Impact of microplastic fibres on direct membrane filtration of low-strength primary wastewater

Wastewater treatment processes, including membrane-based separations, are considered a major barrier preventing the discharge of microplastics into aquatic environments. However, there is currently limited understanding of the effects of microfibres, a common type of microplastics, in direct membrane filtration used as an alternative secondary treatment method. This study investigated the filtration performance and fouling mechanisms during track-etched membrane filtration of low-strength primary wastewater dosed with different types of microfibres. The presence of microfibres (10 mg/L) did not affect the treated water quality (such as biodegradable organics and suspended solids), but accelerated cake fouling, as illustrated by both fouling distribution analysis and model fitting. Shorter microfibres (1 mm) led to higher membrane fouling resistance than long ones (5 and 20 mm). Polyamide (PA, 5 mm) microfibres caused a more porous cake layer but also a higher irreversible fouling resistance compared to polyethylene (PE, 5 mm) and polyethylene terephthalate (PET, 5 mm). Compared to the control (without dosing microfibres), the presence of negatively charged PET, PA, and PE microfibres allowed more deposition of soluble fulvic acid-like matter on the membrane but reduced the accumulation of soluble tyrosine-like and tryptophan-like aromatic proteins possibly due to the stronger interactions of these proteins with the microfibres. However, the organics-cations matrix did not correlate with the cake resistance, suggesting that the microfibres affected the cake structure during cake development. Overall, this study investigated for the first time the microfibres-foulants-membrane interactions in primary wastewater treatment, highlighting the impact of microfibres on the efficiency of membrane-based processes.