Desal Glossary

A B C D E F G H I J K L M N O P Q R S T U V W Z

A

absolute purity water
Water with a specific resistance of 18.3 megohm-cm at 25°C.
acid
(1) A substance that can react with a base to form a salt.
(2) A substance which can donate a hydrogen ion, or proton.
acidity
The capacity of an aqueous solution to neutralize a base.
acre-foot
The volume of water that would cover a one acre area one foot deep. Equivalent to approximately 1,233.6 cubic metres or 325,900 gallons.
aftercondenser
A condenser installed as the last stage of an evaporator venting system to minimize atmospheric steam discharge.
AFY
Acre-foot per year.
aggressive water
Water having corrosive qualities.
AI
Aggressivity index.
alkalinity
The ability of a water to neutralize an acid due to the presence of carbonate, bicarbonate and hydroxide ions.
Angstrom
(Å) A unit of measure equivalent to one ten-thousandth of a micron.
anhydrite
See “calcium sulphate”.
anion
A negatively charged ion that migrates to the anode when an   electrical potential is applied to a solution.
anion exchange
An ion exchange process in which anions in solution are exchanged for other anions bound to an ion exchange   resin or medium.
anode
The positive electrode where current leaves an electrolytic solution.
aqueous solution
A solution in which water is the solvent.
aquifer
A subsurface geological formation containing a large quantity of water.
array
A staged arrangement of membrane elements and pressure vessels in a system.
asymmetric membrane
A membrane that is not reversible, and can only desalinate efficiently in one direction.
atmosphere
(1) The gaseous region that surrounds the earth. (2) A unit of pressure equal to 1.0333 kg/sq cm, or 14.7 psi. Abbreviation: “atm”.

B

back pressure turbine
A turbine which expands steam to a constant condition which is above atmospheric pressure, converting thermal energy into shaft rotation.
bar
A unit of pressure defined as 105 Pascals and equal to 0.9869 atmospheres, 106 dynes per square centimetre, and 14.5 pounds per square inch.
battery limit
The boundary limits of equipment or a process unit that defines interconnecting points for electrical piping or wiring.
bicarbonate alkalinity
Water alkalinity caused by bicarbonate ions.
biofoul
Undesirable presence and growth of organic matter in a water system.
boil out
An evaporator cleaning process where wash water is boiled in an evaporator to remove scale deposits.
boiler
A vessel in which water is continually vaporised into steam by the application of heat.
boiler feedwater
Water which, in the best practice, is softened and/or demineralized and heated nearly to boiler temperature and deaerated before being pumped into a steam boiler.
boiling point
The temperature at which a liquid's vapour pressure equals the pressure acting on the liquid.
boiling point elevation
(BPE) The difference between the boiling point of a solution and the boiling point of pure water at the same pressure.
boiling point rise
(BPR) see “boiling point elevation”.
Boyle's Law
The volume of a gas varies inversely with its pressure at constant temperature.
brackish water
Water containing a low concentration of soluble salts, usually between 1,000 and 10,000 mg/L.
brine
Water saturated with, or containing a high concentration of salts, usually in excess of 36,000 mg/L.
brine concentrator
Term used to describe a vertical tube falling film evaporator employing special scale control techniques to maximize concentration of dissolved solids.
brine heater
The heat input section of a multi-stage flash evaporator where feedwater is heated to the process' top temperature.
brine staging
See “reject staging”.
British Thermal Unit
(BTU) The quantity of heat required to raise the temperature of one pound of water by one degree Fahrenheit.
BTU
See “British Thermal Unit”.
bubble point
The pressure at which air first passes through a wet membrane; the path being the channel of greatest pore size.
buffer
A substance that stabilizes the pH value of solutions.
BWRO
Brackish water reverse osmosis.

C

CA membrane
Cellulose acetate membrane.
calandria
The heating element in an evaporator consisting of vertical tubes which act as the heating surface.
calcium carbonate
A white, chalky substance which is the principle hardness and scale-causing compound in water. Chemical formula is CaCO3.
calcium carbonate equivalent
(mg/L as CaCO3) A convenient unit of exchange for expressing all ions in water by comparing them to calcium carbonate, which has a molecular weight of 100 and an equivalent weight of 50; signifies that the concentration of a dissolved mineral is chemically equivalent to the stated concentration of calcium carbonate.
calcium hardness
The portion of the total hardness attributed to calcium compounds.
calcium hydroxide
See “hydrated lime”.
calcium sulphate
A white solid known as the mineral “anhydrite” with the chemical formula CaSO4, and as “gypsum” with the formula CaSO4-2H2O.
calorie
The amount of heat required to raise the temperature of one gram of water by one degree Celsius, also known as a “small calorie”. A “large calorie” or “kilocalorie” is the amount of heat required to raise the temperature of a kilogram of water by one degree Celsius.
capacitive deionization
An electrically regenerated electrosorption process capable of desalinating saline water.
cartridge filter
A filter unit with cylindrical replaceable elements or cartridges.
cation
A positively charged ion that migrates to the cathode when an electrical potential is applied to a solution.
cation exchange
The ion exchange process in which cations in solution are exchanged for other cations bound to an ion exchange resin or medium.
CCPP
Calcium carbonate precipitation potential.
cellulose acetate
(CA) A plastic material used to make the cellulosic-type semipermeable reverse osmosis membranes.
Celsius
(C) The SI temperature scale on which 0 degrees is the freezing point and 100 degrees is the boiling point of water. Often referred to as the “centigrade” scale.
centigrade
Colloquial term for the Celsius scale of temperature measurement.
Charles' Law
The volume of gas at constant pressure varies in direct proportion to the absolute temperature.
chloramines
Disinfecting compounds containing nitrogen, chlorine and hydrogen, formed by the reaction between hypochlorous acid and ammonia and/or organic amines in water. Also called “combined available chlorine”.
chloride
(1) The ionic form of the element chlorine where the atom has gained one electron. Chemical symbol: Cl-.
(2) Any salt containing the Cl- anion.
chlorine
An oxidant commonly used as a disinfectant in water and wastewater treatment. Chemical formula: Cl2.
chlorine residual
The amount of chlorine remaining in water after application at some prior time; the difference between the total chlorine added and that consumed by oxidizable matter. See “free chlorine residual”.
CIP
See “clean-in-place”.
clean-in-place
(CIP) A method of cleaning process equipment to restore its performance without removing it from the system.
coagulant
A chemical added to initially destabilize, aggregate, and bind together colloids and emulsions to improve settleability, filterability or drainability.
cogen
See “cogeneration”.
cogeneration
A power system that simultaneously produces both electrical and thermal energy from the same source.
colloid
Suspended solid with a diameter of less than one micron that cannot be removed by sedimentation alone.
combined cycle generation
A gas turbine generator system where heat from turbine generator exhaust gases is recovered by a steam generating unit whose steam is used to drive a steam turbine generator.
compressor
A mechanical device used to increase the pressure of a gas or vapour.
concentrate
The water containing the dissolved solids removed during desalination.
concentration
(1) The amount of a substance dissolved or suspended in a unit volume of solution. (2) The process of increasing the amount of a substance per unit volume of solution.
concentration factor
1) A number indicating the number of times a solution has been concentrated from its initial condition.
2) A number indicating the number of times a solution may be concentrated.
concentration polarization
A phenomenon in which solutes form a dense, polarized layer next to a membrane surface which eventually restricts flow through the membrane.
condensate
Water obtained by evaporation and subsequent condensation.
condensation
The change in state from vapour to liquid; the opposite of evaporation.
condenser
A heat exchanger device used to cool steam and convert it from the vapour to the liquid phase.
condensing turbine
A turbine which expands steam into a condition which is below atmospheric pressure, converting thermal energy into shaft rotation.
conductance
(1) A measure of a solution's electrical conductivity that is equal to the reciprocal of the electrical resistance.
(2) A rapid method of estimating the dissolved-solids content of a water sample by determining the capacity of a water sample to carry an electrical current.
conversion
See “recovery”.
cooling water
Water used, usually in a heat exchanger, to reduce the temperature of liquids or gases.
critical point
The combination of pressure and temperature at which point a gas and a liquid become indistinguishable.
crystallizer
Common term for a forced circulation evaporator.
CTA
Cellulose triacetate.
cubic centimetre
(cc) A volume measurement equal in capacity to one millilitre (mL). One US quart is approximately 946 cubic centimetres.
cubic metre
(m3) A volume measurement equal to 1,000 litres or 264. 2 US gallons. One cubic metre of water weighs one tonne.
cycles of concentration
(COC) The ratio of the total dissolved solids concentration in a recirculating water system to the total dissolved solids concentration of the makeup water.

D

DB
Design-Build
DBB
Design-Bid-Build
DBO
Design-Build-Operate
DBOOT
Design-Build-Own-Operate-Transfer
DDW
Distilled deionized water.
deaerator
Device used to remove dissolved air from a solution.
decarbonator
A device used to remove alkalinity from a solution by conversion to CO2 prior to air stripping.
degasifier
Device used to remove dissolved gases from a solution, usually by means of an air stripping column.
deionization
(DI) The process of removing ions from water, most commonly through an ion exchange process.
delta T
Differential temperature.
demineralizing
The process of removing minerals from water, most commonly through an ion exchange process.
desalination
The process of removing dissolved salts from water.
desalinization
See “desalination”.
desalting
See “desalination”.
dialysis
The separation of substances from solution on the basis of molecular size by diffusion through a semipermeable membrane.
diffusion
The movement of suspended or dissolved particles from a more concentrated to a less concentrated area.
diffusion dialysis
An ion exchange membrane process that separates ionic, nonionic or colloidal species from solution based on differing diffusion rates, using concentration differentials across the membrane as the driving force.
dilution
(1) Lowering the concentration of a solution by adding more solvent. (2) The engineered mixing of discharged water with receiving water to lessen its immediate aesthetic and/or biochemical impact.
dilution ratio
(1) The volumetric ratio of solvent to solute. (2) The ratio of the water volume in a stream to the waste volume introduced into the stream. The dilution ratio gives an indication of the capacity of a water body to assimilate waste.
dissociation
The process of ionization of an electrolyte or salt upon being dissolved in water, forming cations and anions.
dissolved organic carbon
(DOC) The fraction of TOC that is dissolved in a water sample.
dissolved organic matter
(DOM) The fraction of organic matter in water that passes through a 0.45 micron filter.
dissolved oxygen
(DO) The oxygen dissolved in a liquid.
dissolved solids
Solids in solution that cannot be removed by filtration with a 0.45 micron filter. See “total dissolved solids”.
distil
See “distillation”.
distillate
A liquid product condensed from vapour during distillation.
distillation
The process of boiling a liquid solution, followed by condensation of the vapour, for the purpose of separating the solute from the solution.
DO
See “dissolved oxygen”.
DOC
See “dissolved organic carbon”.
DOM
See “dissolved organic matter”.
drinking water
Water 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.
dry bulb temperature
The air temperature measured by a conventional thermometer.
dual purpose
Colloquial term for a facility designed to produce both electrical power and desalinated water.
dynamic membrane
A transient membrane formed on the surface of an established membrane by solids filtered from the feed stream.

E

EAR
Engineer-At-Risk
economy
In thermal desalination, the ratio of kilograms of distilled water produced per 2,326 kJ of energy input.
ED
See “electrodialysis”.
EDI
See “electrodeionization”.
EDR
See “electrodialysis reversal”.
EDS
European Desalination Society.
effect
One of several units of an evaporator, each of which operates at successively lower pressures.
EIR
Environmental impact report.
ejectocompressor
See “thermocompressor”.
electrodialysis
(ED) The separation of a solution's ionic components through the use of semipermeable, ion-selective membranes operating in a DC electric field.
electrodialysis-reversal
(EDR) A variation of the electrodialysis process using electrode polarity reversal to automatically clean membrane surfaces.
electrodeionization
(EDI) A water treatment process combining an electrodialysis membrane process with an ion exchange resin process to produce high purity, demineralized water.
electrolysis
The passage of electric current through an electrolyte, resulting in chemical changes caused by migration of positive ions towards the cathode, and negative ions to the anode.
electronic-grade water
Water used in the production of microelectronic devices which meets ASTM D-19 standards for resistivity, silica concentration, particle count and other criteria.
element
1) One of the fundamental substances that consist of atoms of only one kind and that constitute all matter.  2) A common term for an individual flat sheet RO membrane rolled into a cylindrical shape around a permeate tube.
elutriator
An extension in an evaporator vapour body to thicken the solids slurry to minimize the loss of liquor.
energy recovery
The retrieval of waste energy for some beneficial use.
energy recovery turbine
(ERT) A device used to recover pressure energy from reverse osmosis brine streams.
enthalpy
The total heat content of a liquid, vapour, or body.
entrainment
(1) The incorporation of small organisms, including the eggs and larvae of fish and shellfish, into an intake system. (2) The carryover of droplets of water with vapour produced during evaporation.
EPA
Environmental Protection Agency.
EPC
Engineer-Procure-Construct.
ER
Energy recovery.
ERD
Energy recovery device.
ERT
See “energy recovery turbine”.
evaporation
The process in which water is converted to a vapour that can be condensed.
evaporation rate
The mass quantity of water evaporated from a specified water surface per unit of time.
evaporator
A device used to heat water to create a phase change from the liquid to the vapour phase.
exchange capacity
An ion exchanger unit's limited capacity for storage of ions.
exhaustion
That condition that results when activated carbon, ion exchange resin, or other absorbents have depleted their capacity by using all available sites.
extraction turbine
A condensing turbine which has been designed to allow some steam to be removed for other uses after expansion to an intermediate pressure. Steam is removed at extraction points which are controlled at predetermined pressures.

F

Fahrenheit
(F) A temperature scale on which water's freezing point is plus 32 degrees and boiling point is plus 212 degrees.
falling film evaporator
An evaporator with vertical heat transfer surfaces where liquor falling down the surfaces is heated by steam condensing on the other side of the surface.
filtrate
Liquid remaining after removal of solids through filtration.
flash distillation
See “multi-stage flash evaporation”.
flash evaporator
A distillation device where saline water is vaporized in a vessel under vacuum through pressure reduction. See also “multi-stage flash evaporation”.
flashing
The process of vaporizing a fluid by pressure reduction rather than temperature elevation.
flux
(1) Flow rate per unit area. (2) Heat transfer rate per unit area.
forced circulation evaporator
An evaporator in which circulation is maintained by pumping the liquid through the heating element with relatively low evaporation per pass.
fouling allowance
see “fouling factor”.
fouling factor
A design criterion used to allow for some variation of equipment performance due to fouling.
Francis turbine
A centrifugal turbine-type energy recovery device. May simply be a reverse running pump.
free available chlorine
(FAC) The amount of chlorine available as dissolved gas, hypochlorous acid, and hypochlorite ions that is not combined with ammonia or other compounds in the water.
free available chlorine residual
The concentration of residual chlorine remaining at the end of a specific contact time which is available as dissolved gas, hypochlorous acid or hypochlorite ions, not combined with ammonia or in another less readily available form.
free chlorine
See “free available chlorine residual”.
freeze distillation
Production of distillate by freezing a saline solution and washing salts from the pure water crystals prior to melting.
fresh water
Water that usually contains less than 1,000 mg/L of dissolved solids.

G

gain output ratio
(GOR) A measure of evaporator performance which represents the ratio of mass flow of distillate to steam input.
gallon, Imperial
A unit of volume for liquid substances approximately equal to 1.2 gallons (US) or 4.546 litres.
gallon (US)
A unit of volume for liquid substances equal to 231 cubic inches and approximately equivalent to 3.785 litres.
gas blanketing
1) The use of an inert gas in an enclosed tank's vapour space to minimize vapour formation or keep out unwanted air and moisture. 2) The action of non-condensible gases impeding heat transfer as a result of poor venting.
gradient
The rate of change of an elevation, velocity, pressure, temperature, or other parameter.
grain
(gr) A unit of mass equal to 0.0648 gram.
greenhouse gas
(GHG) Gases including carbon dioxide, methane, nitrous oxide, and Chlorofluorocarbons (CFCs) which have been recognized to contribute to the greenhouse effect.
GOR
see “gain output ratio”.
groundwater
Subsurface water found in porous rock strata and soil.
gypsum
The mineral consisting primarily of fully hydrated calcium sulphate. Chemical formula: CaSO4-2H2O.

H

H2S  See “hydrogen sulphide”.
hard water
Alkaline water containing dissolved salts that interfere with some industrial processes and prevent soap from sudsing. Moderately hard water has a “hardness” of 75 to 150 mg/L, while hard water has a “hardness” above 150 mg/L.
hardness
The total concentration of calcium and magnesium ions in water expressed as calcium carbonate.
heat balance
An accounting of the distribution of a system's heat loss and heat gain.
heat of condensation
The amount of heat released when a vapour changes state to a liquid.
heat of vaporization
The amount of heat required to change a volume of liquid to a vapour.
heat recovery steam generator
(HRSG)  A boiler device most typically installed on a gas turbine to recover heat and make steam using the hot exhaust gases.
hollow fibre membrane
Type of reverse osmosis and ultrafiltration membrane formed into small diameter tubes.
HRSG
See “heat recovery steam generator”.
hybrid
A 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 integrated for some process benefit to qualify as hybrid.
hydrogen sulphide
A toxic gas formed by the anaerobic decomposition of organic matter containing sulphur. Chemical formula: H2S.
hydrologic cycle
The natural cycle of continuous evaporation and condensation.
hydrolysis
A chemical change or decomposition of matter produced by combination with water.
hydrophilic
Having an affinity for water.
hydrophobic
Having an aversion to water.
hydroxide ion
A negatively charged ion consisting of a hydrogen atom and an oxygen atom. Chemical formula: OH-.
hyperfiltration
Filtration using a dense membrane that is often used synonymously with “reverse osmosis”.

I

IDA
International Desalination Association.
I/E
Impingement and entrainment.
Imperial gallon
See “gallon, Imperial”.
impingement
The entrapment of fish and other marine life on the surface of an intake screen when a high water velocity prevents escape.
impulse turbine
An energy recovery device used to recover pressure energy from reverse osmosis brine streams. Also known as Pelton Wheels.
indirect potable reuse
The use of water from streams or aquifers that have received discharges from wastewater treatment plants.
indirect reuse
The beneficial use of reclaimed water after releasing it for storage or dilution into natural surface waters or groundwater.
infiltrate
The penetration of water into the pores or interstices of the soil.
insoluble
A compound that has very low solubility.
intercondenser
A condenser used between stages to reduce steam consumption in the steam jet vacuum system in an evaporator system.
inverse solubility
The characteristic attributed to a substance that becomes less soluble with increasing temperature.
ion
An electrically charged atom, molecule or radical.
ion exchange
(IX or IE) A chemical process involving the reversible exchange of ions between a liquid and a solid.
ionic strength
A measure of solution strength based on both the concentrations and the valencies of the ions present.
IPP
Independent power project.
IX
See “ion exchange”.
IWP
Independent water project.
IWPP
Independent water and power project.

J

jelly roll
A descriptive term commonly used for a spiral wound membrane element.
joule
(J) An SI unit of energy or work equal to the work done by the application of one newton acting through a distance of one metre.

K

kilogram
(kg) A unit of mass equivalent to 1,000 grams or approximately 2.205 pounds.

L

Langelier Saturation Index
(LSI) A measure of the degree of saturation of calcium carbonate in water based on pH, alkalinity and hardness. A positive LSI indicates that calcium carbonate may precipitate from solution to form scale.
Larson ratio
(LR) A ratio used to determine impacts of chlorides and sulphates on corrosion in iron pipes.
latent heat
The heat required to cause a change of state at constant temperature, such as the vaporization of water, or the melting of ice.
leakage
(1) The presence of an ionic species in ion exchanger effluent that usually indicates bed exhaustion. (2) The uncontrolled loss of water from a tank or aquifer.
lime
The term generally used to describe ground limestone (calcium carbonate), hydrated lime (calcium hydroxide), or burned lime (calcium oxide).
lime softening
The addition of sufficient lime to raw water to achieve a reduction of carbonate hardness.
limestone
A sedimentary rock composed primarily of calcium carbonate.
litre
(L) A unit of volume equal to 1,000 cubic centimetres, or 1.057 quarts. One litre of water weighs 1,000 grams.
LNG
Liquefied natural gas.
long ton
A unit of weight equal to 2,240 pounds.
LR
See “Larson ratio”.
LSI
See “Langelier Saturation Index”.
LTA
Low temperature additive.

M

MAF
Million acre feet.
makeup water
Fluid introduced into a recirculating stream to maintain an equilibrium of temperature, solids concentration or other parameter(s).
marginal water
Water which is of questionable economics to treat for any further purpose.
ME
Multiple effect. See “multiple effect distillation”.
MEB
Multiple effect boiling. See “multiple effect distillation”.
MED
See “multiple effect distillation”.
megalitre
(ML) A unit of volume equal to 1 million litres.
membrane
A thin barrier that permits the passage of particles of a certain size or of particular physical or chemical properties.
membrane processes
Processes including reverse osmosis, electrodialysis, and ultrafiltration that use membranes to remove dissolved material or fine solids.
membrane softening
A water softening process that utilizes semi-permeable nanofiltration or reverse osmosis membranes to remove hardness constituents such as calcium and magnesium from water.
metre
The basic SI unit of length equivalent to approximately 39.37 inches, or the distance travelled by light in a vacuum in 1/299,792,458 seconds.
metric ton
A unit of mass equal to 1,000 kilograms or approximately 2,204 pounds. Also called “tonne”.
MF
See “microfiltration”.
MGD
Million gallons (US) per day.
MIGD
Million gallons (Imperial) per day.
microelectronic water
See “electronic-grade water”.
micromho
A unit measure of conductivity equal to one millionth of a mho.
micron
(µ) A unit of length equal to one-millionth of a metre. Also called “micrometre”.
milligram
(mg) A unit of mass equal to one-thousandth of a gram.
milligrams per litre
(mg/L) A common unit of measurement of the concentration of a material in solution.
millilitre
(mL) A unit of volume equal to one cubic centimetre.
mist eliminator
A device used to remove entrained droplets of water from a vapour stream produced during evaporation.
mixed bed demineralizer
Ion exchange demineralizer containing strong-acid and strong-base resins in a single vessel.
mother liquor
The concentrated solution that remains after evaporation or crystallization.
motive steam
High pressure steam which is used to operate a steam-jet ejector or thermocompressor.
MSF
See “multi-stage flash evaporation”.
MSF-BR
Multi-stage flash evaporation, brine recirculation.
MSF-OT
Multi-stage flash evaporation, once-through.
multiple effect distillation
(MED) A thin film evaporation process where the vapour formed in a chamber, or effect, condenses in the next, providing a heat source for further evaporation.
multi-stage flash evaporation
(MSF) A 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.
MVC
Mechanical vapour compression. See “vapour compression evaporation”.
MVR
Mechanical vapour recompression. See “vapour compression evaporation”.

N

nanofiltration
A specialty membrane filtration process which rejects solutes larger than approximately one nanometer (10 angstroms) in size.
National Pollutant Discharge Elimination System
(NPDES) US programe to issue, monitor, and enforce pretreatment requirements and discharge permits under the Clean Water Act.
net driving pressure
(NDP) The net feed pressure of a reverse osmosis system plus the osmotic pressure of the permeate, minus the permeate line pressure and osmotic pressure of the feedwater.
neutralization
The chemical process that produces a solution that is neither acidic nor alkaline.
NF
See “nanofiltration”.
noncarbonate hardness
(NCH) The hardness in water caused by chlorides, sulphates and nitrates of calcium and magnesium.
noncondensable gas
(NCG) Gaseous material not liquefied when associated water vapour is condensed in the same environment.
normal solution
A solution that contains one equivalent weight of a substance per litre of solution.
NTU
Nephelometric Turbidity Unit.

O

Offtaker
The organization which purchases water or power produced under a long-term WPA, PPA or PWPA.
osmosis
Movement of water from a dilute solution to a more concentrated solution through a permeable membrane separating the two solutions.
osmotic pressure
Excess pressure that must be applied to a concentrated solution to produce equilibrium and prevent the movement of a more dilute solution through a semipermeable membrane into the more concentrated solution.
OSW
Office of Saline Water.
outfall
The location where a storm or sanitary sewer or effluent is discharged into a receiving water body.
oxidation
(1) A chemical reaction in which an element or ion loses electrons. (2) The biological or chemical conversion of organic matter into simpler, more stable forms.

P

PA membrane
A polyamide membrane.
parts per million
(ppm) A common unit of measure used to express the number of parts of a substance contained within a million parts of a liquid, solid, or gas. Generally interchangeable with “milligrams per litre” in dilute solutions and water treatment calculations.
parts per thousand
(ppt) A unit of measure used to express the number of parts of a substance contained within a thousand parts of a liquid, solid, or gas. Generally used to specify a water's salinity and commonly indicated by the symbol “0/00”.
Pelton wheel
An impulse hydraulic turbine that may be used as an energy recovery device in high head applications such as seawater reverse osmosis.
performance ratio
A unit of measurement used to characterize evaporator performance, expressed as the mass of distillate produced per unit of energy consumed.
permanent hardness
Hardness associated with sulphates, chlorides and nitrates of calcium and magnesium which remain after boiling.
permeate
The liquid that passes through a membrane.
permselective membrane
See “semipermeable membrane”.
pezodialysis
Membrane process to remove salt from solution where salt, rather than water, passes through the membrane.
pH
The reciprocal of the logarithm of the hydrogen ion concentration in gram moles per litre. On the 0 to 14 pH scale, a value of 7 at 25°C (77°F) represents a neutral condition. Decreasing values indicate increasing hydrogen ion concentration (acidity), and increasing values indicate decreasing hydrogen ion concentration (alkalinity). Full form is “potential of hydrogen”.
pharmaceutical-grade water
See “USP-purified water”.
phase
The state of a substance; solid, liquid or vapour.
phase change
A change in the state of a substance such as from liquid to vapour (evaporation) or solid to liquid (melting)
plugging factor
See “Silt Density Index”.
polyamide
A molecular chain polymer made of amide linkages used in the construction of thin film composite reverse osmosis membranes.
polysulphone
A synthetic thermoplastic polymer used in the manufacture of ultrafiltration membranes and in thin film composite and charged polysulphone reverse osmosis membranes.
post chlorination
Addition of chlorine after completion of other treatment processes.
post-treatment
The treatment processes following desalination, usually employed to stabilize water and reduce its corrosivity and improve its taste.
potable water
See “drinking water”.
power-to-water ratio
1) A parameter comparing electric power production to desalinated water production. 2) A parameter comparing electric power demand to desalinated water demand.
PPA
Power purchase agreement.
ppm
See “parts per million”.
precipitate
A solid that separates from a solution.
pressure vessel
1) A tank or chamber designed to contain a fluid at a different pressure from its surroundings. 2) The cylindrical vessel which contains one or more membrane elements.
process water
Water that is used for, or comes in contact with an end product or the materials used in an end product.
product staging
Reverse osmosis process configuration where the product from one stage is used as feedwater on a subsequent stage to improve product water quality.
product water
Water produced as a result of treatment or desalination processes.
pseudo-hardness
The action exhibited by sea, brackish and other waters containing high concentrations of sodium that interferes with the normal behaviour of soap.
PWPA
Power and water purchase agreement.

Q

quicklime
A calcium oxide material produced by calcining limestone to liberate carbon dioxide, also called “calcined lime” or “pebble lime”, commonly used for pH adjustment. Chemical formula: CaO.

R

raw water
Untreated surface or groundwater.
reboiler
An evaporator-condenser unit which produces secondary steam after condensation of primary steam. Used to isolate steam systems and avoid cross-contamination of boiler chemicals.
reboiler
see “steam transformer”.
recarbonation
The reintroduction of carbon dioxide into water, usually during or after lime-soda softening.
recharge
The addition of water from another source to replenish a groundwater supply.
reclaimed water
Wastewater that has been treated to a level that allows for its reuse for a beneficial purpose.
reclamation
The process of improving or restoring the condition of land or other material to a better or more useful state.
recovery
In reverse osmosis processes, recovery indicates the amount/percentage of product water recovered from the feed stream. Also known as “conversion”.
regenerant
A chemical solution used to restore the exchange capacity of an ion exchange resin.
regeneration
The process of restoring the exchange capacity of an ion exchange material.
reject
The waste stream containing impurities rejected in a treatment process, most commonly applied to reverse osmosis, electrodialysis, and ultrafiltration systems.
reject staging
Reverse osmosis process configuration where the reject from one stage is used as feedwater on a subsequent stage to increase water recovery. Also called “brine staging”.
rejection
In reverse osmosis, the percentage of dissolved solids removed from the feedwater as it passes through the semipermeable membrane. Also called “salt rejection”.
relative humidity
The total amount of water vapour present in the air, expressed as a percentage of the maximum amount that the air could hold at a given temperature.
repurification
Treatment of wastewater to a high quality, usually involving a desalination process.
resin
A material having ion exchange properties, used in ion exchange systems.
resistivity
A measure of resistance to the flow of electricity, used as an accurate measure of a water's ionic purity.
reuse
Using a product or component in its original form more than once.
reverse osmosis
(RO) A method of separating water from dissolved salts by passing feedwater through a semipermeable membrane at a pressure greater than the osmotic pressure caused by the dissolved salts.
RFP
Request for proposals.
RFQ
Request for qualifications.
rising film evaporator
An evaporator using vertical heat transfer surfaces where liquor on one side of the surface is boiled by steam condensing on the other side, causing vapours to rise, carrying the liquid upwards as a film.
RO
See “reverse osmosis”.
ROWPU
Reverse osmosis water purification unit.

S

saline
Containing or resembling sodium chloride or similar salts.
saline water
See “salt water”.
salinity
(1) The concentration of dissolved salts in water. (2) The total dissolved solids in water after all carbonates and organic matter have been oxidized.
salinization
The accumulation of salts in a soil to the extent that plant growth is inhibited, usually occurring as a result of excessive irrigation in an arid area. Also known as “salination”.
salt
A class of ionic compounds formed by the combination of an acid and a base, of which sodium chloride is one of the most common examples.
salt flux
The amount of dissolved substances that are able to pass through a reverse osmosis membrane.
salt rejection
In reverse osmosis, the ratio of salts removed to the original salt concentration.
salt water
Water containing a dissolved salt concentration greater than 10,000 mg/L.
salt water intrusion
The intrusion of salt water into a body of fresh surface water or groundwater.
salting out
A procedure in which salt is added to a solution to cause an organic compound to precipitate so that it can be physically removed.
saturated steam
Vapour in equilibrium with water at the boiling temperature, containing no liquid.
scale
A mineral deposit or precipitate that forms on the interior surface of process equipment or water lines as a result of a heating or other physical or chemical change.
SDI
See “Silt Density Index”.
seawater
General term for sea or ocean water, with a typical total dissolved solids concentration of 35,000 mg/L.
sedimentation
The removal of settleable suspended solids from water or wastewater by gravity in a quiescent basin or clarifier.
seed
(1) Crystalline particles added to a supersaturated solution to induce precipitation. (2) Well-digested sludge used to seed a sludge digester.
semipermeable membrane
A membrane that does not have measurable pores, but through which smaller molecules can pass.
sensible heat
Heat measurable by temperature alone.
shell side
The space surrounding the tube bundle and contained by the pressure vessel of a heat exchanger. Generally for a condenser, the shell side is where the condensing vapour is contained. For liquid-liquid heat exchangers, the fluid which is less likely to foul or scale occupies the shell side.
Silt Density Index (SDI)
A measure of the fouling tendency of water, based on the timed flow of a liquid through a membrane filter at a constant pressure.
simple cycle
A gas turbine which is operating without heat recovery from the exhaust gases.
sodium cycle exchange
Ion exchange water softening process in which sodium on the ion exchange resin is exchanged for hardness and other ions in water. Sodium chloride is commonly used as the regenerant.
sodium hexametaphosphate
An alkaline metallic element which is water soluble and commonly used as a sequestering or dispersing agent.
sodium hydroxide
Caustic soda. Chemical formula is NaOH.
sodium metabisulphite
A crystalline form of sulphur dioxide used to remove chlorine. Chemical formula: Na2S2O5.
soft water
Any water containing less than 17.1 mg/L (1 gpg) of calcium or magnesium expressed as calcium carbonate.
softening
Treatment process that involves the removal of calcium and magnesium ions from water.
solar pond
Pond used to accomplish evaporation via direct solar heating.
solar still
A distillation device that utilizes solar energy.
solids balance
A mathematical representation of a treatment system that defines the amount of solids entering and exiting each unit treatment process.
solubility
The tendency of a substance to dissolve in a solution under a given set of conditions.
solubility product
The equilibrium constant that describes the reaction by which a precipitate dissolves in pure water to form its constituent ions.
soluble
Capable of being dissolved in a fluid.
solute
A substance dissolved in a fluid.
solution
A liquid that contains dissolved solute.
solvent
Liquid capable of dissolving or dispersing one or more substances.
sparingly soluble compounds
Term used to describe compounds with solubility ranges from near zero to a few thousand milligrams per litre.
specific conductance
The measure of electrical conductance of water or a water solution, and the reciprocal of specific resistance. Usually stated in micromhos per cm.
specific energy consumption
1) The total energy consumed per unit volume of desalinated water. 2) The total of a specific type of energy, such as electricity, consumed per unit volume of desalinated water.
specific resistance
A measure of total ionized solids concentration determined by the resistance of a one cm cube of water to the passage of electricity under standard conditions.
spent regenerant
Wastes from the regeneration of an ion exchange system.
SPV
Special purpose vehicle.
stage
One of several units of a flash evaporator, each of which operates at a successively lower pressure.
standard seawater
A widely accepted “standard” total dissolved solids concentration of approximately 36,000 mg/L, considered to be typical of most seawaters.
standard solution
A solution whose strength or reacting value per unit volume is known.
steam transformer
A boiler which uses steam as the heat source so as to produce clean steam which cannot be contaminated by the heating steam.
Stiff & Davis Stability Index
Index used to determine the saturation point of calcium carbonate in seawater or other highly saline water.
still
Apparatus used in distillation.
STP
Standard temperature and pressure.
strong acid
An acid that approaches 100 percent ionization in dilute solutions.
strong acid ion exchanger
Ion exchange using a cationic resin that contains exchangeable functional groups derived from a strong acid.
strong base ion exchange
Ion exchange using an anionic resin that contains exchangeable functional groups.
submerged tube evaporator
An evaporator where steam enters a tube bundle submerged in the fluid to be boiled.
sulphate
The divalent, negative SO4 anion, or an ester containing the anion. Also spelled “sulfate”.
sulphide
The divalent, negative S anion, or a salt containing the anion.
sulphur
A flammable, nonmetallic element with many commercial uses. Also spelled “sulfur”. Chemical symbol: S.
superheated steam
Steam with additional heat added after vaporization, increasing its temperature and energy.
supersaturation
Refers to a solution containing more of a dissolved substance that is usually possible at equilibrium.
sweet water
Brackish water that may be used for drinking even though it may not meet potable water standards.
SWRO
Seawater reverse osmosis.

T

TBT
See “top brine temperature”.
TCD
Thermo-compression distillation.
TDS
See “total dissolved solids”.
TFC
See “thin film composite”.
thermocompressor
A device used to increase the pressure of a gas or vapour generally driven by a high pressure source of the vapour to be compressed. Typically an ejector that uses high-pressure steam to increase the pressure of a lower pressure steam.
thin film composite
(TFC) A membrane configuration in which a thin membrane film is supported by a thicker, more porous material.
thin film evaporator
Evaporator where liquid flows or is sprayed over heat transfer surfaces, usually tubes, in a thin, turbulent film.
tide gate
A swinging gate in a sewer pipe that prevents seawater from entering the system during high tides.
TMP
See “transmembrane pressure”.
ton
A unit of mass equal to 2,000 pounds or 907.2 kilograms. Also called “short ton”.
tonne
A unit of mass equal to 1,000 kilograms or approximately 2,204 pounds. Also called “metric tonne”.
top brine temperature
(TBT) The maximum temperature of the fluid being evaporated in an evaporator system.
total dissolved solids
(TDS) The weight per unit volume of all volatile and non-volatile solids dissolved in water or wastewater after a sample has been filtered to remove colloidal and suspended solids.
total solids
(TS) The sum of dissolved and suspended solids in a water or wastewater sample. Matter remaining as residue upon evaporation at 103 to 105°C.
transmembrane pressure
(TMP) The average pressure across a membrane, measured as the hydraulic pressure differential from the feed side to the permeate side.
tubesheet
Flat plate used to secure the ends of tubes in an evaporator, heat exchanger or boiler.
TVC
Thermal vapour compression. See “vapour compression evaporation”.
TVR
Thermal vapour recompression. See “vapour compression evaporation”.
TWPU
Tactical water production (purification) unit.

U

UF
See “ultrafiltration”.
ultrafiltration
(UF) A low pressure (200-700 kPa, 20-100 psi) membrane filtration process which separates solutes in the 20 to 1,000 angstrom (up to 0.1 micron) size range.
ultrapure water
Water with a specific resistance higher than 1 megohm-cm.
USP-purified water
Water meeting USP quality requirements that has been purified by distillation, ion exchange or other suitable process from water complying with US EPA drinking water regulations, and contains no added substances.

V

vacuum deaerator
Device operating under vacuum to remove dissolved gases from a liquid.
vapour
The gaseous phase of a material that is in the solid or liquid state at standard temperature and pressure.
vapour compression evaporation
(VC/VCE) Evaporative system where vapour boiled off in the evaporator is mechanically compressed and reused as the heating medium.
vapour pressure
The pressure at which equilibrium is established between the liquid and gas phases of a substance.
vaporization
The process where a substance changes from a liquid or solid to the gaseous state.
VBNC
See “viable but non-culturable”.
VC
See “vapour compression evaporation”.
VCE
Vapour compression evaporation.
viable but non-culturable (VBNC)
Bacteria commonly found in seawater that cannot be cultured on standard bacteriological media used for culturing heterotrophic bacteria.
VTE
Vertical tube evaporator.
VVC
Vacuum vapour compression.

W

waste heat evaporator
An evaporator that uses the heat of a gas turbine, diesel engine jacket water, or exhaust gas.
Water Factory 21
An Orange County, California treatment plant designed to produce high quality water from municipal wastewater for injection into aquifers, creating a coastal barrier and preventing sea water intrusion of underground water supplies.
water for injection
(WFI) Water purified to USP standards by distillation, ion exchange or other suitable process, containing no additives, and intended for use as a solvent for preparation of parenteral solutions.
Water Purchase Agreement
(WPA) A DBOOT contract is often referred to as the WPA.  A WPA is a long-term water supply agreement between a contractor and offtaker which may be part of an IWP or IWPP.
water purveyor
An agency or person that supplies water.
water reclamation
The restoration of wastewater to a state that will allow its beneficial reuse.
water reuse
The beneficial use of reclaimed water, such as for irrigation, cooling or washing.
Water Tariff
The Water Tariff is defined as the cost the offtaker will pay for provision of a specified water quantity and quality.  The tariff may include a fixed component to cover the capital expenditure and fixed O&M costs, and a variable component to cover energy costs and variable O&M costs. The offtaker may have the option of purchasing additional water at a lower, 'commodity charge' rate.
water vapour
The gaseous form of water.
water-soluble
The description of a material that dissolves in water.
waterbox
The chamber at the inlet end of a condenser tubesheet.
watermaker
A common term for a packaged, vapour compression thermal desalination unit.
weak acid
A poorly ionized acid that dissociates very little and produces few hydrogen ions in aqueous solution.
wet bulb temperature
The temperature reading taken from a thermometer with a wetted wick surrounding its bulb.
work exchanger
A mechanical energy recovery device where the energy of one fluid stream is transferred to another fluid.
WPA
see “Water Purchase Agreement”.
WSTA
Water Science and Technology Association.

Z

zeolites
Minerals or synthetic resins that have ion exchange capabilities.
zeolite softening
Water softening process using a zeolite resin bed to accomplish ion exchange.
zero liquid discharge
(ZLD) A facility that discharges no liquid effluent to the environment.
ZLD
See “zero liquid discharge”.