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Technology New Process May Hold The Key To "Economically Viable" Desalination An energy-saving alternative to reverse osmosis (RO) desalination promises to reduce costs by more than 82 percent. By Neil A. McCarthy T cells within the RO array and corresponding labor in he sea, the blue sea – the phrase reminds one handling them. P-VES technology does not produce of the classic song, "La Mer", written by Charles any of these compounds — P-VES uses polypropylene, Trenet in 1946. It's still blue, it composes 71 which does not allow said compounds to anneal to the percent of the world's surface, but alas it's still surface. full of salt. Desalination is a process that has been used with varying success for centuries; the problem is the cost of turning seawater into fresh water. Reverse Cost Comparison Vs. RO Desalination osmosis (RO) has been the technology most favored Consider the estimate of cost using RO at the Basra, by water suppliers and agencies because of its proven Iraq plant as cited by the National Renewable Energy track record. However, RO is costly. Laboratory. RO primarily uses electricity as its source The Passarell Vapor Exchange System (P-VES) was of power. The cost for this RO per cubic meter (m³) is designed as a highly efficient method of producing US$0.986/m³.1 This cost is consistent with current RO distilled water from saltwater. The simplicity of P-VES installations. lies in its "single flash vapor" exchange. A corresponding study conducted Seawater is raised to a vaporized state by California Polytechnic University, A small amount (steam) and pulled through differential San Luis Obispo (Cal Poly) stated of distillate, along the following: "However, overall, it pressures to an adjoining cell where, as it cools, the vapor recondenses with all the heavier- seems as though RO desalination will to liquid as pure distillated water. A only become economically viable if than-air particles small amount of distillate, along with either the RO process becomes less all the heavier-than-air particles (i.e., (i.e., minerals/salts), expensive or the price of surfaceminerals/salts), is used to heat the delivered water rises above that of is used to heat next batch of incoming seawater. The desalinated water."2 P-VES is the only desalination process A cubic meter (m ³ ) is the the next batch of that recycles the thermal energy. internationally accepted standard incoming seawater. used to measure efficiency, cost, RO requires pretreatment of feed water due to scaling which occurs and amount. One m³ has 264.17205 on RO membranes during desalination. As the gallons. Each gallon of seawater weighs 8.556 pounds concentration of the solids increases, they exceed (lbs.) for a total of 2,260.26 lbs. per m³. A British the RO membranes' ability to absorb them. The thermal unit (BTU) is defined as the amount of heat additional stacking of unwanted solids builds rapidly. required to raise the temperature of 1lb. (0.454 kg) Some of the unwanted compounds found are calcium of liquid water by one degree F (0.56 degree C) at a carbonate (CaCO3), calcium sulfate (CASO4), barium constant pressure. sulfate (BaSO4), and strontium sulfate (SrSO4). This, The P-VES plant is designed to recover all of the in turn, requires the use of sodium hydroxide (NaOH) thermal energy required to operate the process, to remove the soluble Ca and Mg; in addition, sulfuric except loss of thermal energy to the atmosphere. It is acid (H2SO4) or hydrochloric acid (HCl) is used to deal completely insulated to retain all BTUs from being lost, with CaCO3 buildup. The removal of these unwanted as thermal energy is recycled from the concentrated compounds is costly, as it requires downtime of the and removed salt leaving the process. A quantity of 22 wateronline.com ■ Water Online The Magazine