Permeable pavements: A possible sink for tyre wear particles and other microplastics?

In this study, seven roads and parking lots were sampled by a road surface cleaning truck and approximately 100 kg of particulate material was collected per site. Thereafter, the samples were analysed for microplastics, including tyre wear particles. The analyses revealed that tyre wear constituted 0.09 % of the dry mass of the samples on average. Other plastic types were also identified in the samples, but at on average 49 times lower concentrations compared to tyre wear particles. Although the roads and parking lots were used for residential, industrial, and commercial purposes, no correlation between land use and the total concentrations of microplastics was identified. Of microplastics other than tyre wear particles, polypropylene constituted an important fraction in all samples, whereas other polymers were present at various degrees. The contents of heavy metals, sulphur, and total organic carbon were also measured in the samples, but no correlation between them and microplastics was determined. A back-of-the-envelope estimation indicated that the tyre wear material retained by permeable pavements constituted a non-negligible fraction of the total mass of microplastics released on roads and parking lots. Therefore, permeable pavements can serve as a tool for the management of this pollutant.

The occurrence and fate of microplastics in a mesophilic anaerobic digester receiving sewage sludge, grease, and fatty slurries.

The mesophilic digester of a wastewater treatment plant serving 790,000 inhabitants was analyzed for incoming and outgoing microplastics (MPs). The annual MP load on the digester was 7326 kg y-1 and 1.20 × 1014 N y-1, while the digested sludge contained 4885 kg y-1 and 0.85 × 1014 N y-1. The corresponding mean reduction of approximately 30% was though within the variability of the analyzed samples, and size distributions and polymer composition before and after the digester could similarly neither confirm nor deny if MPs were lost in the digester. The load on the digester corresponded to a per capita load of 0.025 g capita-1 day-1 or 4.14 × 105 N capita-1 day-1. In terms of MP numbers, the values were high compared to most previous studies, which most likely was due to differences in analytical methods, where the present study applied FPA-µFTIR hyperspectral imaging with automatized MP recognition in the size range 10-500 µm and ATR-FTIR on all particles in the range 500-5000 µm. The polymer composition was quite diverse, with 15 identified polymer types, of which polyethylene and polyester were the most common in the sludge, while the variation in composition was larger in the grease that came from the plant's grease trap and the fatty slurries which came from grease traps in the drainage area. The load on the digester was finally used to demonstrate how especially the determined mass loads can be used to set upper boundaries for specific sources, for example human excretion.

A complete mass balance for plastics in a wastewater treatment plant - Macroplastics contributes more than microplastics.

A complete plastic particle mass balance was established at Sweden's second-largest wastewater treatment plant. It comprised material collected at its two bar screens, a 20 mm and a 2 mm one, in the influent water after the 20 mm screen, the effluent water, and the digested sludge. Macro- and microplastics above 500 µm were analysed individually applying ATR-FTIR, while microplastics of 10-500 µm were analysed by µFTIR imaging with automated particle recognition. Masses of plastics >500 µm were determined by weighting, while the mass of the smaller microplastics was estimated from the imaging. The total plastic load on the plant was 202.2 kg d-1, of which the two screens retained 73%. The remaining plastic mass was found in the sludge (13.6%) and the effluent (0.4%). The missing 12.7% could be caused by sampling and measuring uncertainties and potentially also fragmentation below the size detection limit of the analytical approach, or by degradation. The bar screens furthermore retained plastics smaller than the screen size, indicating that this material should be taken into account also when solely looking at smaller particles. The overall treatment efficiency of the plant was high: 99.6% considering both macro- and microplastics, and 98.8% considering only microplastics <500 µm.