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Urban stormwater microplastics – Characteristics and removal using a developed filtration system
Summary
This Finnish study tested a concrete filtration system with sand and biochar media to remove microplastics from urban stormwater runoff. Both media showed effective removal, with results suggesting that on-site stormwater filtration could be a practical strategy for reducing microplastic inputs to receiving water bodies.
Microplastics (MP) in urban stormwater runoff and the management methods for their efficient removal are subjects receiving increasing attention. In addition, the means to decrease the microplastic load in stormwater lacks information. This study presents results for this issue from a Finnish case study thus providing a basis to resolve an emerging environmental problem. A concrete-based filtration system with two comparative fine filtration media (sand and biochar) was built to filtrate stormwater before entering a seawater bay in a separate stormwater sewer network in Helsinki, Finland. The research examines the microplastic quality and quantity of three rain-induced stormwater runoff before and after the filtration process. Due to lack of sampling methods for stormwater runoff, a passive sampling device was designed and tested ex-situ and in-situ. Collected samples were processed based on universal enzymatic purification protocol and analyzed with an FPA-based FT-IR spectroscopy and data analysis. From the data, microplastic particle types, quantity and mass estimations were produced with data analysis using spectra correlation. The results indicate that the stormwater runoff contained 29 MP particles (90 micro-m – 5 mm) per litre of stormwater runoff although the concentration varied greatly between the three rain events. Both sand and biochar as fine filtration media removed significantly microplastics from influent stormwater. The results indicated that sand filtration was able to remove up to 96 % of microplastics from stormwater runoff and biochar 93 %. Polyethylene (PE) and polypropylene (PP) accounted for 99 % of all microplastic types in all samples from the three rain events. The average polymer mass was 14.8 micro-g/l for influent stormwater, 0.2 micro-g/l after sand filtration and 1.9 micro-g/l after biochar filtration. To more accurately assess urban stormwater runoff microplastic quality and quantity, regular monitoring from rain events through the filtration system is suggested. However, the received data and results from this study are important information for follow-up studies.