Linking toxicity profiles to pollutants in sludge and sediments
Introduction
Excessive use of natural resources and a large scale synthesis of xenobiotic compounds have generated a number of environmental problems. Waste water treatment plants (WWTPs) have been reported as potential major sources of environmental pollutants. Many xenobiotic compounds and pathogenic organisms remain intact despite wastewater treatment processes and can enter the environment through discharge of wastewater effluents and subsequent usage of sewage sludge [1], [2], [3]. In the European Union, production of sewage sludge is approximately 11.5 Mt dry weight (dw) y−1 [4], [5], up to 70% of which is re-used in agriculture or is disposed of through composting, landfilling or incineration. Sewage Sludge Directive 86/278/EEC sets limits for seven heavy metals, however no general limits currently exist for the presence of persistent organic pollutants (POPs) and the amount of these compounds in land-applied sludge is not regulated. One example of such POP deposition is the release of 24,000–36,000 kg year−1 of polybrominated diphenyl ethers (PBDEs) through sewage sludge land applications in the US [2] and similar quantities across Europe and Asia [6]. This is despite the fact that lower brominated PBDEs were banned in Europe and the USA in 2004 and 2006, respectively [7]. Thus due to the outdatedness of Sewage Sludge Directive 86/278/EEC, several EU countries have implemented stricter values for selected contaminants. The European Commission aims to review this Directive and therefore has dedicated projects to sewage sludge monitoring. In addition to the traditionally monitored inorganic and organic contaminants, such as PTMs, polychlorinated biphenyls (PCBs), polychlorinated dibenzodioxins (PCDDs) and dibenzofurans (PCDFs) as well as PAHs, emerging pollutants are investigated, including brominated flame retardants (BFRs), ingredients of personal care products, pharmaceuticals, some industrial chemicals, etc. The presence of these contaminats in sewage sludge has been documented in several studies [8], [9], [10], [11].
POPs and PTMs tend to bind to organic matter, thus their primary removal from influent during waste water treatment processes occurs via sedimentation and sorption to sludge particles, which then leads to concentration of these contaminants in sewage sludge [12], [13], [14], [15]. However, POPs and PTMs have also been documented in WWTP effluents [12], [15], [16]. Consequently, sedimentation accounts for the primary route of environmental deposition of these POPs, dominating over biotic or abiotic degradation and uptake by aquatic organisms [14], [17], [18].
Multivariate statistical tools have been demonstrated to help in understanding how biological systems respond to toxic stress [19], [20], [21], [22]. In this study, we used multivariate statistics to unfold the relationship between environmental concentrations of organic pollutants and PTMs with their ecotoxicological impacts. We hypothesized that not all pollutants will equally influence the resultant toxicity profiles, with some being significant and others marginally significant or insignificant. In order to examine this hypothesis, we sampled and analyzed two matrices: stabilized sewage sludge, which is often re-used for agricultural purposes, and sediments sampled below WWTPs effluent drains. The analyses included: (i) determining the content of four groups of organic pollutants [BFRs, organochlorinated pesticides (OCPs), PAHs and PCBs] and PTMs (Hg, As, Cd, Cr, Cu, Ni, Pb, Zn); (ii) characterizing the ecotoxicological status of the samples and their water extracts; and (iii) evaluating which chemicals can be used to explain toxicity profiles.
Section snippets
Sample collection
Stabilized sewage sludge was collected from 15 WWTPs located on several Czech rivers (Table 1), surface sediments (0–10 cm) were collected with stainless steel samplers ∼300–500 m below the effluent drain of 12 of the WWTPs (Table 1). Collected samples were transported at 4 °C to the laboratory and stored for a maximum of 3 days at 4 °C prior to the analysis.
Chemicals
The mixture of indicator PCBs (congeners no. 28, 52, 101, 118, 138, 153 and 180) dissolved in isooctane and neat standards of OCPs, including
Distribution of pollutants in sludge and sediments
Concentrations of eight PTMs (As, Cd, Cr, Cu, Ni, Pb, Zn an Hg) and four groups of POPs, including BFRs (HBCD and 11 congeners of PBDEs), OCPs (3 isomers of HCH, HCB, and isomers of DDT and their metabolites DDE and DDD), PCBs (7 congeners) and PAHs (12 congeners), were analyzed in stabilized sewage sludge sampled from 15 different locations across the Czech Republic (Table 2, Table 3, Supplementary Table S1). We also collected sediments from approximately 300–500 m below the WWTP drains. In
Conclusion
Our study demonstrates that statistical correlation of both chemical and ecotoxicological data can provide a detailed understanding of the specific role played by particular pollutants in an ecosystem. We show that some compounds, such as HCH, were present in only small amounts yet proved to have a significant impact on overall sediment and sludge toxicity. To the contrary, PAHs were often present in high amounts, yet proved to be only marginally significant for overall sediment toxicity and
Acknowledgements
Financial support was provided by a grant no. 15-02328S from the Czech Science Foundation. We also acknowledge Dr. Mary-Cathrine Leewis for comments on the manuscript.
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