Biofouling of reverse osmosis membranes: Role of biofilm-enhanced osmotic pressure

A bench-scale investigation of RO biofouling with Pseudomonas aeruginosa PA01 was conducted in order to elucidate the mechanisms governing the decline in RO membrane performance caused by cell deposition and biofilm growth. A sharp decline in permeate water flux and a concomitant increase in salt passage were observed following the inoculation of the RO test unit with a late exponential culture of P. aeruginosa PA01 under enhanced biofouling conditions. The decrease in permeate flux and salt rejection is attributed to the growth of a biofilm comprised of bacterial cells and their self-produced extracellular polymeric substances (EPS). Biofilm growth dynamics on the RO membrane surface are observed using confocal microscopy, where active cells, dead cells, and EPS are monitored. We propose that the biofilm deteriorates membrane performance by increasing both the trans-membrane osmotic pressure and hydraulic resistance. By comparing the decrease in permeate flux and salt rejection upon fouling with dead cells of P. aeruginosa PA01 and upon biofilm growth on the membrane surface, we can distinguish between these two fouling mechanisms. Bacterial cells on the membrane hinder the back diffusion of salt, which results in elevated osmotic pressure on the membrane surface, and therefore a decrease in permeate flux and salt rejection. On the other hand, EPS contributes to the decline in membrane water flux by increasing the hydraulic resistance to permeate flow. Scanning electron microscope (SEM) images of dead cells and biofilm further support these proposed mechanisms. Biofilm imaging reveals an opaque EPS matrix surrounding P. aeruginosa PA01 cells that could provide hydraulic resistance to permeate flux. In contrast, SEM images taken after fouling runs with dead cells reveal a porous cake layer comprised of EPS-free individual cells that is likely to provide negligible resistance to permeate flow compared to the RO membrane resistance. We conclude that "biofilm-enhanced osmotic pressure" plays a dominant role in RO biofouling.