Off the coast of Washington, 32 families on Guemes Island were faced with an aging well that was drawing salt water into its system. Because groundwater is scarce on the island, residents had to rely on what little rain water soaks into the ground. And since the island is an increasingly popular site for permanent residence, concern regarding the water shortage was growing. It soon became clear that seawater was the only available resource that could meet the island's ever-increasing demand for potable water.
Since the chloride level was approximately three times the maximum of state guidelines, the State Health Department was prompted to order a new water source. Osmonics of Minnetonka, Minn., was chosen to manufacture the first desalination plant built by a public agency in Washington.
To reach the water, the Skagit Public Utility District (PUD), which services Guemes Island, drilled a 40-foot well on the beach. After encountering complications with the sand density, engineers tacked on an 80-foot pipe at a T-angle attached to the original well pipe. The extended pipe is perforated to allow water in the channel to seep into the pipe before being pumped up the hill to the desalination system. The system consists of dual-media filtration, 5-micron Hytrex¬ prefiltration, and the desalination RO, followed by a calcite contactor to raise the permeate pH. Within the desalination RO, the water is pumped through a series of polyamide membrane elements that provide salt rejection in excess of 99.5%. According to residents, the end result is water that is clear and good tasting.
