Sewage Derived Microplastic and Anthropogenic Fibre Retention by Integrated Constructed Wetlands 

Abstract

High loads of microplastics and anthropogenic fibres can be discharged from wastewater treatment plants (WWTPs) into surface water bodies. Integrated Constructed Wetlands (ICWs) are potentially well suited to provide a cost-effective mitigation solution at small WWTPs where conventional treatment is prohibitively expensive. ICWs consist of a series of connected ponds (receiving all of their inflow from WWTP effluent) that are planted with diverse native vegetation, and are thus designed to improve downstream water quality. This study aimed to assess the microplastic and anthropogenic fibre retention efficiency of two ICWs (Northrepps and Ingoldisthorpe) in Norfolk (UK) over a 12-month period. Water samples were collected at approximately monthly intervals from the inlet and outlet of each wetland (n = 54) between June 2022 and May 2023, and fine bed sediment samples were collected from the Northrepps ICW (n = 23). Northrepps ICW received on average 351,588 (± 223,986) anthropogenic fibres day-1, with a retention rate of 99.3 %. No seasonal variation was observed in retention efficiency. Ingoldisthorpe ICW intermittently received anthropogenic fibres in low concentrations, with an average of 11,448 (± 518) day-1 and a retention rate of 100 %. Microplastics and anthropogenic fibres were prevalent in sediment samples of the first cell of Northrepps ICW, averaging 10,090 items kg-1 dry sediment, while none were found at concentrations above the limit of detection in the second or third cell. Of the 369 fibres analysed by ATR-FTIR, 55 % were plastic (dominated by polyester). Of the 140 suspected microplastic fragments analysed by ATR-FTIR, 73 % were confidently identified as plastic (mostly polystyrene, polyethylene, or polypropylene). This study demonstrates how ICWs can effectively retain sewage effluent derived microplastics and anthropogenic fibres. However, the accumulation of plastic waste in ICWs may complicate long term management and their cost-effectiveness. Research into the minimum size of the first cell to ensure that microplastics are retained within a small area of the overall wetland is recommended to improve long term management prospects.