# Ecotoxicological Assessment of Microplastics in Freshwater Sources--A Review
## 3 Sources of Microplastics in the Environment  
There are primary and secondary sources of MPs in the environment [5]. Primary include MPs deliberately manufactured for some products; i.e., textile (laundering of synthetic clothes providing \(\sim\)35% of primary microplastics), cosmetics (e.g., microbeads in facial scrubs (\(\sim\)2%) as intentionally added MPs in personal care products), electronic equipment [13; 35; 54]. Furthermore, abrasion of tires through driving (28%), city dust (24%), road making (7%), marine coating (3.7%), and plastic pellets (0.3%) are also considered as the main sources of primary MPs. Most of these enter the environment through wastewater generated during the production or usage phases (Figure 2). Secondary sources include a breakdown of larger plastic fragments such as plastic bags, bottles, or fishing nets introduced in the environment; the breakdown is caused by UV radiation, physical abrasion, chemical oxidation, and possibly biodegradation [13; 35; 55; 56]. Agriculture is one of the major entry points for MPs in the environment [57]; applications of sewage sludge, fertilizers/compost, soil conditioners, and vinyl coverings are the main sources. LDPE films, used in large volumes to protect agricultural crops, suppress weeds, increase temperature, and retain irrigation water in the soil, can be degraded into small fragments and end up in the soil or water resources through irrigation channels. The expanded PS flakes and polyurethane foam are used in horticulture to improve soil quality  
Figure 2: Sources of microplastics in the environment. and as composting additive, respectively [58; 59; 60]. However, significant MPs amounts originate from various industries (through wastewaters or plastic residues disposal) and households (fibers from washing clothes, personal care products usage). Once released into the environment, MPs can be transported by wind, washed from land to surface waters during rainfall (stormwater run-off especially), and be transported in freshwater and seawater [3; 6; 8; 10; 15; 35; 61]. Industrial and municipal wastewaters are also main MPs sources, containing synthetic fibers (e.g., polyester, PES, produced in 2013 at >44.6 MT) [62]. [PERSON] et al. [63] quantified that >1900 PES fibers are released during a single wash. Therefore, wastewaters containing MPs should be treated prior to discharging into the environment. However, only ~60% of municipal wastewaters are treated [64]. Besides, wastewater treatment plants (WWTPs), commonly based on primary and secondary treatment units, are not designed to remove MPs. The tertiary treatment, including disinfection, is optional and irrelevant for MPs removal. After primary treatment application, including removal of material that either floats or readily settle out by gravity, wastewater may contain > 20% of MPs [65]. Secondary treatment is biological; mixed consortium of microorganisms has a key role in forming activated sludge flocs. If MPs are retained within, there is a possibility to be released from flocs due to their instability in the aqueous phase. Although the efficiency of common WWTP for MPs removal is up to 95%, a large MPs fraction is transferred into the activated sludge [65]. Moreover, WWTPs are not effective in removing particles < 10 \(\upmu\)m [15]; these remain in effluent and are consequently discharged into recipient natural aquatic systems. Besides technological, there are legislative issues too; Directive 91/271/EEC [66], which does not consider MPs monitoring in treated effluents or activated sludge, is still in force. Approximately 4-5 MT of activated sludge is applied to agricultural lands annually [14], thus providing additional MPs quantities in the environment. Upon entering freshwater sources, a fraction of MPs would be suspended in the water body, while others would deposit at the bottom of the water column (i.e., sediment). This process depends on the density of MPs; low-density MPs would float, while those with higher density would submerge [59].