Application of low fouling RO membrane elements for reclamation of municipal wastewater

Membrane fouling encountered in reclamation of municipal wastewater represents serious design and operational concern. This is because the municipal effluent, after secondary treatment, contains high concentrations of suspended particles, colloids and high level of biological activity. Application of membrane technology for treatment of municipal wastewater requires very extensive pretreatment prior to the RO process. The conventional multi-step treatment approach, based on disinfection, flocullation, clarification and media filtration, still produces RO feed water with very high fouling potential. Extensive field results from pilot and comnercial RO system operation indicate high fouling rates, regardless of the nature of membrane material: cellulose acetate or composite polyamide. Membrane cleaning has to be applied very frequently in order to maintain the design product capacity. Recently a new pretreatment technology is being used in RO processing of municipal effluent. It consists of backwashable microfiltration and ultrafiltration membrane elements in a capillary configuration. This new membrane pretreatment technology is capable to treat secondary effluent and maintain stable performance of filtrate now and operating pressure. The capillary technology produces RO feed water of a very high quality. The capillary filtrate has a much lower concentration of colloidal and suspended particles than can be produced in a conventional pretreatment process. In reclamation plant that uses membrane pretreatment the fouling rate of the RO membranes operating on capillary effluent has been reduced significantly. The fouling rate has been reduced even more by introduction of new generation of low fouling composite membranes (LFC1). In low fouling membranes the surface of the salt rejection layer has been modified to make it more hydrophilic and reduce its affinity to dissolved organics. Field results of operation of the low fouling membranes in municipal wastewater reclamation systems indicate that the fouling rate is very low, comparable with that observed in RO operation with clean well water. The low fouling rate is attributed. to a lower rate of adsorption of dissolved organics on the LFC1 hydrophilic membrane surface. Apparently, in the low fouling membranes, the bonding between the adsorbed organic layer and the membrane surface is relatively weak. The paper will describe properties of low fouling membrane technology and present results of its application with conventional and capillary pretreatment. Performance in municipal wastewater reclamation applications will be compared with that of conventional membrane technology. Results of operation of capillary UF membrane pretreatment on municipal secondary effluent and optimization of operating parameters will be described as well.