (18 weeks) from the time of the application of biosolids. Nitrile gloves were used when collecting the plant samples, and were changed between plots to avoid cross contamination. The samples were freeze-dried and analysed for metal content using an inductively coupled plasma mass spectrometer (ICP-MS).

Development of risk assessment model

Following a literature review, a suite of 16 contaminants identified in the literature were further analysed in a risk ranking model to include health based risk endpoints. A probabilistic model was constructed in Excel 2010 (incorporating @Risk 6.0) to estimate human exposure to organic contaminants that are contained within biosolids destined for land application. An exposure assessment model was further developed for both metals and E. coli. The model considered exposure to metals and E. coli through surface water abstracted for drinking taking account of surface runoff, dilution and water treatment effects. The likelihood of illness arising from exposure and the severity of the resulting illness was evaluated. Different dose-response relationships were characterised for the different pollutants with reference Lifetime Average Daily Dose (LADD) and Hazard quotient (HQ) used for metals, whilst a worst-case negative exponential dose-response model was used for E. coli.




Results

Metal and PPCP concentrations in sludge

With the exception of Pb (which had a very high concentration in one WWTP), the metal content of the sewage sludge was all under EU regulatory upper limits (Table 1). The metals that are not currently regulated within the EU (Table 1) were generally similar to the concentrations measured in other countries (Healy et al. 2016a). Of the elements considered bio-essential micro-nutrients measured in this study (Se, Fe, Cu and Zn), all were within either EU or international limits (Healy et al. 2016b) (no limits govern Fe).

Table 1: Mean (±standard deviation, SD) metal concentration (mg/kg dry weight) in sludge following anaerobic digestion, lime stabilisation, or thermal drying. n refers to the number of treatments1 (adapted from Healy et al. 2016a)

Metal Anaerobic digestion (n=5)

Lime stabilisation

(n=4)

Thermal drying (n=8)

EU regularity upper limits (EEC, 1986)

Mean Mean MeanRegulated parameters in EUCu 640 (411) 491 (452) 464 (205) 1,750Ni 25 (5) 13 (3) 15 (7) 400Pb 791 (1625) 33 (25) 54 (30) 1,200Cd 11 (1) 13 (1) 10 (3) 40Zn 1,273 (749) 526 (388) 869 (400) 4,000Hg1 <LOD <LOD <LOD 25

Non-regulated parameters in EUAs2 <LOD <LOD <LODSe 3 (2) 3 (1) 2 (1)Sr 162 (61) 183 (75) 114 (36)Mo 5 (2) 4 (1) 5 (1)Ag 11 (2) 11 (3) 8 (3)Sn 55 (57) 23 (4) 23 (5)Sb 20 (5) 17 (3) 17 (4)Cr 51 (43) 25 (15) 16 (12)Fe 32,135 (41,717) 9,654 (7,264) 33,087

(43,373)1Both anaerobic digestion and thermal drying were carried out in one wastewater treatment plant.2Limit of detection (LOD) = 10 ppm 2 LOD = 100 ppm

The TCS and TCC concentrations in the biosolids are shown in Figure 1. There was no trend observed between the concentrations and the PEs in the WWTPs. Further, as there are currently no limits governing the PPCP content of biosolids (Verlicchi and Zambello, 2015), the concentrations measured in this study cannot be compared against regulatory limits.




Figure 1: Triclosan and triclocarban concentrations (ug/g) in treated sludge from 16 wastewater treatment plants in Ireland, ranging (numerically in ascending order) from a population equivalent (PE) of 2.3 million to 6,500.

Surface runoff studies

Appreciable amounts of P and N were present in the runoff form the biosolids-amended plots compared to the grassland (study control) plots. Thermally dried biosolids had the highest concentration of total phosphorus (2.2 mg/L) versus the study control (just under 1 mg/L), and comprised mainly DRP and PP. All biosolids-amended plots produced DRP concentrations far in excess of 35 µg/L, the concentration above which eutrophication of receiving waters may be likely (Brennan et al. 2012). However, DCS had the highest runoff of P, which was many orders of magnitude higher than the biosolids-amended plots (Figure 2).

Similar trends were noted for the N, which mainly comprised NH4-N, in the surface runoff, and tended to reduce in concentration from the first to the final rainfall event. Similar to the P analyses, the concentration of DCS in the runoff was higher than the biosolids-amended plots (results not displayed). Considering the metal analyses of surface runoff, with the exception of Cu from the LS plots, all metals measured were below their respective surface water standards intended for human consumption (S.L.549.21). With the exception of Cu, the surface runoff of all the parameters measured was of the same order of, or much less than the DCS.

The ADUK-amended plots produced runoff with the lowest number of total coliforms (averaged over the three rainfall simulations), but produced the highest average number of faecal coliforms – 7.1 × 103 most probable number (MPN) per 100 ml during RS1 and RS2 (Peyton et al. 2016). The overall losses from DCS (3.1 × 102 MPN) were greatest and significantly greater than LS, AD and the control. The highest median count of total coliforms and faecal coliforms measured in LS biosolids-amended plots was 5.6 × 105 and 1.5 × 101 MPN per 100 ml, respectively. The highest median loss of total coliforms for DCS-amended plots was 1.5 × 105 MPN per 100 ml.

The surface losses of TCS and TCC were below the limits of detection in most cases. Considering the mass of TCS and TCC applied to the micro-plots versus the mass measured in the surface runoff, less than 0.5% of the total mass applied to each plot was lost in runoff.




Figure 2: Phosphorus concentration (including dissolved reactive phosphorus (DRP), particulate phosphorus (PP), and dissolved unreactive phosphorus (DUP) components over three rainfall events at 24 hr (RS1), 48 hr (RS2) and 360 hr (RS3) after land application. Also included in the graph is dairy cattle slurry (‘slurry’) (adapted from Peyton et al. 2016)

Metal concentration in ryegrass

In general, there was no statistically significant difference in the shoot metal concentration of the biosolids-amended micro-plots and the study control. A downward trend in metal concentration in the shoots was noted, which was attributable to dilution effects arising from the growth of ryegrass. The biosolids did not increase the biomass of the ryegrass relative to the study control, nor did the method used to create the biosolids (anaerobic digestion, lime stabilisation, thermal drying) result in any difference in shoot metal content.

Risk assessment model

The probabilistic model constructed in this study to estimate human exposure to organic contaminants, ranked nonylphenols as the highest risk to human health across all environmental compartments analysed. Triclosan and TCC also ranked high, and may be considered as a potentially greater risk, as they are known to cause adverse health effects.

An exposure assessment model was developed, which considered the likelihood of illness arising from exposure to metals and E. coli through surface water abstracted for drinking taking account of surface runoff, dilution and water treatment effects. Of all the scenarios considered, and with regards to the LADD, results showed that mean Cu exposure concentrations for children were highest in all three rainfall events (mean values 2.07 × 10-2, 2.07 × 10-2 and 1.18 × 10-2 µg kg-1 bw/d) corresponding to the LS treatment. This was followed by adult Cu exposure concentrations (mean value 1.80 ×10-2,




1.31 × 10-3 and 9.21 × 10-3 µg/kg bw/d, for all three rainfall events). The results for the hazard quotient showed that, of all the scenarios considered, the metal Cu and the LS biosolids had the highest HQ for children for all three rainfall events with mean child HQ values 5.59 × 10-4, 4.09 × 10-4 and 3.18 × 10-4 respectively, followed by mean adult HQ values of 4.87 × 10-4, 3.54 × 10-4 and 2.49 × 10-4, respectively. However, these were still below the threshold value of risk (HQ < 0.01, no existing risk).

Discussion

Prior to land application, biosolids need to be characterised for their nutrient (N and P) and metal content. The metal parameters regulated within the EU are Cu, Ni, Pb, Cd, Zn and Hg (EEC, 1986). However, there is a possibility that many other metals, which are currently not governed by legislation, may be applied to the soil without regulation. While the concentrations of the unlegislated metals measured in this study were, in general, of the same order as measured elsewhere (LeBlanc et al. 2008), there is a possibility that they may accumulate in soils, following repeated application. This potential problem also applies to the PPCP content of biosolids, whose concentration in sewage sludge or biosolids is currently unlegislated. This means that there is no way of determining whether the concentrations measured in the current study are safe. Further, as the biosolids examined in this study were collected from WWTPs in Winter (January/February), it is reasonable to assume that the concentrations of metals, TCS and TCC were at their lowest concentrations, due to rainfall/dilution. Therefore, there is a possibility that much higher concentrations may be present in biosolids during other periods of the year, which means that higher applications of these parameters may be inadvertently applied to land. It is also important to realise that the PPCP parameters measured in this study – TCS and TCC – are only a small fraction of the contaminants that may be present in biosolids, and other, more potentially harmful, but yet unknown, contaminants may be present. Although the surface runoff experiments showed that the application of biosolids pose no greater threat to surface water quality than DCS, which is mainly disposed of through land application, and that the metal content of ryegrass of biosolids-amended plots is similar to unamended plots, serious questions still remain about the continued application of biosolids to land. While the potential economic benefits arising from the reuse of municipal sewage sludge on land are evident, many researchers acknowledge that a level of uncertainty still exists about the risk to human health (Smith, 2009; Oun et al. 2014).

Conclusion

The overall conclusion from this study is that although, in general, land applied biosolids pose no greater threat to water quality than dairy cattle slurry and cattle exclusion times from biosolids-amended fields may be overly strict (within the context of current exclusion criteria), a matter of concern is that unlegislated metals and PPCPs, found to be present in biosolids originating from a selection of WWTPs examined in this study, may be inadvertently applied to land. With multiple applications over several years, these may build up in the soil and may enter the food chain, raising concerns over the continued application of biosolids to land in Ireland.

Acknowledgements

The authors wish to acknowledge funding from the Irish EPA (Project reference number 2012-EH-MS-13) and the Department of Communications, Energy and Natural Resources under the National Geoscience Programme 2007 – 2013 (Griffiths Award).




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