Wastewater treatment plants as a source of microplastics to an urban estuary: Removal efficiencies and loading per capita over one year

Wastewater treatment plants serve to collect and treat wastes that are known to include microplastic (MP; synthetic polymer materials <5 mm in size) and other small anthropogenic litter as particles, fibers and microbeads. Here, we determined the microplastic loads and removal efficiencies of three wastewater treatment plants (WWTPs) with different treatment sizes, operations and service compositions discharging to Charleston Harbor, South Carolina, USA over the course of a year. Overall, we found that MP concentrations (counts per L) varied within a factor of 2.5 in influent and 4.8 in effluent at each WWTP, and that neither concentrations nor removal efficiencies demonstrated a seasonal trend. The largest wastewater treatment plant in the study, which also employed primary clarification, had the highest MP removal efficiency of 97.6 ± 1.2%. The other two smaller facilities had average removal efficiencies of 85.2 ± 6.0% and 85.5 ± 9.1%. We demonstrate through source modeling that microplastic fiber loads in influent were consistent with service area populations laundering textiles given previously published rates of microplastic generation in washing machines. Using measured WWTP flow rates and MP counts, we find a combined load of MPs leaving all three WWTPs with discharged effluent totaling 500-1000 million MPs per day. We estimate from this the emission of 0.34-0.68 g MP per capita per year in treated wastewater, which may only account for <0.1% of plastic debris input to this metropolitan area's surface waters on an annual mass basis when land-based (mis)managed plastic waste sources are also considered. However, the potential for sorption of chemicals present in wastewater to microplastics and their small size, which confers immediate bioaccessibility, may present unique toxicological risks for microplastics discharged from WWTPs.