Biofouling in reverse osmosis processes : the roles of flux, crossflow velocity and concentration polarization in biofilm development

Abstract

Biofilm development in a spacer-filled reverse osmosis membrane channel can influence both trans-membrane pressure (TMP) and channel pressure drop (ΔPCH). While current pretreatment methods are unable to completely tackle the biofouling problem, more insights are required to provide strategies to minimize the problem. This study examined the role of operating parameters (i.e. flux and crossflow velocity) to minimize biofouling in RO processes. The experiments were conducted with a lab-scale high pressure flat sheet RO reactor where changes in pressure drop along the channel and across the membrane were measured. The impact of biofouling was measured at constant fluxes, where the TMP rise and ΔPCH rise and the biofoulant was quantified as biovolumes of live and dead bacteria on autopsied membrane and spacer samples by confocal laser scanning microscopy (CLSM).

The results show that TMP rise increased exponentially with increasing flux, and decreased with increasing crossflow velocity. The channel pressure drop, ΔPCH, increased when either flux or crossflow velocity was increased, and was more dependent on crossflow. The biofoulant volume on the membrane increased with flux and was less dependent on crossflow. The biofoulant associated with the spacer was much less than on the membrane and relatively insensitive to flux or crossflow velocity.

The TMP rise could be correlated with the estimated concentration of nutrient at the membrane surface, Cw,N, highlighting the combined roles of flux and crossflow velocity in solute concentration polarization. Previous TMP rise data could also be correlated to the estimated Cw,N values. This observation suggests a biofouling mitigation strategy by controlling both incoming nutrient concentration and operating conditions (flux and crossflow).