Stuck in the Sludge : A Study on Microplastics in Sewage Sludge across Swedish Wastewater Treatment Plants

Sammanfattning

Wastewater treatment plants (WWTPs) are a notable source of microplastic (MP) release into the environment, with MPs present in domestic sewage, industrial effluents, and stormwater. Although WWTPs retain a significant portion of MPs, some are still discharged in treated effluents and sludge, whichis often reused in agriculture. Thus, WWTPs represent a critical pathway forMPs into marine, freshwater, and terrestrial ecosystems. This study aimed to understand the fate and distribution of MPs in sewage sludge and assess the potential release of MPs into soils through sludge application. Sludge samples were collected in triplicate from seven WWTPs in the Mälardalen region, Sweden, and analysed by FTIR imaging with siMPle software. Sixteen MP types were detected, dominated by Polyester, Polypropylene (PP), and Polyethylene (PE). The total average count was 1’311 ± 102 MPs/g total solids. Variations in MP abundance and mass were analysed related to industrial inputs, external sludge sources and polyacrylamide use in sludge digestion. Larger WWTPs showed better MP removal, and finer primary screening meshes correlated with smaller MPs in sludge. Of the 15’365 tonnes of REVAQ-certified sludge applied annually to agricultural soils by these WWTPs, 0.1% consists of MPs, which is a small but environmentally significant fraction highlighting ongoing MP accumulation risks in soils. This study offers a robust method for MP quantification and calls for improved removal technologies, standardized methods, and stricter sludge reuse regulations to reduce environmental contamination.