Fouling of microfiltration membranes by bidisperse particle solutions

Abstract

Fouling of filtration membranes is a key process that degrades performance and reduces membrane lifetime. Nevertheless, obtaining direct information about fouling mechanisms is still a significant challenge, especially fouling that occurs within the internal pore spaces. Here, we combined highly multiplexed single-particle tracking methods with alternating laser excitation, which allowed the direct visualization of the transport of nanoparticles within microfiltration membranes, where the advective motion and retention of 200 nm and 40 nm diameter particles was simultaneously imaged within filtration membranes with 650 nm nominal pore size, as membranes became increasingly fouled as a function of total particle throughput. We found that internal membrane fouling consisted of three stages, fouling site formation, fouling site growth, and fouling site coalescence. Larger particles had a greater chance to be retained initially, nucleating the fouling sites. These sites tended to capture other particles passing through the membrane, causing the fouling sites to grow in size. Eventually, isolated fouling sites grew large enough to coalescence with neighboring sites, blocking a large region of membrane pores, leading to significant fouling. Importantly, the presence of small numbers of large particles significantly increased the retention of small particles and also accelerated the rate of particle retention. This mechanistic information about internal membrane fouling will assist in the design and optimization of filtration processes to reduce membrane fouling and expand the fundamental understanding of complex mass transport of polydisperse particle distributions.