absolute filter rating

Filter rating meaning that 99.9% (or essentially all) of the particles larger than a specified micron rating will be trapped on or within the filter.


The process in which one substance penetrates into the body of another substance, termed the absorbent. An example is the absorption of water into soil.


A substance which releases hydrogen ions when dissolved in water. Most acids will dissolve the common metals and will react with a base to form a neutral salt and water. An acid is the opposite of an alkali, has a pH rating lower than 7.0, will turn litmus paper red, and has a sour taste.


The quantitative capacity of a water or water solution to neutralize an alkali or base. It is usually measured by titration with a standard solution of sodium hydroxide, and expressed in ppm or mg/L of its calcium carbonate equivalent.

activated carbon

A water treatment medium, found in block, granulated, or powdered form, which is produced by heating carbonaceous materials, such as coal, wood, or coconut shells, in the absence of air, creating a highly porous adsorbent material. Activated carbon is commonly used for dechlorination, organic chemical reduction and radon reduction.


The physical process occurring when liquids, gases, or suspended matters adhere to the surfaces of, or in the pores of, an adsorbent media such as activated carbon. Adsorption is a physical process which occurs without chemical reaction.

air check

A device which allows water, but not air, to pass through it. An air check is a typical component of a treatment system using a regenerant eductor.

air gap

A clear vertical space through the free atmosphere between the lowest opening of any pipe or faucet conveying water or waste to a tank, plumbing fixture receptor, or other device and the flood level rim of the receptacle. An air gap is used to prevent cross connection between a water treatment device and a possible source of wastewater thereby preventing a reverse flow of water from the sewer into the water supply system. Without an air gap, such reverse flow could occur due to an increase in the pressure in the sewer system or the creation of a negative pressure in the water supply line. Local plumbing codes usually require the air gap to be twice the diameter of the inlet with a minimum width of 1 1/2 inches.


Plant life (green scum) containing chlorophyll is usually found in stagnant surface water. Sometimes it may be controlled in a pond by the addition of Potassium Permanganate. In a water supply system, chlorination followed by dechlorination is often used.


A substance which creates a bitter taste and a slippery feel when dissolved in water and will turn red litmus paper blue. An alkali has a pH greater than seven and is the opposite of an acid. Highly alkaline waters tend to cause drying of the skin. Alkalis may include the soluble hydroxide, carbonate, and bicarbonate salts of calcium, magnesium, potassium, and sodium. A hydroxide alkali may also be called a base.


A negatively charged ion in solution, such as bicarbonate, chloride, or sulfate. An anion [such as chloride (Cl-)] may result from the dissociation of a salt, acid, or alkali.

anion exchange

An ion exchange process in which anions in solution are exchanged for other anions from an ion exchanger. In demineralization, for example, bicarbonate, chloride and sulfate anions are removed from solution in exchange for a chemically equivalent number of hydroxide anions from the anion exchange resin. Please call Toll Free 877-262-5191, International 316-262-5191 for a product solution.


Natural underground reservoirs of porous layers of sand, rock or gravel.


The process in which solids are worn down by friction, often between particles of the same material. Filter media and ion exchange materials are subject to attrition during backwashing, regeneration and service.

automatic water softener (or automatic filter)

A water softener (or filter) that is equipped with a clock timer, meter, or sensor which automatically initiates the backwash and/or regeneration process at the preset intervals of time. A predetermined number of gallons of water usage or as determined by a sensor. All operations, including bypass of treated or untreated water (depending upon design), backwashing, brining, rinsing, and returning the unit to service are performed automatically.


Abbreviation for American Water Works Association. Most municipal water treatment plants hold membership in this association.



Flow of water in a pipe or line in a direction opposite to the normal flow; often associated with back siphonage or the flow of possibly contaminated water into a potable water system.

backflow preventor

A device or system installed in a water line to stop backflow from a non-potable source.

back pressure

Pressure which creates resistance against the flow of water.


The upflow or counter-current flow of water through a filter or ion-exchange medium, lifting the mineral bed and flushing away to the drain the particles of foreign matter that have been filtered from the water supply during the service cycle.


Bacteria are single cell microorganisms capable of replicating on their own. They can be divided into two broad categories, aerobic (requiring oxygen) and anaerobic (not requiring oxygen). Bacteria can live in a very broad range of habitats. Some, for example pseudomonads, can thrive in environments containing a very low level of nutrients. These bacteria are frequently slime producers and are a major problem in water treatment systems. Other bacteria, which adhere to surfaces, secrete a gelatinous material which serves to protect the bacteria from chemical disinfectants. This combination of bacteria and their protective coating is sometimes referred to as biofilm. The concentration of bacteria in water is commonly given in terms of colony forming units (cfu) per ml.A colony forming unit is a viable bacterium able to replicate to form a whole colony when incubated in a given environment.


An alkali that releases hydroxyl ions when dissolved in water. Bases reset with acids to form a neutral salt and water. In general they taste bitter rather than sour, and feel slippery and reverse the color changes produced by acids in indicators. For example, they turn litmus paper blue.


A mass of ion exchange resin particles or filter media contained in a column.

bed depth

The height of the resin or filter media in the column after it has been properly conditioned for effective operation, usually expressed in inches. This depth excludes any supporting bed.

bed expansion

The effect produced during backwashing: the resin particles become separated and rise in the column. The expansion of the bed due to the increase of the space between resin particles may be controlled by regulating backwash flow.


A chemical which can kill or inhibit the growth of lining organisms such as bacteria, fungi, molds, and slime. Biocides can be harmful to humans.

brackish water

Water containing bacteria between 1.000 and 15,000 ppm of dissolved solids.


The first appearance in the solution flowing from an ion exchange unit of unabsorbed ions similar to those which are depleting the activity of the resin bed. Breakthrough is an indication that regeneration of the resin is necessary.


A strong solution of salt(s), such as the sodium chloride or potassium brine used in the regeneration of ion exchange water softeners, but also applied to the mixed sodium, calcium and magnesium chloride waste solution from regeneration.

brine ejector (eductor)

A device used to draw a solution such as brine from a storage tank and force it into a cation or anion water treatment unit.


A chemical which causes a solution to resist changes in pH, or to shift the pH to a specific value.


A connection or a valve system that allows untreated water to flow to a water system while a softener or filter is being regenerated, backwashed or serviced.



Calcium carbonate (CaCO3). A tradename for finely ground limestone, very high in calcium carbonate, which is used to raise the pH of acidic water.

calcium (Ca)

One of the primary elements of the earth’s crust commonly found in water as a dissolved solid. The presence of calcium in water is a factor contributing to the formation of scale and insoluble soap curds which are means of clearly identifying hard water. It is sometimes referred to as lime.

calcium carbonate equivalent

All forms of water hardness and other salts are commonly expressed in terms of calcium carbonate equivalents. This is necessary so that minerals of varying weight can be expressed in chemically equivalent terms.


In a softener or deionizer it is the adsorption activity possessed in varying degree by ion exchange materials. This quality may be expressed as kilograins per cubic foot, gram-milliequivalents per gram, pound-equivalents per pound, gram-milliequivalents per milliliter, etc., where the of these ratios represent the weight of the ions adsorbed and the denominators, the weight or volume of the adsorbent. It can also refer to the ability of any media to take up a specific contaminant and is rated by time over gallons. As to flow rates, it is the maximum or minimum flow obtainable under given conditions of media, temperature, pressure, velocity, etc.

carbon dioxide

Water with a low pH value usually contains free carbon dioxide. Its presence is caused generally by absorption of carbon dioxide (CO2) from the air as water falls as rain, or by decay of organic matter in the earth. Well water containing substantial quantities of CO2 has a resultant low pH and corrosive qualities. Carbon dioxide in water forms a weak carbonic acid.


A positively charged particle or ion.

caustic soda

The common name for sodium hydroxide and often used as a regenerant of anion resin in deionization systems.

cellulose acitate

A synthetic polymer derived from naturally occurring cellulose and widely used in the fabrication of membranes. The polymers used for water purification membranes may be diacetate, triacetate or blends of these materials.


The flow of water or regenerant taking the line of least resistance through a media bed, as opposed to the usual distributed flow through all passages of the bed. Channeling may be due to fouling of the bed, poor distribution design, low flow rates, or insufficient backwash.


Chemicals used to disinfect municipal water. They are formed by reacting ammonia and free chlorine and may occur naturally when free chlorine combines with ammonia arising from the breakdown of vegetation. Chloramines are strong oxidants.


Widely used in the disinfection of water and as an oxidizing agent for organic matter, iron, hydrogen sulfide, etc. It is available as a gas, as a liquid in sodium, hypochlorite, or as a solid in calcium hypochlorite. In water chlorine reacts with organics to form trihalomethanes (THM) which can cause cancer.


A material such as alum, which will form a gelatinous precipitate in water, and gather finely divided particles into larger ones which can then be removed by settling and/or filtration.


The undesirable physical compression of a reverse osmosis or ultra filtration membrane which results in reduced flux rates. The phenomenon is accelerated at higher temperatures and pressures.


A term used in distillation, electrodialysis, reverse osmosis, and ultrafiltration to describe that portion of the incoming feedwater that has passed across the membrane but has not been converted to product water and is being sent to the drain.


Water which has liquefied from steam.


The ability of an aqueous solution to carry electric current depends on the presence of ions in the solution. Conductivity is a quantitative measure which describes this ability. Solutions of inorganic ions are relatively good conductors (and exhibit high conductivity), whereas solutions of organic molecules are rather poor conductors (and exhibit low conductivity). Highly purified water is also a poor conductor. Conductivity is expressed in units of Siemen/cm (also known as mhos/cm). Conductivity measurements are are frequently encountered in monitoring the performance of reverse osmosis equipment. Conductivity is temperature dependent and should be measured with a temperature-compensated meter. The usual reference temperature is 25°C. Conductivity measurements are sometimes used to estimate total dissolved solids in water. While convenient, this practice is imprecise. (See also resistivity.)

contact time

The actual time which water remains in contact with an oxidizer, regenerant, or water conditioning media within a water treatment system. The amount of contact time determines the effectiveness of the system. Also called retention time.


The addition of any physical, chemical, biological or radiological substance to water which reduces the value of the water, or interferes with its intended use.


The destructive disintegration of metals by electromechanical means. Corrosion of iron and steel is commonly called rusting.

critical bed depth

The minimum depth of an adsorbent bed requited to contain the mass transfer zone.


A waterborne protozoan that forms cysts and causes acute illness in humans. This type of organism is resistant to chlorine and ultraviolet light but can be removed by one micron filtration.


Cellulose triacetate. Used to manufacture reverse osmosis membranes.



The removal from solution of inorganic salts by means of adsorption by ion exchange resins of both the cations and the anions that comprise the salts.


The removal of excess or free chlorine from a water supply by adsorption with activated carbon or by catalytic type filter media.


The removal of dissolved gasses from water such as carbon dioxide, methane, hydrogen sulfide, and oxygen. This can by done by subjecting the water to below atmospheric pressure, or by passing air through the water at atmospheric pressure.


The removal of the ionized minerals and salts (both organic and inorganic) from a solution by a two-phase ion exchange procedure. First, positively charged ions are removed by a cation exchange resin in exchange for a chemically equivalent amount of hydrogen ions. Second, negatively charged ions are removed by an anion exchange resin for a chemically equivalent amount of hydroxide ions. The hydrogen and hydroxide ions introduced in this process unite to form water molecules. The term is often used interchangeably with demineralization. The cation resin is regenerated with an acid and the anion resin is regenerated with sodium hydroxide (caustic soda).

delta p

The pressure drop or loss in psi between the inlet and the outlet of a water conditioner as the water flows.


The removal of dissolved inorganic solids (salts) from a solution such as water to make it free of dissolved salts. Typically accomplished by reverse osmosis, distillation, or electrodialysis.


Disinfection is the process of killing micro-organisms, usually by one of a variety of chemical agents, such as formaldehyde and sodium hypochlorite. Disinfection lowers the number of micro-organisms without necessarily killing all those present. Although total killing of all organisms is virtually impossible, Sterilization can generally only be achieved routinely by heat, gamma irradiation, ethylene oxide, and, in certain cases, special filtration.


Abbreviation for Drainage, Waste, and Vent.



The effectiveness of the operational performance of an ion exchanger. Efficiency in the adsorption of ions is expressed as the quantity of regenerant required to effect the removal of a specified unit weight of adsorbed material, e.g., pounds of acid per kilogram of salt removed.


The outflow of a water treatment device. Sometimes used to mean the product water of a given water conditioning device or system.


A chemical compound which dissociates or ionizes in water to produce a solution which will conduct an electric current. Could be an acid, base, or salt.


The stripping of adsorbed ions from an ion exchange material by the use of solutions containing other ions in concentrations higher than those of the ions to be stripped. The process of washing out adsorbed material, especially by use of a solvent.

empty bed contact time

The empty bed contact time (EBCT) is used as a measure of how much contact occurs between particles, such as activated carbon, and water as the water flows through a bed of the particles. As the EBCT increases, the time available for the particles to adsorb solutes from the water also increases, as does the amount of solute removed from the water during its transit through the bed. EBCT is calculated from: EBCT=Vm/Q
Where Vm is the volume of particles in the bed and Q is the volumetric flow rate. A consistent set of units must be used when calculating EBCT with this equation. For example, if Vm is given in ft, then Q must be expressed in ft/min for the EBCT to have units of minutes. Values of Q can be converted from other units, such as GPM, to ft/min using the conversion factors.


The end point is that point in the exhaustion run of a water conditioner such as a softener or deionizer at which the water quality has dropped below an acceptable level.

exchange sites

Locations on ion exchange resin beads which hold mobile ions that are available for exchange with other ions in a solution passing through the bed. These sites are also called functional groups.

exchange velocity

The rate with which one ion is displaced from an exchanger in favor of another.


The state of the adsorbent such as activated carbon, a water softener, or a deionizer that is no longer capable of the removal of a specific pollutant or of useful ion exchange. The exhaustion point is determined arbitrarily in terms of: (a) the presence or increase of an adsorbent contaminant as chlorine; (b) a value in parts per million of ions in the effluent solution; (c) the reduction in quality of the effluent water determined by a conductivity bridge which measures the resistance of the water to the flow of an electric current.


ferric iron

Small solid iron particles containing trivalent iron, usually as gelatinous ferric hydroxide or ferric oxide (Fe2O3), which are suspended in water and visible as “rusty” water. Ferrous (iron in solution) is readily converted to ferric iron by exposure to oxygen found both in water and air. Ferric iron can by removed by filtration, but not by ion-exchange.

ferrous iron

Usually ferrous hydroxide which when dissolved in water produces a clear solution. Often called clear water iron, it can be removed by ion-exchange.


A device used to clean water by removing iron, silt, taste, odor, color, etc., before it is fed into the softener or supply lines of the consumer. Includes mechanical, adsorptive, oxidizing and neutralizing filters. Available as media beds in tanks or as cartridge type devices.


The tradename for aluminum silicate (pumicite) granular product used as a general purpose filter medium. Lighter in weight, it requires a lower backwash rate. Typically removes suspended solids down to the 20-40 micron range.


The process of passing water through a porous substance to remove solids in suspension. Available as media beds in tanks or as cartridge type devices.


Smaller than the specified size or particles of ion exchange or filtration materials. An excess of fines can create undesirable pressure drop in the system.

flash distillation

A distillation process in which hot water is introduced into a low pressure chamber causing some of the water to flash or quickly turn to steam.


A substance, used in combination with coagulants, which causes submicroscopic suspended matter (colloids) to aggregate into larger particles which can be removed by settling or filtration.

flow controller

An in-line self pressure adjusting or orifice to regulate the flow of water or regenerant through a water conditioner.

flow rate

The volume of solution which passes through a given quantity of resin within a given time. Flow rate is usually expressed in terms of gallons per minute per cubic foot of resin, or as milliliters per minute per milliliter of resin. If the flow rate is greater than it should be, the water will not be completely softened or filtered.


In crossflow filtration, it is the product flow rate through a reverse osmosis, electrodialysis or ultrafiltration membrane. It is usually given in terms of volume unit per time per membrane area.


The deposition of insoluble materials, such as bacteria, colloids, oxides and water-borne debris, onto the surface of a reverse osmosis or ultra filtration membrane. Fouling is associated with decreased flux rates and may also reduce the rejection rates of reverse osmosis membranes.


The vertical distance between a bed of filter media or ion exchange material and the overflow or collector for backwash water; the height above the bed of granular media available for bed expansion during backwashing. It may be expressed either as a linear distance or a percentage of bed depth.



A common unit of liquid volume; the US gallon has a volume of 231 cubic inches or 3.78533 liters; the British (Imperial) gallon has a volume of 277.418 cubic inches or 4.54596 liters.


Granular activated carbon.

galvanic action

A form of corrosion which occurs when dissimilar metals in contact with each other and with an electrolyte causes on e of the metals to dissolve and go into solution. An example would be the result of connection copper to steel without an insulating (plastic) coupling or union. The anode metal with the higher electrode potential corrodes and the cathode is protected.


(gr) A unit of weight equal to 1/7000th of a pound or 0.0648 gram.

grains per gallon (GPG)

An expression of concentration of material in solution. One grain per gallon is equivalent to 17.1 parts per million. This is the common reference for hardness of water.

gravel support bed

A layer or layers of graded gravel and course sand placed around and above the underdrain metalwork of a water treatment system. It facilitates even distribution and collection of both product water and backwash flow.


A natural mineral, primarily composed of complex silicates, which possess ion exchange properties. Greensand was the original material used in domestic and commercial water softeners and is the base product in the production of manganese greensand.


The term describing all subsurface water and the source of well water. It can be found in aquifers as deep as several miles.



A family of elements that includes bromine, chlorine, fluorine, astatine, and iodine. They are very active chemically. They are commonly found as the ionic component in compounds with various other elements.


A characteristic of natural water due to the presence of dissolved calcium and magnesium; water hardness is responsible for most scale formation in pipes. Hardness is usually expressed in grains per gallon, parts per million, or milligrams per liter, all as calcium carbonate equivalent. Calcium sulfate, magnesium sulfate, and the chlorides of these two metals cause permanent hardness.

hardness leakage

The presence in the effluent of the type of ions present in the water being treated. Leakage may be caused by incomplete regeneration, channeling, excessive service water, low temperature, high concentrations of sodium or interfering TDS in the feedwater.

hard water

Water with a total hardness of one grain per gallon or more, as calcium carbonate equivalent.

head loss

The reduction on liquid pressure associated with the passage of a solution through a bed of exchange material; a measure of the resistance of a resin bed to the flow of the liquid passing through it.


A central piping system with two or more side outlets located at the bottom of a water conditioning system. It’s purpose is to both collect product water as well as to distribute backwash water.

heme iron

Organically bound iron that can give water a pinkish cast. It is found only in groundwater supplies and cannot be removed by filtration. Like soluble iron, heme iron stains fixtures with a rust or orange coloring. It may draw clear and turn yellow or pink when exposed to oxygen.


The process of purifying a kidney patients blood by means of a dialysis membrane. In this process bodily waste is transferred from the blood into a hemodialysis grade water which is beyond the membrane.


A chemical, such as sodium hexametaphosphate, added to water to increase the solubility of certain ions and to inhibit precipitation of certain chemicals. Known as a sequestering agent, it forms a thin film that protects metals from corrosion.

hydraulic classification

The rearrangement of resin particles in an ion exchange unit. As the backwash water flows up through the resin bed, the particles are placed in a mobile condition wherein the larger particles settle and the smaller particles rise to the top of the bed.

hydro static pressure

A measurement of structural strength and ability to hold water pressure. Hydrostatic pressure is more challenging to a system than air pressure because air will compress and absorb impact, whereas water will not.

hydrogen cycle

A complete course of cation exchange operation in which the cation medium is regenerated with acid and them all cations in the water are removed by exchange with hydrogen ions.

hydrogen sulfide

A corrosive and flammable gas produced from decaying organic matter, commonly known as “sulfur.”

hydrologic cycle

The water cycle, including precipitation of water from the atmosphere as rain or snow, flow of water over or through the earth, and evaporation or transpiration to water vapor in the atmosphere. It is natures great water conditioner since all contaminants are left behind on the earth.


Pertaining to a substance which readily absorbs water. (“water-loving”)


Pertaining to a substance which does not readily absorb water. (“water-hating”)


The term used to describe the anionic hydroxide radical (OH-) which is responsible for the alkalinity of a solution.



The water entering a water treatment devise.

inorganic matter

Matter which is not derived from living organisms and contains no organically produced carbon; includes rocks, minerals and metals.

in-parallel flow

A piping arrangement which directs separate streams through two or more water treatment units in a balanced manner, providing equal flow to each device. The inlets of two or more units are connected together and the outlets are connected together such that water will flow through the units simultaneously.

in-series flow

A piping system in which all of the effluent flow of one unit in a water treatment system is fed to a second and succeeding unit. This arrangement achieves a greater reduction of contaminants than can be obtained by the passage through a single unit.

iodine number

A measure of the ability of activated carbon to adsorb substances with low molecular weights. It is the milligrams of iodine that can be adsorbed on one gram of activated carbon.


An atom, or group of atoms in a solution which function as a unit, and has a positive or negative electrical charge, due to the gain or loss of one or more electrons. It is smaller than a colloid.

ion exchange

A reversible process in which ions are released from an insoluble permanent material in exchange for other ions in a surrounding solution; the direction of the exchange depends upon the affinities of the ion exchanger for the ions present and the concentration of the ions in the solution. The ion exchanger media is an insoluble permanent solid medium.


The dissociation of molecules into simpler, electronically charged particles. It is related to the gaining or losing of electrons causing the atoms to become electronically charged.


An element often found dissolved in ground water (in the form of ferrous iron) in concentrations usually ranging from zero to 10 ppm (mg/l). It is objectionable in water supplies because of the staining caused after oxidation and precipitation (as ferric hydroxide), because of tastes, and because of unsightly colors produced when iron reacts with tannins in beverages such as coffee and tea. As little as 0.3 ppm of iron can cause staining. (See also ferrous iron, ferric iron, and heme iron).

iron bacteria

Organisms which are capable of utilizing ferrous iron, either from the water or from steel pipe, in their metabolism, and precipitating ferric hydroxide in their sheaths and gelatinous deposits. These organisms tend to collect in pipe lines and tanks during periods of low flow, and to break loose in slugs of turbid water.

iron fouling

The accumulation of iron on and within an ion exchange resin or filter bed resulting in a reduced capacity of the media.



A unit of weight; one thousand grains, 17100 ppm, or 0.1429 pounds.


The study of the relationships between temperature, motion, and the velocity of very small particles. It is used to describe the rate of ion exchange reactions.


Langelier index

A calculated number that gives and indication of the tendency of water to form a protective film of calcium carbonate scale, to dissolve it or be in equilibrium with it. It does not take into account the quantities of film formed, the effect of velocities, oxygen, carbon dioxide, ammonia, silicon or natural inhibitors in the water. Therefore, it is sometimes erroneously assumed that any water that tends to dissolve calcium carbonate is automatically corrosive.


The phenomenon in which some of the influent ions are not adsorbed and appear in the effluent. It is usually caused by an under-regenerated exchange resin bed or by excessive flow rate.


A series of bacteria, including legionella pneumophila, which can cause pneumonia-like illness called Legionnaires disease after the American Legion convention in Philadelphia where the disease first drew attention. These bacteria have been found growing in hard water scale and thrive below 140 degrees Fahrenheit in water heaters, showers, humidifiers, etc. Infection is obtained by inhalation.


The common name for calcium oxide (CaO); hydrated lime is calcium hydroxide, Ca(OH)2.

lime scale

Hard water scale containing a high percentage of calcium carbonate. Insoluble scale is commonly formed when water containing calcium carbonate is heated. It also forms in cold water but precipitates at a higher pH.

lime softening

Often used by municipalities for partial reduction of water hardness. After the addition of baked lime, soda ash is added to form an insoluble precipitate which is filtered from the water. This method leaves five or more grains of hardness.


macroporous resin

Ion exchange resins produced in both cation and anion versions with 12 percent or higher cross-linkage. They offer a higher resistance to oxidation and organic fouling.


One of the elements making up the earth’s crust, the compounds of which when dissolved in water make the water hard. The presence of magnesium in water is a factor contributing to the formation of scale and insoluble soap curds.

manganese (Mn)

An element sometimes found dissolved in ground water, usually with dissolved iron but in lower concentrations. It causes black stains in laundry and plumbing fixtures at concentrations higher than 0.05 mg/l. It is removed the same way as iron, by ion-exchange or oxidation and filtration.

mechanical filter

A filter primarily designed for the removal of suspended solid particles, as opposed to filters that remove contaminants by chemical means.


Membranes are thin films made with structures designed to provide selective transport of solutes. In general, the selectivity of a membrane is based on its ability to pass or exclude species according to their size. Membrane structures may become homogeneous or asymmetric. Homogeneous membranes have structures which are uniform in cross-section, at least to a magnification of 100x. Most homogeneous membranes have been developed for micro-filtration and hemodialysis.
Membranes reduce not only the flow of undesirable solutes, but also the flow of solvent. In order to minimize the reduction in solvent flow, asymmetric membranes have been developed. These membranes are made with asymmetric cross-sections, that is, they consist of two parallel layers. The resistance to flow of the skin layer, which gives the membrane its filtration selectivity, is minimized by reducing its thickness. The resistance to flow of the ticker support layer, which provides structural strength, is minimized because of its open pore structure. These different layers may be made from the same material, as in asymmetric cellulose acetate membranes, or from different materials, as in thin-film composite membranes.
Membranes used in water treatment equipment are fabricated in two forms, as flat sheets or as hollow fibers.

microgram per liter

Also known as parts per billion (ppb). The common symbol for microgram per liter is µg/l.


One millionth of an ohm. A unit of measurement used to test the electrical resistance of water to determine its purity. The purer the water, the greater its resistance to conducting an electrical current. Water of absolute purity has a resistance of eighteen million ohms across one centimeter at a temperature of twenty-five degrees Celsius.


One millionth of a mho. Used to measure the conductivity and the approximate TDS content of water. Absolute pure water has a conductivity of 0.055 micromhos per centimeter at 25 degrees Celsius. Also known as micro Siemens. The specific conductance is the reciprocal of resistance, therefore MHO is OHM spelled backwards.


A linear measure equal to one millionth of a meter, or .00003937 inch. The symbol for the micron is the Greek letter “µ”. The smallest particle visible to the human eye is 40 microns. Most types of bacteria range from 0.05 to 10.0 microns in size.

micron rating

The term applied to a filter or filter medium to indicate the particle size above which all suspended solids will be removed, throughout the rated capacity. As used in industry standards, this is an “absolute,” not “nominal” rating.

milligram per liter

(mg/l) A unit concentration of matter used in reporting the results of water and wastewater analyses. In dilute water solutions, it is practically equal to the part per million, but varies from the ppm in concentrated solutions such as brine. As most analyses are performed on measured volumes of water, the mg/l is a more accurate expression of the concentration, and is the preferred unit of measure.


A term applied to inorganic substances, such as rocks and similar matter found in the earth’s strata, as opposed to organic substances such as plant and animal matter. Minerals normally have definite chemical composition and crystal structure. The term is also applied to matter derived from minerals, such as the inorganic ions found in water. The term has been incorrectly applied to ion exchangers, even though most of the modern materials are organic ion exchange resins.


The simplest combination of atoms that will form a specific chemical compound; the smallest particle of a substance which will still retain the essential composition and properties of that substance, and which can be broken down only into atoms and simpler substances.



A membrane process that treats water between reverse osmosis and ultrafiltration the filtration/separation spectrum. It can remove particles in the 300 to 1,000 molecular weight range such as humic acid and organic color found in water. Nanofiltration may be used for selective removal of hardness ions.

nephelometric turbidity unit (NTU)

The standard unit of measurement used to measure turbidity in water. It makes use of a light scattering effect of fine suspended particles in a light beam. The NTU has replaced the Jackson Turbidity Unit (JTU) as the standard of measurement.


A common designation for alkaline materials such as calcite (calcium carbonate) or magnesia (magnesium oxide) used in the neutralization of acid waters. Alkaline water can also be neutralized by the addition of an acid. The neutral point of the pH scale is 7.0, indicating the presence of equal numbers of free hydrogen and hydroxide ions.



A unit of measure determining the resistance to passage of an electrical current. In a solution, it is related to the electrolyte concentration in the solution.

operating pressure

The range of pressure, usually expressed in pounds per square inch, over which a water conditioning device or water system is designed to function. Usually 30-100 psi.


Having the characteristics of or being derived from plant or animal matter, as opposed to inorganic matter derived from rocks and minerals. Organic matter is characterized by its carbon-hydrogen structure.

organics (i.e., organic chemicals)

Term used to describe any or all of the compounds with chemical structures based on carbon. Examples are hydrocarbons, wood, sugars, proteins, methane, petroleum-based compounds, solvents, pesticides, herbicides, trihalomethane (THM) and trichloroethylene (TCE).


A process of diffusion of a solvent such as water through a semi-permeable membrane which will transmit the solvent but impede most dissolved substances. The normal flow of solvent is from the dilute solution to the concentrated solution. Osmosis causes the stronger solution to become more diluted and tends to equalize the opposing solutions.

osmotic pressure

The pressure and potential energy difference that exists between solutions on either side of a semi-permeable membrane. This pressure is caused by the tendency of water to flow in osmosis. Every 100 ppm (mg/L) of TDS produces about one pound per square inch of osmotic pressure. Osmotic pressure must first be overcome by water pressure in the reverse osmosis process.


chemical process in which electrons are removed from an atom, ion or compound. The addition of oxygen is a specific form of oxidation. Combustion is an extremely rapid form of oxidation, while the rusting of iron is a slow form. Oxidation never occurs alone but always as a part of the oxidation-reduction (redox) reaction.

oxidizing agent

A chemical substance that brings about the oxidation of other substances in chemical oxidation and reduction reactions. Examples of oxidizing agents include oxygen, ozone, chlorine and peroxide.

oxidizing filter

A type of filter used to change the valence state of dissolved molecules, making them insoluble and therefore filterable. For example, a filter that oxidizes ferrous iron, manganous manganese, and/or anionic sulfur by use of a catalytic media such as manganese oxide and then filters the oxidized precipitant out of the water.


An unstable form of oxygen (O3), which can be generated by sending a high voltage electrical discharge through air or regular oxygen. It is a strong oxidizing agent and has been used in water conditioning as a disinfectant. It can be also produced by some types of ultraviolet lamps and during lightning storms.



A term used to describe visible sediment particles, used as both singular and plural.

parts per billion (ppb)

A basis for reporting the results of water and wastewater analysis, indicating the number of parts by weight of a dissolved or suspended constituent, per billion parts by weight of water or other solvent. One part per billion is equal to one microgram per liter, the preferred unit.

parts per million (ppm)

A common basis for reporting the results of water and wastewater analysis, indicating the number of parts by weight of water or other solvent. In dilute water solutions, one part per million is practically equal to one milligram per liter, which is the preferred unit. 17.1 ppm equals one grain per US gallon. One ppm equals one pound per million pounds of water.

pH (potential of Hydrogen)

An expression of the acidity of a solution; the negative logarithm of the hydrogen ion concentration (pH 1, very acidic; pH 14, very basic; pH 7, neutral). e.g., pH 5 is 10 times the acidity of 6 and 100 times the acidity of 7. pH is a measure of intensity and not capacity. It is the logarithm of the reciprocal of the hydrogen ion concentration of a solution. The neutral point of 7 indicates the presence of equal concentrations of free hydrogen and free hydroxide ions.

pharmaceutial grade water

The definition of six grades of water by the U.S. Pharmacopoeia is as follows: 1.) Purified water 2.) Water for injection 3.) Bacteriostatic water for injection 4.) Sterile water for inhalation 5.) Sterile water for injection 6.) Sterile water for irrigation.


An acid-base indicator which produces no color in an acid solution but turns pink or red in an alkaline solution.

physical stability

The quality which an ion exchange resin must possess to resist changes that might be caused by attrition, high temperatures, and other physical conditions.

point of entry (POE)

A water treatment device which installs at the main inlet to a building and acts as centralized treatment.

point-of-use (POU)

A water treatment system designed to connect at the actual point-of-use for water; countertop or undersink treatment systems.


A treatment stage placed at the end of other treatment to bring the water to a more highly conditioned and more perfect state. For example, a mixed bed of ion exchange media installed as the final treatment step in the deionization process to remove last traces of undesirable ions.


A synthetic polymer of the nylon family used in the fabrication of reverse osmosis and ultra filtration membranes.


A sequestering agent used to tie up hardness and iron in solution. As a coating agent, it forms a thin passivating film on metal surfaces to control corrosion.


A synthetic polymer used in the fabrication of reverse osmosis and ultra filtration membranes which are characterized by extreme thermal stability and chemical resistance.

polyvinyl chloride (PVC)

A thermoplastic material produced by the polymerization of vinyl chloride. Used extensively in the U.S. for piping, food packaging, and injection molded plastic parts.


The complex network of channels in the interior of a particle of a sorbent.

portable exchange

Water softeners, deionizers, and filters which are designed for removal from its point of application for transport to a central station or plant for regeneration or servicing.

positive charge

The electrical potential acquired by an atom which has lost one or more electrons; a characteristic of a cation.

potable water

Water which is considered safe and fit for human consumption, culinary and domestic purposes and meets the requirements of the health authority having jurisdiction.


The abbreviation for “parts per billion.”


The abbreviation for “parts per million.”


To cause a dissolved substance to form a solid particle that can be removed by settling or filtering. The term also refers to the solid thus formed.

pressure drop

A decrease in water pressure during its flow due to internal friction between molecules of water, and external friction due to irregularities or roughness in surfaces past which the water flows.


Any water treatment step performed prior to the primary treatment process, such as filtration prior to deionization.


The removal of undesirable matter from water or wastewater. It is the disinfection of water by the killing of microbial contaminants, such as coliform bacteria. A strict definition means the removal from water of all contaminants.


Biological decomposition of organic matter by microbes with the production of ill-smelling products. Usually takes place when there is a deficiency of oxygen.


A super oxidation media serving as a catalyst in the removal of iron, hydrogen sulfide and manganese. It works best at or above a pH of 6.5 and requires no regeneration. Adequate backwashing is necessary to provide at least 20 per cent bed expansion of this 120 lb. per cubic foot media.


quartz sleeve

Also called a quartz jacket, it is a clear, pure quartz sleeve that is installed around the high intensity ultraviolet lamp in an ultraviolet system. It retards less than 10 percent of the radiation dosage in contrast to the poorer results offered by glass.



The abbreviation for “reverse osmosis.”


Naturally occurring radioactive elements such as radium 226 and radium 228 created in the decay of the uranium and thorium series. It can be removed from water by cation exchange softening.


A short lived radioactive gas produced from decaying uranium that is soluble in water. Can be effectively removed by activated carbon filtration or serration. Radon is considered carcinogenic when inhaled by humans.

raw water

Untreated water from wells or from surface sources or any water before it reaches a water treatment device or process.

reactivation (revivification)

Oxidation processes for restoring the adsorptive properties of a spent sorbent such as activated carbon.

recovery (percent recovery)

A measurement applied to reverse osmosis and ultra filtration equipment which characterizes the ratio of product water to feed water flow rates. The measurement is descriptive of reverse osmosis or ultra filtration equipment as a system and not of individual membrane elements. Expressed as a percentage, recovery is defined as: % Recovery = (Product flow rate/feed flow rate) x 100


– A shortened term for oxidation-reduction. A reaction where electrons are gained or lost and new elements are formed.


The solution used to restore the activity of an ion exchanger. Acids are employed to restore a cation exchanger to its sodium form. The anion exchanger may be rejuvenated by treatment with an alkaline solution. Potassium permanganate is used to regenerate a manganese greensand iron and manganese iron and manganese removal filter.


The process of returning the sodium ions to the mineral after it has exchanged all its sodium ions for calcium and magnesium from hard water. This is accomplished by first back-washing the mineral bed to free it of all foreign matter, them passing salt brine through the mineral. The sodium ions attach themselves to the mineral, and the calcium and magnesium combine with the chloride from the brine to form calcium and magnesium chlorides, which are rinsed down the drain. All water softeners using the ion-exchange process are regenerated with these basic steps. In similar fashion cation and anion components of a demineralizer as well as manganese greensand are recharged with comparable sequences.

rejection (percent rejection)

A measure of the ability of a reverse osmosis membrane to remove salts. Expressed as a percentage, rejection is defined as:
% Rejection = (1-Product concentration/Feed concentration) x 100


The amount of a specific material remaining in the water following a water treatment process. It may refer to material remaining as the result of incomplete removal such as hardness leakage, or to a substance meant to remain in the treated water such as residual chlorine.


Synthetic organic ion exchange material, such as the high capacity cation exchange resin widely used in water softeners. Technical name: sulfonated co-polymer of styrene and divinyl benzene.


Resistivity is a measure of the current-resisting characteristics of a substance when an electrical charge is applied (and is the reciprocal of conductivity). The standard unit of resistance is the Ohm. Because of the variable nature of water, a distance between measuring probes must be maintained if accurate measurements are desired. The almost universal standard distance for this is the centimeter, hence the “Ohm-cm.” Resistivity measurements, like conductivity measurements, can be used in many ways to improve the management of a water purification system, and are commonly used to asses the quality of water produced by deionizers. Because temperature effects resistivity of water, temperature compensating devices are frequently used. These adjust the resistance meter to indicate what the water resistance would be at one temperature, usually 25°C.


The ability of an adsorbent to resist desorption of an adsorbate.

reverse deionization

The use of an anion exchange unit ahead of a cation exchange unit – in that order – in a deionization system.

reverse osmosis

A process for the removal of dissolved ions from water, in which pressure is used to force the water through a semi-permeable membrane, which will transmit the water but reject most other suspended and dissolved materials. It is called reverse osmosis because mechanical pressure is used to force the water to flow in the direction that is the reverse of natural osmosis, namely from the dilute to the concentrated solution.



The common name for the specific chemical compound sodium chloride (NaCl), used in the regeneration of ion exchange water softeners. In chemistry, the term is applied to a class of chemical compounds which can be formed by the neutralization of an acid with a base.

sand filter

A treatment device or structure for removing solid or colloidal material of a type that cannot be removed by sedimentation. Such filters can be gravity rapid-rate or enclosed pressure type.

saturated solution

A solution containing the maximum amount of the dissolved substance that such a solution can hold at this temperature.


In reference to reverse osmosis equipment, scaling is the precipitation of sparingly soluble salts, such as calcium carbonate, onto the surface of a membrane. Scaling is associated with decreased flux and reduced reverse osmosis rejection rates.


A polymer matrix or ion exchanger used to remove organics from feedwater prior to a deionization process.


The process by which solids are separated from water by gravity and deposited on the bottom of a container or basin.

selective ion exchange

The use of a selective ion exchange medium with the property of removing specific ions from a solution.


Descriptive of a material, such as a reverse osmosis or ultra filtration membrane, which allows the passage of some molecules and prevents the passage of others.

service run

That portion of the operating cycle of a water conditioning unit during which treated water is being delivered, as opposed to the period when the unit is being backwashed, recharged or regenerated.

silt density index

The silt density index (SDI) is a measure of the ability of water to foul a membrane or plug a filter. SDI is measured using an apparatus which typically consists of an inlet pressure regulator and pressure gauge followed by a filter holder containing a 0.45 micron microporous membrane filter. Commercial test kits, complete with instructions on how to calculate the index, are available.

soda ash

The common name for sodium carbonate, a chemical compound used as an alkaline builder in some soap and detergent formulations, to neutralize acid water, and in the lime-soda ash water treatment process.

sodium hydrosulfite

A strong reducing agent used as the main ingredient of several resin cleaners used to clean iron fouled in ion exchange resin beds.

softened water

Any water that is treated to reduce hardness minerals to 1.0 GPG (17.1 mg/L) or less, expressed as calcium carbonate.


The liquid, such as water, in which other materials (solutes) are dissolved.


A physical or chemical process that reduces the number of organisms to a safe predetermined level.


The expansion of an ion exchange bed which occurs when the reactive groups on the resin are converted from one form to another. This property is reversible and indeed, some resins shrink in the exhausted state.



The abbreviation for “total dissolved solids.”

thin-film composite membrane (TFC)

Reverse osmosis membrane produced with polyamide-based polymer. It is resistant to bacteria and can withstand a wide pH range. However, it cannot tolerate chlorine.

throughput volume

The amount of solution passed through an exchange bed before exhaustion of the resin is reached.

total acidity

The total of all forms of acidity, including mineral acidity, carbon dioxide, and acid salts. Total acidity is usually determined by titration with a standard base solution to the phenolphthalein endpoint (pH 8.3).

total alkalinity

The alkalinity of a water as determined by titration with standard acid solution to the methyl orange endpoint (pH approximately 4.5); sometimes abbreviated as “M alkalinity.” Total alkalinity includes many alkalinity components, such as hydroxides, carbonates, and bicarbonates.

total chlorine

The total amount of chlorine is a solution, which includes the combined chlorine as well as the free available chlorine.

total dissolved solids

The sum of all organic, inorganic and ionic contents in a solution (excluding all dissolved gasses). Since a TDS meter cannot measure organic content of water, most TDS readings are an approximation. TDS measurements are widely used in the water and waste water industries to monitor final water quality. The TDS meter derives its values from resistivity and conductivity measurements of the product water.

total hardness

The sum of all hardness components in a water, expressed as their equivalent concentration of calcium carbonate. Primarily due to calcium and magnesium in solution, but may include small amounts of metals such as iron which can act like calcium and magnesium in certain reactions. These minerals are scale forming, affect taste and color of certain foods and react with soap to form insoluble soap curds.

total organic carbon

The measurement of carbon dioxide produced from organics when a water sample is atomized into a combustion chamber. The amount of carbon covalently bound in organic compounds in a water sample.

total solids

The weight of all solids, dissolved and suspended, organic and inorganic, per unit volume of water; usually determined by the evaporation of a measured volume of water at 105 degrees Celsius in a pre-weighed dish.

trihalomethanes (THM’s)

A group of organic chemicals to known to be carcinogenic in more than trace amounts which are produced from chlorination. They reduce the germicidal activity of chlorine in alkaline water.


Turbidity is a measure of the presence of colloidal matter in the water that remains suspended. Suspended matter in a water sample, such as clay, silt, or finely divided organic and/or inorganic material will scatter the light from an incident light beam. The extent of scattering is expressed in Jackson or Nephelometric turbidity units (JTU and NTU, respectively).



A membrane type system that removes small colloids and large molecules from solutions. Ultrafiltration removes particles in size range between 0.002 to 0.1 micron range. The process falls between reverse osmosis and microfiltration as far as the size of particles removed is concerned.

ultrapure water

No standards exist describing ultrapure water, though it is not considered to be sterile. It is water that has been deionized and provides high resistivity and contains no organics.

ultraviolet light

Radiation having a wavelength shorter than 4000 angstroms (visible light) down to 100 angstroms on the border of the x-ray region. Ultraviolet light is used as a disinfectant.


The operation of an ion exchange unit in which solutions are passed in at the bottom and out at the top of the container.



A tube with a tapered throat which causes an increase in velocity thus a decrease in pressure of the fluid passing through it. It is the common item used to educt or suck a regenerant into a water conditioning system.


The smallest form of life known to be capable of producing disease or infection, usually considered to be of large molecular size. They multiply by assembly of component fragments in living cells, rather than be cell division, as do most bacteria. Being parasitic infectious microbes, they are much smaller than bacteria.

void area

The space between the resin beads in an ion exchange bed or the space between the particles of filter media bed. Also can be defined as the space between the chunks of salt in a brine tank.

volatile organic compounds (VOC’s)

Synthetic organic chemicals that vaporize at relatively low temperatures.


water conditioning

Virtually any form of water treatment designed to improve the quality of water, by neutralization, inhibition or removal of undesirable substances.

water hammer

The shock wave produced by the abrupt change of water flow through a piping system. Water hammer produces an instantaneous multiple increase in the pressure normal to the system. The installation of a water hammer arrestor will absorb these shock waves.

water softening

The reduction or removal of calcium and magnesium ions which are the principle cause of hardness in water.



A volatile organic chemical (VOC) commonly used in industry as a solvent.



Naturally occurring or synthetic hydrated sodium alumina silicate with ion exchange properties. Zeolites have been largely replaced with synthetic organic cation ion exchange resins.
Modified zeolites can be selectively charged with exchange minerals such as potassium and used to remove undesirable elements such as iron, hydrogen, sulfide, and manganese.

zero soft

Water with a total hardness less than 1.0 grain per US Gallon (17.1 ppm), as calcium carbonate.