Microplastic Mitigation in Urban Stormwater : Assessing Horizontal-Flow Filters of Bark, Biochar and Sand as Remediation Strategies

Abstract: Urban stormwater has been suggested as a significant vector through which microplastics (MPs) reach aquatic environments. Currently, only a small fraction of stormwater in Sweden undergoes any form of treatment. Therefore, a considerable portion of the runoff reaches and pollutes recipients with microplastics, which have demonstrated adverse effects on aquatic life. In recent years, there has been a growing interest in the decentralized treatment of stormwater, shifting the treatment of stormwater upstream and close to the runoff source through filtration-based techniques. Filtration-units can be incorporated into urban environments prone to high runoff coefficients– such as roads, parking lots, and other paved surfaces. At present, there is an opportunity to also shape decentralized treatments towards sustainable and environmentally positive approaches, by utilizing forestry by-products as filter media. Due to the promising characteristics of large specific surface areas and high porosity, the carbon-rich bark and biochar materials have been suggested as potential filter media. This thesis aims to evaluate MP pollution in urban environments, with the city of Uppsala as a case study and parking lots as sampling sites. Then, the thesis assesses the current capabilities of removing MPs in traditional sand media, followed by an evaluation of the potential of bark and biochar, as filter media for removing MPs in stormwater.Road dust and spiderwebs were sampled from parking lots in Uppsala City, to assess the ground-based and airborne MP pollution. A wide range of MP types were identified, such as polyurethane (PU), polypropylene (PP), polyester (PET), Acrylic (PMMA), polyethylene (PE), polystyrene (PS), polyamide (PA), and cellulose acetate. The PU was especially dominant in the indoor parking and PET fibers were commonly found to be airborne. Overall, the road dust and spiderwebs contained 5.78–4951 and 2500–505000 MPs/g, respectively. Horizontal-flow filters of 25, 50, and 100 cm were established to evaluate the removal efficacy of MPs using sand, bark and biochar as filtration media. The filters were introduced to a heterogeneous mix of MPs, including PET and PP fibers, spherical PA, PE and PS, and fragmented PP, representative of the polymer types identified in Uppsala city. The analysis revealed a substantial removal, >97%, of individual MPs for all filter media, achieving an overall MP retention exceeding 99%. The high removal was achieved even with filter lengths of 25 cm, with a slight increased removal with increased filter length. This thesis has proved that MPs are present in a multitude of surfaces in the urban environment, including air, and showed that sand, bark, and biochar should be considered as potential media for filtration-based remedies to improve MP mitigation in urban stormwater.