Influence of wastewater treatment plant discharges on microplastic concentrations in surface water
Graphical abstract
Introduction
Worldwide plastic production has been growing since 1950 (Plastics Europe, 2013). Consequently, millions of tons of plastics enter oceans and landfills each year (Gourmelon, 2015). All oceans have been affected by plastic pollution (Wright et al., 2013). Plastics entering aquatic environments have a wide size distribution, ranging from micrometers to meters (Hidalgo-Ruz et al., 2012). Microplastics are defined as plastic particles smaller than 5 mm (Arthur et al., 2008) derived from larger particles (secondary microplastics) or manufactured in small size (primary microplastics) (Hidalgo-Ruz et al., 2012). In the United States it is estimated that 8 trillion microplastic beads enter the aquatic environment daily (Rochman et al., 2015). Due to slow rates of plastic degradation, microplastics persist in the environment (Eerkes-Medrano et al., 2015). The presence and consequences of microplastics in the marine environment have been studied since 1970 (Carpenter et al., 1972). However, less is known about microplastic abundance in the freshwater environment (Eerkes-Medrano et al., 2015).
Accumulation of microplastic in lakes (Faure et al., 2012, Eriksen et al., 2013), estuaries (Sadri and Thompson, 2014), and rivers (Lechner et al., 2014, McCormick et al., 2014) has been reported. There is a high concentration of microplastics in WWTP influent (104 to 105microplastic/m3) and incomplete removal during the treatment process (70–100%) can result in microplastic pollution in the receiving water (Magnusson and Wahlberg, 2014). WWTP effluent has been identified as one of the sources of microplastics in the freshwater environment (Magnusson and Norén, 2014). WWTP effluent resulted in an increase in the concentration of microplastic in Chicago River (McCormick et al., 2014). However, the cumulative impact of WWTP effluents along a river has not been demonstrated. In addition, insufficient removal of microplastics <300 μm in WWTPs has been reported (Magnusson and Wahlberg, 2014). Further, most microplastic studies focus on plastics larger than 330 μm, overlooking the smaller size classes which are potentially important sources of microplastic pollution in the freshwater environment.
The objective of this study was to investigate the abundance of microplastic and the impact of municipal WWTP effluents on the microplastic concentration in the Raritan River. In this study, the presence of microplastics in a wider size range (125 μm-2mm) is reported. Based on morphology, microplastics were categorized into primary and secondary groups to aid in identification of the sources of microplastic contamination. Moreover, correlations between distances downstream and microplastic concentration were tested to provide insight into the fate and transport of microplastic in the river environment.
Section snippets
Sampling
Sampling was performed on the Raritan River, located in central New Jersey (NJ), US (Fig. 1). The river basin covers 2850 km2 and provides water for drinking, irrigation, agriculture, recreation, and industry. The River has two branches, north and south, that meet then flow into the Raritan Bay. The primary land use of the river main stem is urban and suburban, (51.3%) and the primary land use of the south and north branches are agricultural and forest (61.3%) (Newcomb et al., 2000). More than
Results
Microplastics of all size categories were observed in all samples, including those collected at the background site (Fig. 3). Of the four major WWTPs sampled, microplastic concentrations increased downstream of WWTP-A1, A2, and M/C in the 125 μm size category (p = 0.024–0.048), and for WWTP-A2 and M/C in the 250 μm size category (p = 0.0012–0.0052) compared to upstream samples. No differences were observed in concentration of microplastics in samples collected downstream of the WWTP discharge
Discussion
This study provides insight into the impact of wastewater treatment plant discharges on the prevalence of a wide size distribution of microplastics in the freshwater environment. Increases in microplastic concentrations were observed for the 125–250 μm and 250–500 μm size categories downstream of select wastewater treatment plants on the Raritan River. This result is consistent with results of McCormick et al. (2014) who observed microplastic concentrations (including fibers, which were not
Conclusions
This study demonstrated the spatial pattern of primary and secondary microplastics in the Raritan River. The results of this study indicated that microplastic concentration in select size categories, particularly primary microplastics, increased downstream of several WWTP outfalls. Additionally, the presence of microplastics at the background location showed that WWTPs are not the only source of microplastic contamination in the river. Correlations between distance from the Raritan Bay and
Acknowledgements
Thanks to Alessia Eramo and Johnny Quispe for sampling assistance. This research was supported by university start-up funds to NLF, a Presidential fellowship to SE, and a NJ Water Resources Research Institute Graduate Student Grant-in aid.
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