Acre-foot (AF). A unit for measuring the volume of water. One acre-foot equals 325,851 gallons (the volume of water that will cover one acre to a depth of one foot). One million gallons equals 3.07 acre-feet.
Biocide. A chemical used to kill biological organisms (e.g., chlorine).
Brine. Water that contains a high concentration of salt. Brine discharges from desalination plants may include constituents used in pretreatment processes, in addition to the high salt concentration seawater.
Btu (British Thermal Unit). A standard unit for measuring a quantity of thermal energy, either electricity, natural gas or any other source of energy. One Btu is the amount of thermal energy required to raise the temperature on one pound of water one degree Fahrenheit at sea level.
Capacity Factor. An electric utility's annual capacity factor is defined as the annual kilowatt-hour sales divided by the product of the total hours in a year and the rated capacity of the utility in kilowatts.
Coagulation. A pretreatment process used in some desalination plants. A substance (e.g., ferric chloride) is added to a solution to cause certain elements to thicken into a coherent mass, so that they may be removed.
Cogeneration. A power plant that is designed to conserve energy by using "waste heat" from generating electricity for another purpose.
Deaeration. Removal of oxygen. A pretreatment process in desalination plants to reduce corrosion.
Distillation. A process of desalination where the intake water is heated to produce steam. The steam is then condensed to produce product water with low salt concentration.
Electrodialysis. Most of the impurities in water are present in an ionized (electrically-charged) state. When an electric current is applied, the impurities migrate towards the positive and negative electrodes. The intermediate area becomes depleted of impurities and discharges a purified stream of product water. This technology is used for brackish waters but is not currently available for desalting seawater on a commercial scale.
Feedwater. Water fed to the desalination equipment. This can be source water with or without pretreatment.
Infiltration Gallery. A method used for seawater intake. Perforated pipes are arranged in a radial pattern in the sand onshore below the water level. Water in the saturated sand enters the perforated pipes.
Ion Exchange. A water treatment process. An electric charge is used to remove charged particles from solution.
Kilowatt (kW). A thousand watts. The watt is a measure of power used by electricity generating plants. One watt is equivalent to 1 Joule/second or 3.4127 Btu/hour.
Megawatt (MW). A million watts.
Microlayer. The upper few millimeters of the ocean. Fish eggs are sometimes concentrated in the microlayer.
Multiple Effect Distillation (MED). A form of distillation. Evaporators are in series, and vapor from one effect is used to evaporate water in the next lower pressure effect. This technology is in several forms, one of the most common of which is the Vertical Tube Evaporator (VTE).
Multistage Flash (MSF). A form of distillation. Intake water is heated then discharged into a chamber maintained slightly below the saturation vapor pressure of the incoming water, so that a fraction of the water content flashes into steam. The steam condenses on the exterior surface of heat transfer tubing and becomes product water. The unflashed brine enters another chamber at a lower pressure, where a portion flashes to steam. Each evaporation and condensation chamber is called a stage.
Ocean Thermal Energy Conversion (OTEC). A solar, ocean thermal desalination approach where electricity is produced by using the temperature differential between cold, deep waters and warm, shallow surface waters. Water at the ocean surface (at about 70°F) is used to heat liquid ammonia, which vaporizes at this temperature in a vacuum chamber. The ammonia vapor is used to turn a turbine to produce electricity. The vapor is then condensed by using cold water pumped up from the ocean depths (at about 35°F).
Product Water. The desalted water delivered to the water distribution system.
Reverse Osmosis (RO). A process of desalination where pressure is applied continuously to the feedwater, forcing water molecules through a semipermeable membrane. Water that passes through the membrane leaves the unit as product water; most of the dissolved impurities remain behind and are discharged in a waste stream.
Scaling. Salt deposits on the interior surfaces of a desalination plant.
Total Dissolved Solids (tds). Total salt and calcium carbonate concentration in a sample of water, usually expressed in milligrams per liter (mg/L) or parts per million (ppm). The state-recommended Maximum Contaminant Level (MCL) drinking water standard for total dissolved solids is 500 mg/L, the upper MCL is 1,000 mg/L, and the short-term permitted level is 1,500 mg/L.
Vacuum Freezing (VF). A process of desalination where the temperature and pressure of the seawater is lowered so that the pure water forms ice crystals. The ice is then washed and melted to produce the product water. This technology is still being developed, and is not yet commercially competitive.
Vapor Compression (VC). A form of distillation. A portion of feedwater is evaporated, and the vapor is sent to a compressor. Mechanical or thermal energy is used to compress the vapor, which increases its temperature. The vapor is then condensed to form product water and the released heat is used to evaporate the feedwater.
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