Wastewater-induced microplastic biofouling in freshwater: role of particle size and flow velocity†

Microplastics (MPs), discharged from wastewater treatment plants (WWTPs), are found abundantly in freshwater systems. Along with MPs, various microorganisms that evade WWTP disinfection may colonize these particles, leading to biofouling. This study assessed the performance of six bacterial strains isolated from wastewater and the factors influencing biofilm formation using synthetic freshwater and polyethylene (PE) microplastics as a model. The effect of two PE microplastic sizes (180–200 μm and 3–4 mm) and three flow velocities (0.238, 0.11, and 0.077 m s⁻¹) were tested on the isolated strains' microbial growth and biofilm formation. Smaller MPs notably enhanced the growth rate. The treatment with small PE microplastics and a low flow velocity promoted the biofilm formation compared to a higher flow velocity where rapid microbial growth was observed but showed a lower biofilm formation after seven days of cultivation. These findings reveal how MP size and flow velocities influence biofilm development, advancing the understanding of MP-microbial interactions in freshwater aquatic environments.