Transport and retention of laundry microplastic fibres in slow sand filtration systems

This study investigates the transport and retention of microplastic fibres (MFs) in slow sand filtration (SSF) systems using coarse and fine sand media under varying flow conditions. SSF is one of the most energy-efficient and cost-effective tertiary treatment methods. This study examined the performance of SSF for treating washing machine effluents heavily contaminated with MFs, such as those from washing machine discharge. Fine sand filters (with effective grain size of 0.2 mm) consistently retained significantly fewer MFs in the effluent than coarse sand filters (with an effective grain size of 0.6 mm), by a factor of 1.3 ± 0.2, with retention occurring predominantly within the upper 0-10 cm of the filter bed, highlighting the dominant role of the surface layer in MF capture. Between 83.9% and 93.6% of MFs in the filter effluents were 10-50 μm in length, with 10-20 μm fibres forming the dominant fraction across filtration rates of 5, 10, and 20 cm/h. Increasing filtration rate promoted deeper MF penetration into the filter bed, reduced overall retention, and increased MF breakthrough. Dissolved organic carbon removal was slightly higher in fine sand filters than in coarse sand filters, and effluent pH increased modestly with longer retention times. The highest MF removal efficiencies were achieved at the lowest filtration rate (5 cm/h), where removal reached 92.0% and 95.0% for coarse and fine sand, respectively. These trends are consistent with a mechanistic interpretation in which MF transport and retention are governed by interception, straining, and limited diffusion within the porous media, rather than simple size exclusion. SSF therefore represents a promising low-energy treatment option for mitigating MF release from wastewater effluents, particularly for fibres larger than 10 μm.