Investigation of seawater reverse osmosis fouling and its relationship to pretreatment type

Desalination of seawater using reverse osmosis (RO) technology is an important option available to water-scarce coastal regions. A major challenge to seawater reverse osmosis (SWRO) is membrane productivity decline due to fouling. Systematic studies in the area of SWRO fouling are lacking as compared to RO fouling by freshwater. The effect of the type of pretreatment employed ahead of the SWRO process has been recognized to be of critical importance in SWRO fouling. The objective of this study was to evaluate the effect of pretreatment on SWRO performance using bench scale experiments. The effect of different pretreatment strategies on SWRO flux decline was simulated using prefiltration of the SWRO feedwater using different filtration size ranges. The prefiltration size ranges used were selected to mimic the size fractions associated with different SWRO pretreatment processes. It was found that particulate matter greater than 1 mu m (representing media filtration) caused most of the RO fouling. On the other hand, significant reduction in fouling was observed when membrane filtration was used (microfiltration represented by 0.1 mu m prefiltration and ultrafiltration represented by 100 kDa prefiltration). There was no significant difference in flux decline between these two membrane filtration types. The lowest RO flux decline was observed when a tight ultrafiltration membrane (20 kDa) was used as prefiltration. The RO fouling observed was modeled using the gel layer theory, which could be used to satisfactorily describe fouling by different dissolved fractions of seawater. The observed SWRO fouling trends were confirmed using specially adapted attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy of the fouled membrane surface.