Pretreatment of seawater: Results of pilot trials in Singapore

In this paper, performance of different pretreatment systems used in the seawater reverse osmosis desalination process is presented. The different pretreatment techniques analyzed included conventional media-filtration technique and non-conventional membrane filtration techniques. For the membrane filtration techniques, two ultrafiltration and one microfiltration pilot plants were used in the experimental study. During the experiments, the silt density index (SDI) of the filtrate samples was regularly measured to quantify the performance of pretreatment systems in rejecting colloidal particles. Measurements of other important parameters included filtrate flux, transmembrane pressure, total suspended solids, colloidal silica, total organic carbon, etc. According to the experimental findings, the quality of the filtrate produced by the conventional media-filtration technique was inferior and highly inconsistent. SDI of filtrate varied from 2.8 to 3.8 and spikes as high as 6.3 were frequently observed. Membrane pretreatment produced filtrate of a better quality. SDI of the filtrate produced was consistently below 3.0, a prerequisite for proper operation of a RO desalination plant. The ease of operation of the membrane pretreatment processes was also noted. Together with improved maintenance procedures such as filtrate backwashing and air scouring, complete flux and pressure recoveries were achievable with minimal use of chemicals during the experiments.

According to the experimental findings, the quality of the filtrate produced by conventional media-filtration techniques is inferior and highly inconsistent. Unlike a media-filtration technique, membrane pretreatment consistently produces filtrate of a better quality. The ease of operation of the membrane pretreatment processes was also noted. Moreover, membrane pretreatment systems generally require less space and fewer chemicals compared to a conventional pretreatment system. Together with improved maintenance procedures such as filtrate backwashing and air scouring, complete flux and pressure recoveries are achievable with minimal use of chemicals. Successful application of membrane technology was also reported by Taniguchi [18]. Using membrane methods of pretreatment will also go in tandem with the recent trend of increasing packing density of spiral-wound RO elements. With an increasing packing density, the propensity to fouling and pore blockage for the new RO spiral element will definitely be higher and greater emphasis will be placed on pretreatment in producing better quality feed. Therefore, there is an economic incentive to the use and continual improvement and research in the area of membrane pretreatment. The current research also emphasized the importance of pilot testing as measured performance is the combined effect of the membrane, system configuration and the dynamic cake layer that formed on surface of the membrane during filtration.