Occurrence and apparent aqueous-phase concentration reduction of microplastics in municipal wastewater treatment plants in London-Middlesex Region, Canada.

Microplastics (MPs) are emerging contaminants of concern due to their persistence and potential risks to ecosystems and human health. Municipal wastewater treatment plants (WWTPs) act as both sinks and discharge pathways for MPs, receiving contaminants from households and urban sources, while a residual fraction can remain in treated effluent. This study assessed the occurrence and apparent aqueous-phase concentration reduction of MPs across five WWTPs in the London-Middlesex region, Canada, with particular focus on smaller MPs (<25 μm), which are often overlooked in previous research. Total MP concentrations ranged from 380 to 1100 particles/L in primary influents and 190 to 310 particles/L in secondary effluents, with estimated daily discharges of 3.2 × 109 to 31 × 109 particles into receiving waters. Apparent concentration reductions varied across plants from 21% to 74%, with lower reduction during primary treatment. Larger MPs (150-550 μm) contributed 29%-56% of influent MPs, whereas smaller MPs (20-150 μm) comprised 27%-37%, suggesting that analytical size cutoffs can influence reported MP abundance. Fine MPs remained detectable after treatment, showing that analytical size cutoff strongly affects reported abundance and effluent particle loads. The reported reductions represent apparent aqueous-phase concentration changes within the sampled treatment train; plant-wide MP fate also includes transfer to treatment solids.