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MPs can enter freshwater environments by several pathways due to the bulk of plastic wastes in the environment (Figure 1.4). A pathway of MPs entering may be important in one region but less important in another (Lambert and Wagner 2018). For example, MPs applied in personal care products are likely more important in urban than agricultural regions (Lambert et al. 2014). Potential environmental release pathways of MPs can be separated by their primary or secondary sources.

A pathway of primary MPs input to freshwater environments has been found through WWTPs and the utilization of sludge from WWTPs to agricultural lands. Previous studies depicted that 90% of MPs in domestic wastewater are retained within sludge (Magnusson and Norén 2014; Talvitie and Heinonen 2014). In Europe, sewage sludge is normally composted to produce agricultural fertilizer as well as dispose of the sludge to land. The EU countries apply about four to five million tonnes of sludge to agricultural lands, annually (Willén et al. 2017). MPs that cannot be removed in the treatment process will reach the freshwater environments via effluent (Horton et al. 2017). Another pathway of primary MPs could be from the release of industrial products or processes.

Figure 1.4 Possible exposure pathways of MPs into freshwater environments.

Routes of secondary MPs into freshwater environments are mainly from improper management of plastic wastes. This includes release during collecting, transporting, processing, and landfilling of solid waste. Another route could be the runoff of wastes through drainage canals from agricultural land. Plastic wastes on roads such as vehicle debris, tire wear particles, or fragments of road‐marking paints could overflow with storm water into the freshwater environments (Eriksen et al. 2013). Lighter plastic wastes may be transported into freshwater environments by wind (Zylstra 2013), and synthetic fibers could be carried and accumulated by atmospheric fallout (Dris et al. 2015).