Ecotoxicological and chemical characterization of selected treatment process effluents of municipal sewage treatment plant

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Abstract

The triolein-containing semipermeable membrane devices (SPMDs) were deployed for 4 weeks in a sewage treatment plant in Beijing, China, to sample and concentrate priority hydrophobic organic pollutants in a sewage treatment process. The chemical analyses and ecotoxicities of the residuals of SPMDs dialysate were examined. The data from the chemical analyses by gas chromatography-mass spectrometry selected ion monitoring mode indicated the lower removal for polychlorinated biphenyls (PCB) congeners and polycyclic aromatic hydrocarbons (PAHs) coincided with the persistence of them in the environment. The acute toxicity examined by bioluminescence test with Vibrio fischeri revealed approximately only 20% decrease in the overall toxicity of the influent after the activate sludge treatment process. The ethoxy resorufin-O-deethylase (EROD) induction with a micro-EROD assay in vitro using H4-IIE rat hepatoma cell cultures demonstrated the presence of persistent organics in influent and sequency effluents. Results obtained suggested that integration of the SPMD technique and chemical analyses and bioassay might be a valuable approach for the risk assessment of hydrophobic organic pollutants in water ecosystem. It revealed the necessity for organic pollutants monitoring and ecotoxicities examining of sewage treatment plants.

Introduction

The risk assessment of organic pollutants in aquatic environments is often based on the concentrations detected by analytical chemistry and data on the toxicity of single compounds. However, all pollutants exert their effects as components of complex mixtures. The utility of a battery of biotests is well established for environmental hazard assessment of chemicals and chemical products and used routinely to evaluate the toxicity of complex mixtures such as industrial wastewaters (Baun and Nyholm, 1996). Because concentrations of pollutants in water are usually too low for most short-term bioassays and chemical analysis, samples are often needed to preconcentrate using different extraction techniques. Semipermeable membrane devices (SPMDs) are one of the extraction techniques.

SPMDs can provide a means of determining the dissolved portion of organic chemicals in aquatic environment by mimicking the transfer of organic contaminants through respiratory lamellae of fish (Huckins et al., 1990). It consists of a thin film of neutral lipid (such as triolein) enclosed in a thin-walled lay-flat tubing made of low-density polyethylene which have been used for passive monitoring of hydrophobic organic contaminants in many kinds of water bodies (Huckins et al., 1993). SPMDs were effective for concentrating trace organic pollutants, such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), and organochlorinae pesticides (OCPs). (Bennett et al., 1996; Huckins et al., 1997; Sabaliunas and Sodergren, 1997; Sabaliunas et al., 2000).

Chemical pollutants in the aquatic environment come from various sources. Both PAHs and PCBs are present in industrial and urban wastewaters carried along the coastal and estuarine waters. Some organic compounds in water are known to be involved in several mechanisms of toxicities. Mixed-function oxygenases (MFOs) are a class of inducible microsomal enzymes capable of a number of modification reactions which can result in the conversion of lipophilic compounds to more water-soluble metabolites (Payne et al., 1987). MFOs can be induced by many organic contaminants, including PCBs, PAHs, chlorinated dibenzodioxinx, and chlorinated dibenzofurans (Hansen and Addisson, 1990; Elskus et al., 1992). Extensive studies have suggested that monooxygenases in liver can serve as biomarkers of exposure to PAH and PCB types of pollutants (Narbonne et al., 1991; Goksoyr and Förlin, 1992; Stegeman and Hahn, 1994) and for assessing the risks of environmental samples in vitro (Pesonen and Andersson, 1992). The bioluminescence inhibition assay with Vibrio fischeri is used worldwide to evaluate the toxicological effect of effluents (Riisberg et al., 1996) and wastewaters (Asami et al., 1996). It is not only described as rapid, sensible, and easy to perform, but also as a low-cost bioassay that can be recommended as a valuable test method in water pollution control (Nohava and Vogel, 1995).

The municipal sewage treatment plant studied in Beijing, China, is the largest and most complex treatment plant in Asia. The treatment plant has been designed as a conventional sludge system with secondary biological treatment, with activated sludge process for average aeration detention time of 6 h. It treats annually approximately 50 million mm3 of wastewater serving a population of about 14 million and receiving both industrial wastewater and domestic wastewaters in the city. It consists of pertreatment (screens, grittraps, preaeration), primary clarifers, aeration tanks, and secondary clarifiers. Since the plant was designed as conventional, effluent standards for settleable solids (SS), total suspended solids, COD and BOD5 have been set up. The treatment efficiency seems good since BOD, COD, and SS in treated sewage do decrease to 5–10% of the untreated sewage. However, the treatment efficacy for trace priority organic pollutants in sewage and their ecological effects have either never been studied or only detected sporadically, which should be of great value considering their potential toxicity and environmental risk.

Chemical analysis provides only part of the knowledge necessary to evaluate and assess their toxic potential for wildlife and humans. This is because the bioavailability of the compounds is not considered and each of the compounds has different cytosolic binding activities (Piskorska-Plisczzynska et al., 1986) and biological activities (Brunström et al., 1991). Moreover, the complex interactions between different environmental chemicals are not completely understood and considered when hazard assessments and predictions of possible ecotoxicological effects are made based on concentrations alone. A toxicity evaluation is an important parameter in wastewater quality monitoring, as it provides the complete response of test organism to all the compounds in the wastewater. Biotoxicity tests could be used in parallel to detect changes in toxicity of the differently treated wastewater samples. The use of selective bioanalytical tools, particularly in connection with chemical analysis, can circumvent these limitations. Therefore, bioassay is a good complement to chemical analyses and a useful tool to predicating the ecological effects to environment.

In this study, triolein SPMD technology was applied to sample and concentrate the priority organic pollutants in a sewage treatment process. Chemical analyses were performed for PCBs congeners, PAHs, and substituted benzenes. Bioluminescence test and EROD induction bioassay were introduced to assess the potential ecotoxicological effects of the selected effluents from the sewage treatment plant. The objective is to connect the chemical analyses components with their ecotoxicity to provide a basis for an integrated pollutant monitoring and assessment method.

Section snippets

Assembly of SPMDs and deployment

Semipermeable membrane devices were prepared using lay-flat polyethylene tubing (from Beijing Plastics Company; 2.5 cm wide and 0.05–0.55 mm thick). The tubing was cut into 21-cm-long segments and immersed in cyclohexane for 24 h to remove any contaminants and dried with ultrapure nitrogen. Forty microliters (36.4 mg) of triolein (99% purity, Sigma Chemical, St. Louis, MO) was pipetted into each piece of tubing and coated onto the inner part of the tubing and then sealed the ends.

Prepared SPMDs

GC-MS analysis

PCB congeners, PAHs, and substituted benzenes and other target contaminants identified in SPMD samples in sewage at each deployment site are shown in Table 1, Table 2. It could be seen that 3 PAHs could be detected in all samples; their concentrations in SPMDs ranged from 1 pg/ml triolein to 44 pg/ml. For PCB congeners, 23 congeners could be found in SPMD samples; their concentrations in SPMDs ranged from a low of 0.6 pg/ml triolein to a high of 439 pg/ml. Eight substituted benzenes, including

Conclusion

Screening of the municipal wastewater treatment plant for hydrophobic organic compounds and EROD induction revealed the presence of the persistent pollutants in influent and following selected effluents, even in final effluent in the sewage treatment plant. The bioassays could be developed as a promising technique for discharge control of the treated wastewater from municipal sewage treatment plant. Regarding ecotoxicity and chemical analysis result of persistent organics, the treatment

Acknowledgments

This work was funded by National Natural Science Foundation of China (40171050 and 20337001) and by Federal Ministry for Research and Technology, Germany (BMBF). The authors greatly thank Professor F.J. Wiebel of Toxicological institute of GSF for his valuable suggestion and discussion on EROD test and results.

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