Removal of silica from brackish water by electrocoagulation pretreatment to prevent fouling of reverse osmosis membranes

Desalination of seawater and brackish water by reverse osmosis (RO) has become increasingly important for drinking water supply in a greater part of the world. The presence of high silica concentrations in some brackish water, however, limits the application of RO desalination due to the potential formation of silica scales that irreversibly deteriorate the membrane material and performance. This study investigates the feasibility of electrocoagulation as a pretreatment process to remove silica from the source brackish water. The effects of several electrical parameters, including electrode arrangement, current intensity and hydraulic retention time, were studied on the basis of silica removal efficiency. Bipolar configuration attained greater extent of silica removal as compared to monopolar configuration. Increases in charge loading generally improved the silica removal efficiency, but excessive hydraulic retention time (60 min) was detrimental to the system performance. In this study, with no modification of the source water, silica removal efficiency up to 80% was achieved under a current intensity of 0.5 A and a hydraulic retention time of 30 min. The subsequent nanofiltration studies demonstrated severe flux declines over the first 3 h, yielding only 70% of its initial flux for brackish water containing 100 mg/L silica, and progressively lower with higher silica concentrations. For the pretreated water by electrocogulation, the extent of flux decline was markedly improved, suggesting that the pretreatment was effective for the attenuation of the flux decline. Electron micrograph images of the membrane autopsy also confirmed the lack of scale formation for the pretreated water as compared to those without pretreatment. (C) 2007 Elsevier B.V. All rights reserved.