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Glossary Acre-foot (AF)âThe volume of water that would cover a one acre area one foot deep. Equivalent to approximately 1,233.6 cubic meters or 325,900 gallons. Applied researchâSystematic study to gain knowledge or understand- ing necessary to determine the means by which a recognized and specific need may be met. Available energyâMechanical, electrical energy, or any other energy, which in practice can be nearly completely converted into mechanical work. Basic researchâSystematic study directed toward fuller knowledge or understanding of the fundamental aspects of phenomena and of observ- able facts without specific applications towards processes or products in mind. BiofoulingâPresence and growth of organic matter in a water treatment system that interferes with system performance. Brackish waterâWater with a salinity between that of fresh- and sea- water. BrineâWater with a greater salinity than seawater, usually in excess of 37,000 mg/L. Capacitive deionizationâAn electrosorption process whereby ions are removed from water using an electric field gradient as the driving force. CoagulationâA pretreatment process used in some desalination plants. A substance (e.g., ferric chloride) is added to a solution to cause sus- pended particles to agglomerate and form larger particles which are eas- ier to remove from a solution than small particles. CogenerationâDual-purpose facilities that produce both electricity and water. Cogeneration plants integrate the thermal desalination process with available steam from power generation. 262
Glossary 263 ColloidâSuspended solid with a diameter less than 1 micron that can not be removed by sedimentation alone. ConcentrateâThe water containing the dissolved solids removed dur- ing desalination. Concentrate managementâThe handling and disposal or reuse of waste residuals from the desalination system. Concentration polarizationâA phenomenon in which solutes form a dense, polarized layer next to a membrane surface which eventually re- stricts flow through the membrane. Consumptive useâWater withdrawn from a source and made unavail- able for reuse in the same basin, such as through conversion to steam, losses to evaporation or transpiration, seepage to a saline sink, or con- tamination. Also referred to as irretrievable or irrecoverable loss. DeaerationâRemoval of oxygen. A pretreatment process in desalina- tion plants to reduce corrosion. DemandâAn economic concept that is used to describe a want for wa- ter backed up by a willingness to pay. Demand schedule (or curve)âA summarization of the quantities and qualities of water that consumers are willing to take at different prices. DesalinationâThe process that removes dissolved solids, primarily salts and other inorganic constituents, from a saline water source. Design-Build (DB)âA delivery approach characterized by a single con- tractual relationship between the public water provider and a contractor, who develops the project design and oversees its construction. The pro- ject is then operated by the owner or contract operator. Design-Build-Operate (DBO)âA delivery approach involving a single contractor for design, construction, and operation. Design-Build-Own-Operate-Transfer (DBOOT)âAn expansion of the DBO concept in which the contractor also finances the project and initially owns the facility. DevelopmentâSystematic application of knowledge or understanding, directed toward the production of useful materials, devices, and systems or methods, including design, development, and improvement of proto- types and new processes to meet specific requirements. DewvaporationâA desalination method where a stream of air is hu- midified by a falling film of saline water along one side of a heat transfer surface. The air is partially heated by an external source (e.g., solar, waste heat). The heated air then is swept along the condensing side of heat transfer films, where the vapor condenses to a liquid, which is col- lected as product water DiffusionâThe movement of suspended or dissolved particles or mole- cules from a more concentrated to a less concentrated area.
264 Desalination: A National Perspective DistillationâA process of desalination where the intake water is heated to produce steam. The steam is then condensed to produce product water with extremely low salt concentration. Drinking waterâWater safe for human consumption or which may be used in the preparation of food or beverages, or for cleaning articles used in the preparation of food or beverages. EffluentâWater leaving a desalting process. May be applied to both concentrate or product water. Electrodialysis (ED)âA process of desalination whereby an electrical current is used to separate out salt and impurities in the intake water through the use of semipermeable, ion-selective membranes operating in a DC electric field. Electrodialysis reversal (EDR)âA variation of the electrodialysis process using electrode polarity reversal to reduce and minimize scaling and fouling, thus allowing the system to operate at comparatively higher recoveries. EntrainmentâThe incorporation of small organisms, including the eggs and larvae of fish and shellfish, into an intake system. EvaporationâThe process by which water is converted to a vapor. Evaporation pondsâA concentrate management method where the concentrate is pumped into a shallow lined pond and allowed to evapo- rate naturally using solar energy. Evaporation rateâThe mass quantity of water evaporated from a speci- fied water surface per unit of time. FeedwaterâInput or raw water stream fed into the desalination process. FluxâTerm used in reverse osmosis to indicate the rate of water per- meation through the membrane. Usually expressed as gallons per square foot per day in the U.S., and liters per square meter per hour in metric units. Forward osmosisâA membrane-based separation process which uses osmotic pressure difference between a concentrated âdrawâ solution and a feed stream to drive water through a semi-permeable membrane. FoulingâThe reduction in performance of process equipment (heat transfer tubing, membranes, etc.) that occurs as a result of scale buildup, biological growth, or the deposition of colloidal material. Freeze desalinationâProduction of distillate by freezing a saline solu- tion and separating the salt water from the pure crystalline water prior to melting. Fresh waterâWater that generally contains less than 1,000 mg/L of dissolved solids.
Glossary 265 Gained output ratio (GOR)âA measure of the efficiency of a thermal desalination process, expressed as a ratio of the mass of water produced by the process over the mass of (saturated) steam supplying process heat. If, for example, 8 kg of water are produced from the desalination system for each kg of steam delivered to the process, the GOR is equal to 8. This measure is a dimensionless value. HybridâA system incorporating multiple processes or technologies; for example a desalination facility incorporating both thermal and membrane processes. Generally the technologies should at least be partially inte- grated for some process benefit to qualify as hybrid. ImpingementâThe pinning and trapping of fish or other larger organ- isms against the screens of water intake structures. IntakesâThe structures used to extract source water and convey it to the process system. Ion exchangeâA chemical process involving the reversible exchange of ions between a liquid and a solid. Land subsidenceâA gradual settling or sudden sinking of the Earth's surface owing to subsurface movement of earth materials, sometimes caused by groundwater withdrawal. LouversâA series of vertical panels placed perpendicular to the intake approach flow that serve to create a new velocity field that carries fish away from the intake and towards a fish bypass system. Mechanical vapor compression (MVC)âSee âvapor compression evaporation.â Membraneâ In desalination, used to describe a semipermeable film. Membranes used in electrodialysis are permeable to ions of either posi- tive or negative charge. Reverse osmosis and nanofiltration membranes ideally allow the passage of pure water and allow only minor passage of salts. Microfiltration (MF)âMembranes used to reduce turbidity and remove suspended particles, algae, and bacteria. MF membranes operate via a sieving mechanism under a lower pressure than either UF or NF mem- branes, through membrane pores of 0.03 to 10 Î¼m and an molecular weight cut off of greater than 100,000 daltons. Multiple effect distillation (MED)âA thin film evaporation process where the vapor formed in a chamber, or effect, condenses in the next, providing a heat source for further evaporation. Multi-port diffusionâThe employment of multiple outlet ports, rather than a single pipe, during the process of diffusion, thus allowing the mix-
266 Desalination: A National Perspective ing and dilution of the concentrate to be accelerated and lessening poten- tial impacts in sensitive areas. Multi-stage flash distillation (MSF)âA desalination process where a stream of brine flows through the bottom of chambers, or stages, each operating at a successively lower pressure, and a proportion of it flashes into steam and is then condensed. Nanofiltration (NF)âMembranes used for water softening, organics and sulfate removal, and some removal of viruses. Pressure-driven re- moval is by combined particle size-based sieving and solution/diffusion. Pores in NF membranes are usually smaller than 0.001 Î¼m and a molecu- lar weight cutoff of 1,000 to 10,000 daltons. NeedâThe minimum amount of water required to satisfy a particular purpose or requirement. Passive screenâAn intake screen that can be flushed back with com- pressed air, has no moving parts, and operates with a very low velocity (to mitigate impingement). Performance ratio (PR)âThe ratio of the mass of water produced by a desalination process over a fixed quantity of energy consumed. This ra- tio is useful, as steam may be delivered over a wide range of tempera- tures, and its heat content, both in terms of total enthalpy and in enthalpy of vaporization, differs at each temperature. Additionally, in many prac- tical cases, steam may not be the medium of heat transfer. The Perform- ance Ratio is most commonly defined as the mass, in pounds, of water produced by desalination per 1000 BTU of heat provided to the process. The SI equivalent of this formulation is the number of kg of water pro- duced per 2326 kJ of heat. PermeateâThe liquid that passes through a membrane. Post-treatmentâThe addition of chemicals to the product water to pre- vent corrosion of downstream infrastructure piping. PrecipitateâA substance separated from a solution by chemical or physical change as an insoluble amorphous or crystalline solid. Pre-treatmentâRefers to methods for treatment of water to remove suspended particles and control of biological growth, and to prepare the source water for further processing. Conventionally this involves coagu- lation, settling, and filtration. More recently ultrafiltration or microfiltra- tion can be used to prepare the water. Product waterâWater produced as a result of treatment or desalination processes. RecoveryâIn reverse osmosis processes, recovery indicates the amount/percentage of product water recovered from the feed stream.
Glossary 267 Reverse osmosis (RO)âA method of separating water from dissolved salts by passing feedwater through a semipermeable membrane at a pres- sure greater than the osmotic pressure caused by the dissolved salts. RO operates via a solution/diffusion mechanism whereby the water dissolves into and diffuses through the nonporous membrane, leaving the majority of the salts behind in the concentrate. RO membranes are also capable of removing some larger organic contaminants. Small uncharged species can pass through the membrane. Water that passes through the mem- brane leaves the unit as permeate or product water; most of the dissolved impurities remain behind and are discharged in a brine or waste stream. Ristroph screenâA modified traveling screen with water-filled lifting buckets that collect impinged organisms and transport them to a bypass, trough, or other protected area. Saline waterâWater with dissolved solids exceeding the limits of pota- bility. Saline water may include sea water, brackish water, mineralized ground and surface water, and irrigation return flows. SalinityâThe concentration of dissolved salts in water. ScalingâMineral deposits or precipitates that form on the interior sur- faces of process equipment or water lines as a result of heating or other physical or chemical change. SedimentationâThe removal of settleable suspended solids from water or wastewater by gravity in a quiescent basin or clarifer. Silt Density Index (SDI)âA measure of the fouling tendency of water, based on the timed flow of a liquid through a membrane filter at a con- stant pressure. SolubilityâA measure of the maximum amount of a certain substance that can dissolve in a given amount of water, or other solvent, at a given temperature. Surface intakesâOften called open intakes, they are in direct commu- nication with the ocean or sea where feedwater can be taken directly from the surface or below the surface through submerged intakes. Subsurface intakesâWhere feedwater is taken from below the floor of the ocean using natural occurring sand and geologic formations to pro- vide filtration. Subsurface intakes can be horizontally drilled from cen- tral wells, slant-drilled from onshore beaches, or constructed infiltration beds. Thermal vapor compression (TVC)âSee âvapor compression evapo- rationâ. Total dissolved solids (TDS)âThe mass of all inorganic and organic material per unit volume of water after a sample has been filtered to re- move suspended solids.
268 Desalination: A National Perspective Ultrafiltration (UF)âMembrane removal of high-weight dissolved or- ganic compounds, bacteria, and some viruses. UF membranes operate via a pressure-driven size-based sieving mechanism through a membrane with pores in the range of 0.002 to 0.1 Î¼m with an molecular weight cut off of 10,000 to 100,000 daltons. Vapor compression evaporation (VC/VCE)âThermal desalination processes that utilize heat from compression of water vapor for subse- quent evaporation of feedwater, either with a mechanical compressor (mechanical vapor compression, MVC) or a steam ejector (thermal vapor compression, TVC). Vapor compression processes are particularly useful for small to medium installations. Water reclamationâThe restoration of wastewater to a state that will allow its beneficial reuse. Water reuseâThe beneficial use of reclaimed water, such as for irriga- tion, cooling or washing. WithdrawalâWater removed from a source and used to meet a human need. Zero liquid discharge (ZLD)âA concentrate management approach in which no liquid effluent is discharged beyond the facility boundaries.