Microbubbles and organic fouling in flat sheet ultrafiltration membranes

Organic fouling is the long-known Achilles heel of ultrafiltration UF membranes. Its effect can be minimized either by fouling prevention and growth-control, or by elimination of an existing fouling layer. Air bubbling is instrumental for both approaches, although at present elimination is the commonly main industrial application. Air bubbles are used in membrane bioreactors to minimize organic and biofouling of submerged UF hollow fibers. The present work seeks to advance our understanding of how bubble flows can prevent and control the growth of organic fouling in various membranes. To study various bubble regimes, regular and oscillating bubble flows were generated using a novel air-water vortex device. Oscillating flows were generated via a cylindrical pin installed in the generator exit cavity. The size and velocity of generated bubbles were measured using a laser Doppler velocimeter and particle analyzer. A characterized suspension was fed into a filtration cell operated in crossflow mode. 0.3 g/L bovine serum albumin BSA suspension was used to promote organic fouling in 20 and 50 kDa UF polyethersulfone membranes. Analysis indicates that mechanisms by which air bubbles prevent or control organic fouling vary with bubble size. These range from adsorption of potential foulants on small bubbles ahead of contact with the membrane, to distortion of already formed fouling layer by large bubbles. A flow of 0.5–1.0 mm air bubbles allowed a continuous 3 h run, indicating that these fouling-combat mechanisms can be combined. Small < 0.05 mm air bubbles increased BSA retention but noticeably contributed to flux decline. Large 2 mm bubbles minimized fouling but did not improve BSA retention. These findings suggest that altering bubble sizes can optimize fouling-combat mechanisms for given foulants, thus minimizing fouling while substantially reducing air and energy demands.