Microplastics are commonly defined as plastic debris ranging in size from 1 μm to5 mm. They have been studied in marine and coastal waters since the early 1970s.Several studies have reported that microplastics in the marine environment originatefrom land-based sources and are released with stormwater and wastewater.For this reason, increasing attention is being paid to urban water systems. 

In this project, the pathways of microplastics from terrestrial to aquatic environmentswere investigated and mapped. Special attention was paid to severalaspects of urban stormwater. A conceptual model of a city was developed, toillustrate and map the flows of microplastics in the city and identify the measuresthat could be taken to control these. The following research questions were investigated:1) How does the most common plastic (macro) litter from streets break downinto microplastic particles? 2) What types and concentrations of microplastics arefound in urban stormwater from different catchments? 3) What types and concentrationsof microplastics are retained by commonly used stormwater treatmentfacilities? 4) What types and concentrations of microplastic particles are retainedby, and found in effluents from, on-site and small-scale wastewater treatmentfacilities? 5) Where are microplastics found in urban areas, and what measures canbe taken to control microplastic pollution? 6) How do local public actors view theirown responsibilities with regards to microplastics in stormwater, and which othersocietal actors do they perceive to be responsible? 

The results showed that microplastics concentrations in stormwater runofffrom the road, the parking lot, and the roof top ranged between 267-11400 N/L,95-1690 N/L and 467-1220 N/m3, respectively (where N is the number of particles).The three most common polymer types at all three sites were polypropene (PP) >polyethene (PE) > polyesters (including PET). However, it is reasonable to assumethat other types of microplastics were present in high concentrations, althoughthese were not detected with the applied analytical method: these include tyreand road wear particles, which were not detected because black particles were notincluded with the analytical technique applied to these samples. Concentrations ofmicroplastics in sediments from stormwater gully pots and bioretention systemsvaried between 720-25300 N/100 g dry matter (DM) and <9-17300 N/100 g DM.The four most common microplastics in both plant types were PP, EPDM rubber,ethylene vinyl acetates (EVA), and polystyrenes (PS), and a large fraction of theparticles were black. Four litter items commonly discarded in the urban environmentwere exposed to UV light for up to 56 days, corresponding to approximately2 years of UV radiation in Sweden – a plastic bag (PE-LD), chocolate bar wrapper(PP), a plastic coffee cup lid (PS), and a bottle (PET). The results indicated cleardegradation of PS, PP, and PET, and an increased release of microplastics withlonger exposure times. For the PE-LD item (a grocery bag), degradation due to UVexposure was not observed over the exposure times used because the numbers ofparticles released from exposed and unexposed (control) samples were in the sameorder of magnitude. The emissions of microplastics in the model city wereestimated as 7.2 kg/year originating from treated wastewater and 1kg/year for combined sewer overflows. The estimated load to stormwater was 13 000-17 000kg/year for microplastics and 2 100 kg/year for tyre wear particles. The largestsources were cigarette butts, followed by paint and tyre wear for stormwater andlaundry for wastewater. Tap water, roof runoff, and dust made small contributions.Most of the actors who were identified as having a responsibility could influenceemissions to stormwater, either by influencing the introduction of microplasticsinto society or by affecting the emissions of microplastics to stormwater. Theconcentrations of microplastics in the different greywater treatment plants variedwidely; from non-detected to 1100 μg/L, 130 μg/L, 1000 μg/L, 150 μg/L for PS, PVC,PET and PA respectively. The concentrations in the outgoing water were generallylow, indicating relatively good treatment efficiency. 

The results in this report can be used to identify which plastics are present indifferent parts of the urban environment and will facilitate further efforts to identifyupstream pollution sources. The flow analysis gives an overview of the flowsof microplastics at a city level, highlighting larger and smaller flows, and can beapplied to other cities with different characteristics. 

There are still several uncertainties when estimating sources of microplastics,and the polymers found in the samples were sometimes not consistent with whatwould be expected based on the source estimates. This raises the question ofwhether some sources have been missed, while others might be overestimated.Future research should include field studies of additional treatment techniquesand stormwater from catchments with other land use patterns, and cover all seasonsof a year. Such studies should also include black particles and tyres and roadwear particles in the microplastics analysis, since these represent a significantfraction of the microplastics released from the urban environment.