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) or 1,233 cubic meters. One million gallons equals 3.07 acre-feet.

Bar. A unit for measuring pressure. 1 bar = 14.5 psi = 0.99 atm

Biocide. A chemical used to kill biological organisms (e.g., Sodium Bisulphite).

Brackish. Water that contains less than 10,000 ppm salt - more salt than freshwater but less than the open seawater.

Brine. Water that contains greater than 50,000 ppm salt. Brine discharges from desalination plants may also include constituents used in pretreatment processes.

BTU (British Thermal Unit). A standard unit for measuring a quantity of thermal energy. Electricity, natural gas or any other source of energy can be measured in BTUs. One BTU is the amount of thermal energy required to raise the temperature on one pound of water one degree Fahrenheit at sea level. One thousand BTUs equals 0.29 kilowatt-hours.

Coagulation. A pretreatment process used in some desalination plants. A substance (e.g., ferric chloride) is added to a solution to cause suspended particles to agglomerate and form larger particles which are easier to remove from a solution than small particles.

Cogeneration. A power plant that is designed to conserve energy by using "waste heat" from generating electricity for another purpose, for example, thermal desalination or to warm SWRO feed water.

Deaeration. Removal of oxygen. A pretreatment process in desalination plants to reduce corrosion and fouling.

Dissolved Air Flotation (DAF). A pretreatment process in desalination plants to remove solids and organics.

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.

Efficiency. Energy transfer efficiency expressed as the ratio of the sum of the energy leaving the PX® unit or array divided by the sum of the energy entering, as calculated with the following equation:

Efficiency   =	∑(Pressure x Flow)OUT	x100%
	∑(Pressure x Flow)IN

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.

Fouling. Contamination or biological growth on the reverse osmosis membranes or pretreatment filters.

Freshwater. Water that contains less than 1,000 milligrams per liter (mg/L) of dissolved solids; generally, more than 500 mg/L of dissolved solids is undesirable for drinking and many industrial uses.

High Pressure Flow Differential Pressure (HP DP). The pressure at the high-pressure inlet port of the PX® minus the pressure at the high-pressure outlet port of the PX®.

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 reversible water treatment process. A charged polymer exchanges Na+, H+, Cl-, or OH- for other ions in a solution.

Isobaric. Literally: same pressure. An isobaric energy recovery device, like the PX®, includes chambers wherein the pressure of two volumes of water equalizes.

Low Pressure Flow Differential Pressure (LP DP). The pressure at the low-pressure inlet port of the PX® minus the pressure at the low-pressure outlet port of the PX®.

Lubrication Flow. The flow rate of high-pressure brine required to lubricate the PX®'s hydrodynamic bearing measured as a difference in flow rates according to any of the following equivalent methods:

• Low-pressure feedwater to the main high-pressure pump minus the membrane permeate
• High-pressure brine to the PX® unit minus the high-pressure feedwater from the PX® unit
• Low-pressure brine from the PX® unit minus the low-pressure feedwater to the PX® unit

Kilowatt (kW). One 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.

Kilowatts-hours per cubic meter (kWh/m³). A measure of the power require to produce a cubic meter of permeate.

Megawatt (MW). One million watts.

Minimum Discharge Pressure (MDP). The minimum allowable pressure at the low-pressure outlet port of the PX®.

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.

Nanofiltration (NF). A lower pressure membrane treatment technology sometimes used for pretreating reverse osmosis feedwater.

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).

Permeate. Water purified by reverse osmosis treatment.

Pounds per square inch (psi). A unit for measuring pressure. 1 psi = 0.069 bar = 0.068 atm.

Product Water. The desalted, post-treated water delivered to the water distribution system.

Recovery. Permeate or product water per unit of membrane feed, typically expressed as a percentage.

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 permeate or product water; most of the dissolved impurities remain behind and are discharged in a brine or waste stream.

Salinity Increase. The increase in the salinity of the feedwater to the reverse osmosis membrane caused by the energy recovery device. This is typically expressed as a percentage increase above the salinity of the system feedwater according to the following equation:

Salinity Increase   =	membrane inlet salinity - system feedwater salinity	x100%
	system feedwater salinity

Seawater Reverse Osmosis (SWRO). Reverse Osmosis desalination of seawater.

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.

Volumetric Mixing. The ratio of the difference between the salinity of the feed to the membranes and the salinity of the feedwater supply divided by the salinity of the feedwater at 40% recovery in a standard PX®-booster pump system according to the following equation:

Volumetric Mixing   =	membrane feed salinity - feedwater salinity	x100%
	feedwater salinity