Microplastic removal and environmental emissions from municipal wastewater treatment plants
Simone Cavazzoli, Francesca Murari, Massimo Donegà, Werner Tirler, Gianni Andreottola
Abstract
Wastewater treatment plants (WWTPs) play a crucial role in removing microplastics (MPs) originating from urban areas. However, despite high removal efficiencies, WWTPs systematically release MPs into the environment through effluents. Beyond their direct ecological impact, MPs can act as carriers of micropollutants and pathogens. This study quantifies and characterizes MPs, including polyethylene, polypropylene, polystyrene, polyvinyl chloride, polymethyl methacrylate, and bio-based polymers, in the influent and effluent of five municipal WWTPs employing different treatment technologies. The selected WWTPs include a conventional activated sludge (CAS) system, a moving bed biofilm reactor, two plants with tertiary filtration (pile cloth and stainless-steel filters), and a hybrid CAS/membrane bioreactor (MBR). The study aims to systematically assess MPs removal efficiencies by applying a standardized method for sampling, preparation, and analysis. In-situ sampling was conducted using a pumping and filtration system capable of capturing MPs as small as 2 μm. Samples were treated with Fenton oxidation and density separation to eliminate potential interferences before polymer identification via thermal desorption gas chromatography-mass spectrometry (TD-GC/MS). MPs concentrations varied across WWTPs but remained within a relatively stable range (215.0–761.5 μg/L), corresponding to daily MP loads of 1.2–8.8 kg entering the plants. Polyethylene was the most abundant polymer in wastewater. Removal efficiencies varied by treatment technology, with MBR and tertiary filtration achieving the highest removal rates (>99 %), unexpectedly followed by the CAS WWTP, which exhibited an MPs removal efficiency of approximately 99 %. Per capita MPs loads ranged from 26.1 to 296.8 mg/day in influents and 0.2–8.3 mg/day in effluents, depending on plant capacity and daily load. Despite high removal rates, WWTPs continue to discharge significant MPs quantities, particularly given that major polymers such as polyethylene terephthalate and polyamides were excluded from this analysis. This highlights the need for continuous MPs monitoring and improved removal strategies. Additionally, this study did not investigate sludge samples. However, MPs removal in WWTPs is strongly associated with their accumulation in sewage sludge, raising concerns about their fate during sludge disposal and potential environmental impacts. Further research on MPs present in both WWTP effluents and sludge is essential to fully assess their risks to human health and ecosystem integrity.