(415f) Mineral Scaling of RO Membranes in the Presence of Biofilm

Membrane fouling is one of the major factors that limit recovery in reverse osmosis desalination of brackish water. Fouling of the membrane surfaces can lead to significant reduction in membrane performance (flux reduction and salt rejection impairment) and shortening of membrane life. Surface fouling may occur by accumulation and adsorption of inorganic particles, microorganisms, macromolecules and corrosion products on membrane surfaces. For organic and biological fouling (biofouling), even though significant fractions of macromolecules, organisms and organics in the feed stream are often removed during the pre-treatment processes, one can still encounter problems with this type of fouling depending on the source water compositions. Mineral salt scaling is another major foulant that limits the recovery in reverse osmosis desalination processes. The primary mineral salt scalants that are typically of concern in inland water desalination are calcite, gypsum, and barite. Mineral salt scaling is more severe at the exit of the membrane chancel due to the effects of concentration polarization. In contrast, biofouling is more severe, and occurs earlier, at the entrance of the membrane channel due to the rapid adsorption of macromolecules when feed solution is introduced. However, these regions may overlap and in order to properly mitigate membrane fouling, one must understand how mineral salt scaling is affect by the presence of biofouling. In order to address the above coupled fouling and scaling problem, a systematic surface scaling study was carried out, with gypsum as the model scalant, to evaluate the kinetics of surface crystallization in the presence of a biofilm. Secondary wastewater effluent (from Orange County Sanitation District) was used as the source water to simulate biofouling and create biofilm on RO membrane surfaces, followed by gypsum surface crystallization. An ex-situ observation detection (EXSOD) system was use to observed the growth of gypsum crystals on membranes surfaces in real-time and to monitor flux decline. The kinetics of gypsum surface crystallization in the presence of biofilm growth were analyzed using a single crystal growth model along with a population growth model. Imaging of the biofouled regions were also obtained to assess the interplay between scaling and biofouling. The results of this study demonstrated that the presence of biofilm can have a significant impact on both single crystal growth rates and the distribution of crystals on the biofouled membrane surface.