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Seeking Potential Anomalous Levels of Exposure to PCDD/Fs and PCBs through Sewage Sludge Characterization | OMICS International
ISSN: 2155-6199
Journal of Bioremediation & Biodegradation

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Seeking Potential Anomalous Levels of Exposure to PCDD/Fs and PCBs through Sewage Sludge Characterization

Rada EC1*, Schiavon M1,2, Ragazzi M1
1Department of Civil Environmental and Mechanical Engineering, Via Mesiano, University of Trento, Italy
2Fondazione Trentina per la Ricerca sui Tumori, Corso III Novembre 162, I-38122–Trento, Italy
Corresponding Author : Rada EC
Department of Civil Environmental and Mechanical Engineering
University of Trento, Via Mesiano 77
I-38123 Trento, Italy
E-mail: [email protected]
Received September 02, 2013; Accepted October 21, 2013; Published October 27, 2013
Citation: Rada EC, Schiavon M, Ragazzi M (2013) Seeking Potential Anomalous Levels of Exposure to PCDD/Fs and Pcbs through Sewage Sludge Characterization. J Bioremed Biodeg 4:210. doi: 10.4172/2155-6199.1000210
Copyright: © 2013 Rada EC, et al. This is an open-a ccess article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Abstract

An approach to detect anomalies in the exposure to Persistent Organic Pollutants (POPs) throughout the food chain is presented. The proposed method is useful also for preventing soil contamination by POPs that would require a remediation intervention. A steel making plant and its surrounding area were selected as a case-study. To investigate the possible effects of the plant on the settled population, sewage sludge samples from four wastewater treatment plants (WWTPs) were taken: one of these was chosen as reference for the population exposed to the emissions of the mill; the remaining three plants were chosen to provide background information about the POP content in sludge. No clear anomalies in dioxins (PCDD/Fs) were detected for the potentially exposed population. In terms of Polychlorinated Biphenyls (PCBs), the steel plant-influenced WWTP showed a total concentration between 2.7 and 4.8-time higher than the other plants; in terms of equivalent toxicity, only slightly higher concentrations were found for the steel plant-influenced WWTP. Therefore, if considering acceptable the daily intake from the diet of the unexposed population, the absence of a dioxin and dioxin-like emergency in the area of the mill is demonstrated. This method represents an innovative and technically simple tool to assess situations of permanent exposure to POP levels that are higher than the background.

Keywords
Pcbs; PCDD/Fs; Pops; Exposure; Sewage sludge; Diet; Food chain
Introduction
Within the large family of Persistent Organic Pollutants (POPs), organochlorine substances like polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and polychlorinated biphenyls (PCBs) are considered the most toxic and widespread compounds [1]. PCDD/Fs are commonly named dioxin, and are composed of a total number of 210 congeners, who’s the most toxic for humans and animals could be grouped in 17 congeners: 7 for PCDDs and 10 for PCDFs [2]. PCBs are known to be composed of a total of 209 congeners, but the most toxic ones are the co-planar PCBs, which are 12, and present a toxic behavior similar to PCDD/Fs; for this reason, these congeners are named dioxin-like PCBs. The exposure to such compounds has been object of important concern in the last decades [3], especially in the light of their health effects. Acute toxicity does not represent the most important hazard; the primary risk is related to chronic exposures at lower concentrations. 2,3,7,8-TCDD, the most toxic PCDD/F congener, was classified as carcinogenic to humans by the International Agency for the Research on Cancer (IARC). This compound is the most known cancer promoter, since increased risks for lung cancer, soft-tissue sarcoma, non-Hodgkin lymphoma and other malignant neoplasms were reported in several cohort studies [4,5]. There is also clear evidence that PCBs cause cancer in animals, especially liver and thyroid neoplasms [6,7]. In addition, the results of a number of epidemiological studies raise concerns for the potential carcinogenicity of PCBs on humans [7]. For these reasons, the IARC classified PCBs in the Group 2A, as potential carcinogenic to humans [8].
More than the 90% of the average dioxin daily intake is estimated deriving from food consumption, primarily dairy products, followed by cereals and vegetables, meat and fish [9,10]. In fact, POPs have lipophilic properties and bioaccumulation represents the prevailing way of contamination of the food chain: atmospheric deposition of POPs coming from various sources (e.g. waste treatment plants, production of chemicals, metal industry) firstly contaminates soil, hay, vegetables and fruit [11]; contaminated soil and hay transfer the accumulated POPs to the adipose tissues of the cattle, and following the consumption of meat and dairy products to the humans, whilst contaminated vegetables and fruit may transfer their POP content to the cattle, but also directly to humans. Thus, it is clear the importance of methods for preventing the alteration of soils by POPs, avoiding remediation interventions. The absorption of PCDD/Fs and PCBs from the diet can result higher than 80% in situations of elevated POP intake [12]. The absorption is followed by excretion from the body, which does not depend on fluctuations of the concentration in the diet, but only on the concentration in the body: in fact, the faecal elimination occurs continuously at a rate that depends on the blood concentration [12]. The body, thus, seems to be able to equalize the effects of peak intakes of POPs.
On European scale, it was estimated that steel making plants would constitute the most important source of PCDD/Fs [13]. The metal sector is also an important PCB source, as demonstrated by a study on scrap metal recycling plants, whose wastewater showed a maximal dioxin-like PCB concentration of 3.61 ngI-TEQ L-1 [14].
After entering the human body and leaving the organism by excretion, POPs reach the wastewater treatment plants (WWTPs) and concentrate in sewage sludge. Sludge contamination is then object of great concern, if the reuse of sewage sludge in agriculture is an option [15]. Two important studies focused on the mass balance of POPs throughout the human body [16,12], showing the significant role of bioaccumulation.
The presence of organic [17,18] and inorganic [19,20] hazardous substances in sewage sludge has been object of investigation in several studies. Sewage sludge has also been studied as potential source of energy [21-26], and as raw material for conventional [27,28] and nonconventional products [29,30]. In addition to these applications, sewage sludge can be also used as source of information to seek potential anomalous levels of exposure of a population to POPs: indeed, the dominant exposure route of POPs released by the most important sources (e.g. steel making plants) is the emission into the atmosphere, the atmospheric deposition to farmlands and pastures, the consequent contamination of the food produced, the intake by humans, the excretion, the transportation to WWTPs and the POP concentration in sewage sludge; thus, analyses on the POP content of sludge samples offer an alternative, inexpensive and technically simple approach to assess the existence of critical situations of exposure to POPs. Several studies focused on the ambient air and deposition monitoring in areas where steel making plants were present and highlighted their evident influence in terms of PCDD/F contribution in the surroundings [31-33]. The present study is then intended to propose a methodology, in order to detect anomalies throughout the food chain, as a consequence of the release of POPs in air from significant sources and their subsequent deposition to farmlands and pastures.
This study focuses on a steel making plant located in a West-East oriented Alpine valley in the North of Italy. Three domestic WWTPs located outside the area of influence of the plant, were chosen as background reference for assessing the content of POPs in their sewage sludge. Another domestic WWTP, which receives the wastewater from the villages located in the vicinity of the steel making plant, was chosen as representative of the population exposed to the intake of POPs released by the mill. One sewage sludge sample was taken from each WWTP. The samples were analyzed and the results are discussed, in order to understand the potentialities of this methodology in assessing the presence of critical levels of exposure to POPs. In addition, a sensitivity analysis of the method is presented and the results are interpreted by moving from the considerations presented [12], and from the findings of a previous study, which reported that a permanent exposure to normal levels of PCDD/Fs, following a higher exposure in the past can produce a PCDD/F excretion that is twice higher than the intake [34]. Considered the limited number of samples, the application of this approach to a specific case study has the purpose of explaining how this methodology can be applied in different contexts, with a sufficient number of samples to perform a statistical analysis and better interpret the results.
Experimental Section
To highlight the effect of PCDD/F and PCB emissions from the steel making plant on the food chain and to reconstruct the fate of these compounds into the environment, an analysis of four sewage sludge samples from four different WWTPs was performed. One of them was chosen as reference to assess the direct impact of the emissions from the steel making plant on the food chain of the population that lives inside the area of influence of the mill. The choice of this WWTP started from previous dispersion simulations of PM emitted by the plant, whose results allowed the detection of the population potentially exposed to contamination, through direct inhalation and assumption via food intake after deposition [35]. The region surrounding the plant hosts several cultivated lands, cattle and dairy farms. The consumption of locally produced food was assumed equal to 10% of the total diet of the local population, according to a previous study [36].
The steel plant-influenced WWTP located inside the area of influence of the steel making plant, was finally chosen from those plants whose catchment area includes the villages directly exposed to the emissions, according to the dispersion simulations [35]. The input for this WWTP is composed for 75% by the wastewater from the potentially exposed population (about 12,670 inhabitants); the remaining 25% is composed by wastewater from other residential unexposed populations [37]; moreover, the WWT line receives liquid streams from the thermal drying of provincial sewage sludge that can be supposed poor of dioxin. In fact, the liquid streams are dominated by the liquid phase, unlike domestic wastewater that has a higher content of suspended solids (or organic matter): indeed, a positive association between dioxin levels and suspended solids was found in a previous study, which supports the theory that dioxin-like compounds will partition almost exclusively onto the organic matter in sludge in preference to water [38].
As a term of comparison, three more WWTPs were chosen as representative of the population living outside the study area (about 39,690 inhabitants), hence providing background information on the PCDD/F and PCB levels in the food chain. Similarly, these three plants were chosen so that their catchment areas include villages located outside the area of influence of the plant and far from significant emitters. The consultation of the local emission inventory excluded the presence of other important sources of POPs. Locations of the WWTPs and their distances from the steel making plant are presented in Figure 1.
All the WWTPs here considered are equipped with an oxidation stage and a secondary sedimentation tank; the sludge is then conditioned with the addition of polyelectrolytes and is sent to mechanical dehydration. The choice of taking sludge samples instead of wastewater samples is related to the fact that POPs are more concentrated in sludge than in water. As a matter of fact, the activated sludge treatment that is applied to WWTPs removes hydrophobic compounds like PCDD/Fs and PCBs from the wastewater by sorption to sludge [39]. The four sewage sludge samples (sample volume of 2 l each), one for the steel plant-influenced WWTP and three for the background WWTPs, were taken on the 12.12.2011, between 9 am and 11 am. The choice of this period of the year is crucial and is due to the need of avoiding contributions from tourist peaks in the region, which usually occur during summer or during Christmas holidays. In fact, by taking the sludge samples during the low season, only the contribution of the resident population is counted. If sampling had been carried out during a tourist period, the results would have shown the diluting effect of a differently exposed foreign population. Since POP levels in sewage sludge change slowly, due to the slowness of the process of accumulation in the food chain, the number of samples taken at each plant is not influent on the characterization of the population exposure in a specific period (tourist periods excluded). On the other hand, primary importance should be given to the choice of the WWTPs that must be representative of the exposed and the unexposed population and must clearly distinguish between them.
The four sludge samples were taken before the chemical conditioning with polyelectrolytes. Since polyelectrolytes are synthesis products, the possibility exists that such substances may contain dioxin compounds, and thus, they may increase the dioxin content of the unconditioned sludge. The choice of sampling sewage sludge before conditioning gives the certainty that no additional sources of dioxin (other than anthropic input) influenced the sludge samplings. Furthermore, cross-contamination was prevented by the adoption of glass vases as containers for the samples. Hence, the only possible source of dioxin and dioxin-like compounds could be represented by the indoor air, whose quality is kept under control and possible dioxin levels are anyway negligible if compared with the concentration in the samples.
The moisture of the samples was detected by calculation of the dry residual mass after evaporation at 105°C, according to the EN 14346:2007 method [40]. The content of PCDD/Fs was measured in accordance with the EPA 1613 method: the sample was extracted in a Soxhlet/Dean-Stark (SDS) extractor; the extract was concentrated for cleanup and 37Cl4-labeled 2,3,7,8-TCDD was added to measure the efficiency of the cleanup process; the extract was then concentrated to near dryness and injected into the gas chromatograph (GC) after addition of internal standards; the analytes were detected by high resolution mass spectrometry (HRMS); isotope dilution and internal standard techniques were used for quantitative analyses, according to the congeners to be measured [41]. The content of dioxin-like PCBs in the sludge samples was determined following the EPA 1668B methodology: the sample was extracted by an SDS extractor and the extract was cleaned up by back-extraction with sulphuric acid; the extract was concentrated to 20 μL and charged with internal standards; the analytes were detected by HRMS and quantitatively measured by isotope dilution [42].
Results and Discussions
The results of the analysis on the four sludge samples from the WWTP taken as reference for the population potentially exposed to contamination (A) and for the three background WWTPs (B1, B2 and B3) are presented in Table 1 and 2, for PCDD/Fs and PCBs, respectively. For some PCDD/F congeners (e.g. 2,3,7,8-TCDD and 1,2,3,7,8,9-HxCDF), the concentration measured in sewage sludge is lower than the instrumental detection limit (DL) for all the WWTPs considered. Conventionally, concentrations below the DL are assumed as half the DL itself (Table 1 and 2).
As the results show, the total PCDD/F concentrations measured at the steel plant-influenced WWTP (A) are slightly higher than those measured elsewhere, since the congeners 1,2,3,4,6,7,8-HpCDD and OCDD are predominant. Moreover, such compounds are two of the most important congeners that characterize the emissions from an electric arc furnace [43], which is the technology adopted by the steel making plant under investigation. On the other hand, in terms of total WHO-TEQ concentrations, the A plant shows the lowest dioxin content with respect to the others. This behavior is due to the fact that the toxicity of the dominant congeners (1,2,3,4,6,7,8-HpCDD and OCDD) is lower than the others (Table 1). Considering that the WHO-TEQ concentrations are related to the toxicity for humans, no anomalies in PCDD/Fs were detected in the food chain for the potentially exposed population.
These results are in agreement with the fact that the PCDD/F deposition in the surroundings of the steel making plant, in terms of WHO-TEQ values, is substantially low and can be considered similar to that normally found in rural areas, as demonstrated by a monitoring campaign carried out between 2010 and 2011 [44]. The use of wood as a source for domestic heating, which is typical of mountainous regions where this material is abundant, may have had a certain influence in making the results of the four samples comparable. The PCDD/F concentrations found in the samples are within the range 1.2–15.3 ng WHO-TEQ kg−1 DM measured in a previous study on Australian WWTPs [45], and are lower than the average concentrations found in two German WWTPs, expressed in terms of International Toxic Equivalency (25 and 10 ng I-TEQ kg−1DM) [46].
With regard to PCB concentrations, some considerations that slightly differ from the case of PCDD/Fs can be made: the total concentration at the A plant is between 2.7 and 4.8-time higher than the other plants; PCBs are typical compounds emitted by steel making plants [14], and this can explain such a finding. The same sludge sample shows a WHO-TEQ concentration that is higher than the B1 and B2 samples, but is similar to the B3 sample, where the concentration of the most toxic congener (PCB 126) is also higher. The different urbanization, compared to the B1 and B2 plants, can explain this aspect, since the B3 case is characterized by a moderate presence of small industrial activities.
Some considerations about the sensitivity of the methodology can also be expressed. If considering that 75% of the stream is composed by wastewater from the exposed population, the ratio between the POP concentrations visible in the sewage sludge after and before the increase of exposure (ΔPOPsludge) can be calculated by the following equation:
                                       (1)
Where ΔPOPfeces is the ratio between the POP concentrations occurring in the feces after and before the exposure increase and ΔPOPother is the ratio between the POP concentrations (after and before the exposure increase) in the sludge from the unexposed population whose wastewater is collected at plant A.
In conditions of permanent exposure, ΔPOPfeces is considered equal to the ratio between the intakes by the human body after and before the exposure increase (ΔPOPintake) [12], which is the sum of the ratio between the POP concentrations in the locally produced food (ΔPOPfood,loc), the ratio of the POP concentrations in the non-locally produced food (ΔPOPfood,nloc) and in air (ΔPOPair) after and before the exposure increase:
          (2)
Where Cfood,loc is the percentage of the consumption of locally produced food on the total diet, Cfood,nloc is the percentage of the consumption of non-locally produced food, IPOP,food is the percentage of POPs taken in by food consumption, and the remaining part is attributed to the intake of POPs by inhalation (IPOP,inhal). Cfood,loc is assumed equal to 10%, according to Cernuschi [36]; consequently, Cfood,nloc is equal to 90%; following the findings of [9], IPOP,food is assumed equal to 90%, and consequently, IPOP,inhal is equal to 10%. If considering ΔPOPintake only addressed to the consumption of local food, ΔPOPfood,nloc and ΔPOPair are equal to 1. Thus, by applying (2) an increase of concentration of 100 times in the locally produced food (ΔPOPfood,loc=100) would result in an increase of 9.91 times in the POP content in feces (ΔPOPfeces=9.91). If assuming the POP concentration in the sludge from the unexposed population whose wastewater is collected at plant A as unvaried (ΔPOPother=1), by applying (1), a 7.7-time higher concentration should be observed in the sewage sludge at the steel plant-influenced WWTP.
In this specific case, the assumption of permanent exposure cannot be made, since in 2009, the steel making plant significantly modified the off-gas treatment line in order to decrease its emissions. In this case, the sensitivity of the method can be assessed by considering that, in conditions of permanent exposure to normal levels following a higher exposure in the past, the PCDD/F excretion can result twice higher than the intake [34]. In this case, ΔPOPfeces=2ΔPOPintake, and if considering a past concentration in the locally produced food that was 100 higher than the background (ΔPOPfood,loc=100), ΔPOPsludge would be approximately equal to 15.
It should be taken into account that the 2.7-4.8 time higher PCB concentrations found at the A plant can be also explained by the higher exposure that occurred in the past.
On the other hand, the method is not able to detect acute episodes of exposure, since as previously stated, in such situations, even only 20% of the assumed POPs can be excreted [12]: in this case, ΔPOPfeces=0.2 ΔPOPin and ΔPOPsludge would become comparable with that one achievable in permanent conditions, if the concentration in the locally produced food is 12-time higher than the background, thus giving misleading results.
Given that the contribution of food consumption in the intake of POPs is dominant and assuming acceptable, the daily intake of the population living in the areas B1, B2 and B3 (for the absence of significant sources of POPs), the fact that the concentrations of POPs in the sewage sludge at the A plant are similar to those found in the unexposed areas allows excluding the absence of a dioxin and dioxinlike emergency in the area of the steel making plant, even if the presence of the steel making plant is visible by the detection of some congeners characteristic of the emissions from electric arc furnaces. The positive results can be attributed also to the technology adopted, as the arc furnace has a less impact in terms of dioxin emissions with respect to other technologies [13].
Possible wastewater contaminations can derive from surface runoff, but this contribution was found to be limited to about 10% in a previous work [46]. Laundry washing is an additional source of PCDD/Fs, and its contribution can be comparable with the one of domestic wastewater [46]. However, if considering the habits unchanged over the years, the contribution of laundry washing can be considered constant; thus, a hypothetical 100-time higher dietary exposure would be anyway visible in terms of POP concentration in sludge.
Conclusion
A novel, inexpensive and technically simple methodology to detect anomalies in the exposure to POPs throughout the food chain was presented. This methodology, suitable also for preventing soil contamination, was applied to the case of a steel plant and the residential population living in the surroundings, but the approach can be applied to any case where a population is exposed to contamination by POPs. Given the limited number of samples, the application to the present case study has the purpose of better explaining how the methodology works, without providing definitive results on the exposure assessment of the target population. For future applications and for a better reliability and solidity of the method, increasing the number of samples at every WWTP is necessary. Three samplings under the same conditions at each site will allow a statistical analysis of the results.
In spite of the limited number of samples taken in this study, no anomalies in PCDD/Fs were detected for the sludge samples representative of the potentially exposed population, giving credit to the favorable results obtained during a previous deposition monitoring campaign. In terms of PCBs, the WWTP taken as reference for the exposed population showed a total concentration between 2.7 and 4.8-time higher than the other plants; in terms of WHO-TEQ concentrations, only slightly higher PCB levels were found for the WWTP representative of the exposed population, even though the most toxic congener (PCB 126) presented a higher concentration at the B3 plant. In order to understand the meaning of the 2.7-4.8 time higher PCB sludge concentrations found at the A plant, it must be remarked that in 2009 the steel making plant adopted the Best Available Technologies for the air pollution control system, and the levels nowadays measured can still incorporate the effects of the higher exposure occurred in the past. However, considering acceptable the daily intake from the diet of the population living in the areas not affected by the steel plant, the absence of a dioxin and dioxin-like emergency in the area of the steel plant can be deducted by the similar concentrations of PCDD/Fs and PCBs in the sewage sludge samples.
The method is capable of well assessing situations of long periods of exposure of the population to POP levels that are higher than the background, even if difficulties in the interpretation of the results can occur in presence of acute episodes.
Acknowledgements
The Authors wish to thank the Fondazione Trentina per la Ricerca sui Tumori, and especially to the De Luca family for the support to this research. The Authors are also grateful to the Environmental Protection Agency of the Province of Trento (APPA), and especially, Mr. Maurizio Tava for his precious contribution. Special thanks to Dr. Paola Foladori for her important support in the premises of this activity, Ms. Roberta Villa and Mr. Alessandro Chistè for their assistance in the sampling campaign, Mr. Paolo Andreatta for the precious data about the wastewater treatment plants, Dr. Werner Tirler and his research group for the analysis of the samples.
 
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