Seawater Desalination

Seawater desalination is a permanent answer for coastal settlements, hotels and islands with limited freshwater. Reverse osmosis (SWRO) brings roughly 35,000 mg/L salinity down to potable level: under high pressure water passes through the membrane while salt is left behind. The stage that governs plant life and operating cost, however, is not the membrane itself but pre-treatment. Capturing suspended solids, algae and organic load before they reach the membrane directly sets the fouling rate and therefore the chemical cleaning frequency. A plant built with inadequate pre-treatment may hit its design figures in year one but soon faces membrane replacement cost. Pre-treatment selection depends on the intake. Open sea intakes carry a high risk of algal bloom and suspended matter, calling for multimedia filtration or ultrafiltration. Beach well intakes benefit from natural filtration and present a lower load. The second decisive item is energy. SWRO needs high pressure and electricity is the largest component of operating cost. Energy recovery devices, which transfer pressure from the concentrate stream to the feed, cut that consumption markedly and are considered standard on modern plants. The third issue is concentrate discharge: at roughly double salinity, this stream must be released through a diffuser arrangement that achieves adequate dilution, with due regard for the marine ecosystem.