Direct filtration of microfibre-containing wastewater using nanofibre membranes: combined effects of mode of filtration and type of microfibre

Microfibre pollution derived from wastewater discharges containing synthetic fibres less than 5 mm in length poses significant environmental risks. Direct membrane filtration (DMF) offers an alternative solution for treating microfibre-containing wastewater, such as greywater and textile industry wastewater. In this study, the effects of microfibres on DMF performance, especially the interactions between microfibres and other potential foulants, were examined. Three microfibre types, polyethylene terephthalate (PET), polyamide (PA), and polyethylene (PE), were dosed at 10 mg/L to low-strength municipal wastewater, which was used as the feed water for DMF operated with polyacrylonitrile nanofibre membranes under either crossflow or dead-end filtration mode. The results revealed that the microfibres influenced membrane fouling mechanisms, with their behaviours associated with the microfibre material and the filtration mode. Under crossflow conditions, PET and PA microfibres increased the fouling resistance by 25-50% compared to the control, because they promoted the deposition of 8-39% more scalant cations, 17-23% more fulvic acid-like and 23-34% more humic acid-like matter on the membrane. Conversely, PE microfibres mitigated fouling by 45% due to their strong binding capacity with both soluble fluorescent organics and cations. Under dead-end filtration, the three microfibre types led to a reduction in membrane fouling by 23-46%, indicating that the perpendicular driving force allowed the deposited microfibres to interfere with the organics-cations gel-like matrix formation, resulting in less cake resistance. By elucidating the effects of microfibres in nanofibre membrane filtration, this study supports the development of resilient membrane processes for the treatment of microplastic-containing wastewater.