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Learn about the different types of contaminants that might put your family at risk.



Generally means 100% retention of particulates of the size equal to the filter rating.


To take up or drink in, as a sponge imbibes water.  The process of assimilation of molecules into the structure of a solid.  One substance taken into the body of another substance. 


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. 


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 terms of its calcium carbonate equivalent. 

activated carbon (AC)

Adsorptive particles or granules usually obtained by heating carbonaceous material in the absence of air or in steam and possessing a high capacity to selectively remove trace and soluble components from solution. 

activated carbon adsorption

Removal of soluble components from aqueous solution by contact with highly adsorptive granular or powdered carbon.

activated carbon treatment

Treatment process in which water is brought into contact with highly adsorptive granular or powdered carbon to remove soluble components.  Process may be applied to raw water, primary effluent, or chemically clarified wastewater for nonspecific removal of organics, or to secondary effluent as a polishing process to remove specific organics. 

activated silica

A material usually formed from the reaction of a dilute silicate solution with a dilute acid and used as a coagulant aid. 


A material, usually solid, capable of holding gases, liquids, and/or suspended matter at its surface and in exposed pores.  Activated carbon is a common adsorbent used in water treatment. 


The process in which matter adheres to the surface of an adsorbent. 


The process in which air is brought into intimate contact with water, often by spraying water through air, or by bubbling air through water.  Aeration may be used to add oxygen to the water for the oxidation of matter such as iron, or to cause the release of dissolved gases such as carbon dioxide or hydrogen sulfide from the water. 


The coalescence of dispersed suspended matter into larger flocs or particles which settle rapidly. 

air gap

A clear, vertical space between a water or drain line and the flood level of a receptacle used to prevent backflow or siphonage from the receptacle in the event of a negative pressure or vacuum.  Most plumbing codes require the air gap to be a least twice the diameter of the water or drain line with a minimum of 1 ½ inches. 

air stripping

A technique for removal of volatile substances from a solution.  Employs the principles of Henry’s Law to transfer volatile pollutants from a solution of high concentration into an air stream of lower concentration.  The process ordinarily is designed so that the solution containing the volatile pollutant contacts large volumes of air. 


Small primitive plants containing chlorophyll commonly found in surface water.  Excessive growths may create taste and odor problems and consume dissolved oxygen during decay. 


A group of water soluble mineral compounds usually considered to have moderate strengths as bases as opposed to the caustic or strongly basic hydroxides, although this differentiation is not always made.  In general, the term is applied to the carbonates, borates, phosphates, and silicates when these are present in the water or solution.


The quantitative capacity of a water or water solution to neutralize an acid.  It is usually measured by titration with a standard acid solution of sulfuric acid and expressed in terms of its calcium carbonate equivalent. 


A common name for aluminum sulfate (Al2(SO4)3, used as a coagulant. 


A small, single-celled animal or protozoan.


A negatively charged ion in solution such as bicarbonate, chloride, or sulfate.


The positive pole of an electrolytic system; the metal which goes into solution in a galvanic cell.  Anodes of metals such as magnesium or zinc are sometimes installed in water heaters or other tanks to deliberately establish galvanic cells to control corrosion of the tank through the sacrifice of the anode.


A layer or zone below the surface of the earth which is capable of yielding a significant volume of water. 


The smallest particle of an element that can exist either alone or in combination with similar particles of the same element or of a different element. 


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



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

backflow preventer

A device or system installed in a water line to stop backflow from a nonpotable source. 


The process in which beds of filter or ion exchange media are subjected to flow opposite to the service flow direction; to loosen the bed and to flush suspended matter (collected during the service run) to waste. 


Unicellular microorganisms which typically reproduce by cell division.  Although usually classed as plants, bacteria contain no chlorophyll. 


A substance which releases hydroxyl ions when dissolved in water.  Bases react with acids to form a neutral salt and water. 


The filter media or ion exchange resin in a column or other tank or operational vessel. 

bed depth

The height of the filter media or ion exchanger in the vessel after preparation for service expressed in inches or centimeters. 

bed expansion

The increase in volume of a bed of the ion filter or exchanger media during backwashing due to lifting and separation of the bed material.  Usually expressed as the percent of increase of bed depth. 

bicarbonate alkalinity

The alkalinity (HCO3) of a water due to the presence of bicarbonate ions. 


An oxidizing agent formulated to break down colored matter; includes the widely used hypochlorites, as well as perborates and other special purpose materials. 


The appearance in the effluent from a water conditioner of the material to be removed by the conditioner, such as hardness in the effluent of a softener, or turbidity in the effluent of a mechanical filter.  An indication that regeneration, backwashing, or other treatment is necessary for further service. 


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

buffer action

The action of certain ions in solution in opposing a change in hydrogen-ion concentration. 

buffer solution

A solution containing two or more substances which, in combination, resist any marked change in pH following addition of moderate amounts of either strong acid or base. 


A connection or a valve system that allows untreated water to flow to a water system while a water treatment unit is being regenerated, backwashed, or serviced; also applied to a special water line installed to provide untreated water to a particular tap, such as a sill cock. 



One of the principal elements (Ca) making up the earth’s crust, the compounds of which when dissolved make the water hard.  The presence of calcium in water is a factor contributing to the formation of scale and insoluble soap curds which are a means of clearly identifying hard water. 

calcium carbonate equivalent

A common basis for expressing the concentration of hardness and other salts in chemically equivalent terms to simplify certain calculations; signifies that the concentration of a dissolved mineral is chemically equivalent to the stated concentration of calcium carbonate. 

calcium hypochlorite

A chemical compound, [Ca (CLO)24H2O], used as a bleach and a source of chlorine in water treatment.  Specifically useful because it is stable as a dry powder and can be formed into tablets. 


An expression of the quantity of an undesirable material which can be removed by a water conditioner between servicing of the media, i.e., cleaning, regeneration or replacement, as determined under standard test conditions.  For ion exchange water softeners, the capacity is expressed in grains of hardness removal between successive regenerations and is related to the pounds of salt used in regeneration.  For filters, the capacity may be expressed in the length of time or total gallons delivered between servicing. 

carbon dioxide

A gas (CO2) present in the atmosphere and formed by the decay of organic matter; the gas in carbonated beverages; in water, it forms carbonic acid. 


The CO3 ion.

carbonate alkalinity

Alkalinity (CO3) due to the presence of the carbonate ion. 

carbonate hardness

Hardness due to the presence of calcium and magnesium bicarbonates and carbonates in water; the smaller of the total hardness and the total alkalinity. 


A material substance that induces excessive or abnormal cellular growth cancer in an organism. 


Any removable, preformed, or prepackaged component containing a filtering media or ion exchanger.  Also referred to as “element”.


A substance that increases the rate or yield of a chemical reaction without being consumed itself.

catalyzed oxidation

Enhancement of chemical or biological oxidation by the addition of catalytic agents that promote higher rates of reaction. 


The negative pole of an electrolytic system; an electrode where reduction occurs.

cathodic protection

A corrosion control system in which the metal to be protected is made to serve as a cathode either by the deliberate establishment of a galvanic cell or by impressed current. 


An ion with a positive electrical charge, such as calcium, magnesium, sodium, iron, lead, and manganese. 

cation exchange

Ion exchange process in which cations in solution are exchanged for other cations from an ion exchanger. 

cationic flocculant

A polyelectrolyte with a net positive electrical charge. 


Any substance capable of burning or destroying animal flesh or tissue.  The term is usually applied to strong bases such as lye. 

caustic soda

The common name for sodium hydroxide, (NaOH).


The flow of water or other solution in a limited number of passages in a filter or ion exchanger bed instead of distributed flow through all passages in the bed.  May be due to fouling of the bed and plugging of many passages, poor distributor design, flow rates which are too low, faulty operational procedures, or other causes. 


To form a complex chemical compound in which an ion, usually metallic, is bound into a stable ring structure. 

chelating agent

A chemical compound sometimes fed to water to tie up undesirable metal ions, keep them in solution, and eliminate or reduce the normal effects of the ion. 

chemical feeder

A mechanical device designed to introduce chemicals into a water system, more or less accurately in proportion to water flow. 


A combination of chlorine and a small amount of ammonia, chloramine is a disinfectant used by some water utilities. The addition of the ammonia helps to make the solution more stable and longer lasting. Chloramines can cause an adverse effect on the taste and odor of water.


A mechanical device specifically designed to feed chlorine gas or solutions of its compounds, such as hypochlorites, into a water supply in proportion to the flow of water. 


A gas used by many water utilities for the disinfection of water and as an oxidizing agent for organic matter and some metals. It imparts a noticeable taste and odor to water, and may contribute to the formation of trihalomethanes (THM).

chlorine demand

A measure of the amount of chlorine which will be consumed by organic matter and other oxidizable  substances in a water before a chlorine residual will be found; the difference between the total chlorine fed and the chlorine residual. 


The removal of small quantities (<2%) of fine particulate matter (solids) from a liquid (water) to improve the product liquid.  Generally, clarifiers will remove particles from 2 to 100 micrometers in size.  Clarification methods include filtration, gravity and centrifugal sedimentation, and magnetic separation. 


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

coagulant aid

A material which is not a coagulant but which improves the effectiveness of a coagulant often by forming larger or heavier particles, speeding the reactions, or by permitting reduced coagulant dosage. 


The process in which very small, finely divided solid particles, often colloidal in nature, are agglomerated into larger particles.

co-current flow

(See downflow.)


A device or system designed to collect backwash water from a filter or ion exchange bed.  May also be used as an upper distributor to spread the flow of water in downflow column operation. 


Very finely divided solid particles which will not settle out of a solution; intermediate between a true dissolved particle and a suspended solid which will settle out of solution.  The removal of colloidal particles usually requires coagulation to form larger particles which may be removed by sedimentation and/or filtration.


The shade or tint imparted to water by substances in true solution, and thus not removed by mechanical filtration; most commonly caused by dissolved mineral matter.

color throw

The discharge of color to the effluent of a filter or ion exchange system by any component.  It usually occurs after a period of standing which allows slowly soluble colored matter to accumulate in the system. 

column operation

The process in which the solution to be treated is passed through a vessel containing a bed of filter media or ion exchange; may be either upflow or downflow. 

combined available chlorine

The chlorine present as chloramines or other derivatives in a water, but still available for disinfection and the oxidation of organic matter.  The combined chlorine compounds are more stable than free chlorine forms, but are somewhat slower in action. 

composite sample

A mixture of a number of single or “grab” samples intended to produce a typical or average sample when wide variations in quality or characteristics may occur; may be made up of equal volumes of individual samples or of single samples proportioned to variations in flow or usage. 


The addition of foreign matter to a substance which reduces the value of the substance or interferes with its intended use.


In physics, mass motions within a fluid resulting in transport and mixing of the properties of that fluid caused by the force of gravity and by differences in density due to nonuniform temperature. 


The destructive disintegration of a metal by electrochemical means.

countercurrent flow

(See upflow)

cross connection

A direct link between a potable water system and a non potable water system which would permit undesirable substances to be drawn into the potable water.

cross-sectional area

The area of a plane at a right angle to the direction of flow through a tank or vessel; often expressed in square feet and related to the flow rate (Example: 5 gallons per minute per square foot of ion exchanger bed area).


A slang expression sometimes used to mean a cubic foot of ion exchanger or filter media.

cubic foot

The common basis for the measurement of the volume of ion exchangers or loose filter media.  The measurement is made after a specific process including backwashing and settling of the bed and draining excess water from above the bed.  A cubic foot equals 28.3 liters.


A series of events or steps which ultimately lead back to the starting point, such as the exhaustion regeneration cycle of an ion exchange system; sometimes incorrectly used in reference to a single step of a complete cycle.


(See spore.)



The removal of excess chlorine residual, often after super-chlorination.  (See super-chlorination.)


The removal of all 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 icons.  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 removal of ionized inorganic minerals and salts (not organic materials) from a solution by a two-phase ion exchange procedure; similar to deionization, and the two terms are often used interchangeably. 


The mass of a substance per specified unit of volume; for example, pounds per cubic foot.  True density is the mass per unit volume excluding pores; apparent density is the mass per unit volume including pores.

diaphragm pump

A form of positive displacement pump in which the reciprocating piston is separated from the solution by a flexible diaphragm, thus protecting the piston from corrosion and erosion and avoiding problems with packing and seals. 

diatomaceous earth, diatomite

A processed, natural material, chiefly the skeletons of diatoms, used as a filter medium. 

differential pressure

The difference in pressures at two points in a water system; may be due to differences in elevation or to friction losses or pressure drops due to resistance to flow in pipes, softeners, filters or other devices.


The process whereby particles of liquids intermingle as the result of their spontaneous movement in dissolved substances moving from a region of high concentrations to one of low concentration.


A process in which pathogenic (disease producing) bacteria are killed; may involve disinfecting agents such as chlorine or physical processes such as heating.


The separation of molecules into positively and negatively charged ions in water solution.

dissolved solids

The weight of matter in true solution in a stated volume of water; includes both inorganic and organic matter; usually determined by weighing the residue after evaporation of the water at 105 or 180° C. 


(See collector.)


A specific form of limestone (CaMg (CO3)) 2 containing chemically equivalent concentrations of calcium and magnesium carbonates; the term is sometimes applied to limestone with compositions similar to true dolomite.


A slang term sometimes applied to water conditioning equipment designed for residential use.


A term applied to designate the direction (down) in which water or a regenerant flows through an ion exchanger or filter during any phase of the operating cycle.  Also referred to as co-current flow.


Active, alive, or tending to produce motion as opposed to static, resting, or fixed. 

dynamic system

A system or process in which motion occurs, or includes active forces, as opposed to static conditions with no motion. 



A circular movement whirlpool occurring in flowing water caused by currents set up in the water by obstructions. 


A device utilizing a nozzle and throat and installed in a stream of water to create a partial vacuum to draw air or liquid into the stream.  Commonly used to draw brine into a water line for the regeneration of an ion exchange water softener.

effective size

A measure of the size of particles of ion exchanger or filter media, defined as the diameter of a specific particle in a bed, batch, or lot which has 10 percent smaller and 90 percent larger particles.


The stream emerging from a system or process such as the softened water from an ion exchange softener.  The filtrate water from a filter.


(See eductor.)


A fundamental particle found in the atom and which carries a single negative charge.  In a neutral atom, the positive charges of the nucleus are balanced by an equal number of negative electrons in the field surrounding the nucleus.  Ions are formed when atoms gain or lose electrons, thus achieving positive or negative net charges. 


The disposable filtering cartridge itself in a replaceable cartridge-type filter.


The point at which a process is stopped because a predetermined value of a measurable variable is reached; the endpoint of an ion exchanger water softener service run is the point at which the hardness of the softener effluent increases to a predefined concentration, often 1.0 grain per gallon; the endpoint of a filter service run may be the point at which the pressure drop across the filter reaches a predetermined value; the endpoint of a titration is the point at which the titrant produces a predetermined color change, pH value, or other measurable characteristic.


The abbreviation for “equivalent per million.”


The state in which the action of multiple forces produces a steady balance or seeming lack of change; may be due to a true stop in action or due to continuing actions which neutralize each other resulting in no net change.

equilibrium reaction

A chemical reaction which proceeds primarily in one direction until the concentrations of reactants and products reach an equilibrium which usually can be expressed as ratio or other mathematical relationship.

equivalent per million

A unit of concentration used in chemical calculations, calculated by dividing the concentration in ppm or mg/L by the equivalent weight.

equivalent weight

The weight in grams of an element, compound, or ion which would react with or replace 1 gram of hydrogen; the molecular weight in grams divided by the valence.



Specifically, a device or system for the removal of solid particles (suspended solids); in general, includes mechanical, adsorptive, oxidizing and neutralizing filters. (Nonhealth related.)

filter area

The effective area through which water approaches the filter media often expressed in square feet.  Also referred to as surface area.

filter media

(See media.)

filter rating

(See micron rating.)


The effluent liquid which has passed through any style filter.


Extremely small particles of filter media or ion exchange material formed either in the manufacturing process or as a result of breakdown; undesirable in most systems because of high pressure drop.


The agglomeration of finely divided, suspended solids into larger, usually gelatinous, particles; the development of a “floc” after treatment with a coagulant by gentle stirring or mixing.

flow control

A device designed to limit the flow of water or regenerant to a predetermined value over a broad range of inlet water pressures. 

flow rate

The quantity of water or regenerant which passes a given point in a specified unit of time, often expressed in gallons per minute.


The addition of a fluoride compound to a potable water supply to produce the concentration desired for the reduction in incidence of dental caries.


The process in which undesirable foreign matter accumulates in a bed of filter media or ion exchanger, clogging pores and coating surfaces and thus inhibiting or retarding the proper operation of the bed. 

free available chlorine

The concentration of residual chlorine present as dissolved gas, hypochlorous acid, or hypochlorite not combined with ammonia or in other less readily available form. 

free carbon dioxide

Carbon dioxide (CO2) present in water as the gas or as carbonic acid, but not that combined in carbonates or bicarbonates.

free chlorine

(See free available chlorine.)


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; may be expressed either was a linear distance or a percentage of bed depth.



A common unit of liquid volume; the U.S. 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.

grab sample

A single sample of material collected at one place and one time; represents only the specific material at the time and place of sampling.


A natural mineral, primarily composed of complex silicates, which possess ion exchange properties. 


hard water

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


A characteristic of natural water due to the presence of dissolved calcium and magnesium; water hardness is responsible for most scale formation in pipes and water heaters and forms insoluble “curd” when it reacts with soaps.  Hardness is usually expressed in grains per gallon, parts per million, or milligrams per liter, all as calcium carbonate equivalent.


A measure of the pressure at a point in the water system expressed in pounds per square inch or in the height of a column of water which would produce the pressure.

head loss

(See pressure drop.)

high-test hypochlorite

A dry solid, largely calcium hypochlorite [Ca (OH)2], used as a disinfecting agent; has excellent stability as long as kept dry.


The chemical combination of water into a substance.


Referring to water or other fluids in motion.

hydrogen ion concentration

The concentration of hydrogen ions in moles per liter of solution; often expressed as pH.

hydropneumatic system

A system utilizing both air and water in its operation, such as the pressure tank used with many well systems, which utilizes an air chamber to maintain pressure on the water when the pump is not operating.

hydrostatic test

A pressure test procedure in which a vessel or system is filled with water, purged of air, sealed, subjected to water pressure, and examined for leaks, distortion, and/or mechanical failure.


A chemical compound of an element or elements with the hydroxyl (OH) anion.


The “OH” anion which has a single negative charge and provides the characteristics common to bases.


Calcium and sodium hypochlorites (CLO2) are commonly used as bleaches and as disinfecting agents.



A material which can be used to show the endpoint of a chemical reaction, usually by a color change, or a chemical concentration by a depth or shade of color.


The stream entering a unit or process, such as the hard water entering an ion exchange water softener, or turbid water entering a filter system.

inorganic matter

Substances not derived from living organisms and containing no organically produced carbon; includes rocks, minerals, and metals.


The process of connecting conditioning equipment into the water system and a drain line provided where necessary.  The term is also used to refer to the complete assembly of piping, valves, drain line, water conditioning unit, and related equipment.

intermittent flow

The term usually applied to the interrupted patterns of water usage; also used in reference to specific on-off flow patterns selected to test the performance of water conditioning equipment under standard conditions which may or may not be similar to actual patterns of use. 


An atom, or group of atoms which function as a unit, and have a positive or negative electrical charge due to the gain or loss of one or more electrons.


The process in which atoms gain or lose electrons and thus become ions with positive or negative charges; sometimes used synonymously with dissociation; the separation of molecules into charged ions in solution. 


An element (Fe) 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.

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 to create staining, taste, and odor problems.


There are currently no glossary terms for this letter



A prefix used to indicate 1000 of the succeeding unit.  (Is also sometimes used as an abbreviation for kilogram.)

kilograin (Kgr.)

One thousand grains.

kilogram (Kg.)

One thousand grams.


laminar flow

The form of flow of a fluid in which the flow paths are in smooth, parallel lines with essentially no mixing and no turbulence.


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.


A sedimentary rock, largely calcium carbonate (CaCO3), usually containing significant amounts of magnesium carbonate.  The calcite grade is used in filtration and for pH modification.



One of the elements (Mg) 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.


An element (Mn) sometimes found dissolved in groundwater, usually with dissolved iron but in lower concentrations.  Causes black stains and other problems similar to iron.

manganese greensand

Greensand which has been processed to incorporate in it pores and on its surface the higher oxides of manganese.  The product has mild oxidizing power and is often used in the oxidation and precipitation of iron, manganese and/or hydrogen sulfide, and their removal from water. 

manganese zeolite

Synthetic gel zeolite which has been processed in the same manner as manganese greensand and used for similar purposes.

mechanical filter

A filter primarily designed for the removal of suspended solid particles as opposed to filters with additional capabilities.


The selected materials in a filter that form the barrier to the passage of certain suspended solids or dissolved molecules.


Singular form of media.


The abbreviation of milligrams per liter.

microfiltration (MF)

The separation or removal of particulates of more than 0.02 μm or less than 10.0 μm size from liquids.


A linear measure equal to one millionth of a meter, or .00003937 inch.  The symbol for the micron is the Greek letter “μ.”

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.


One thousandth of an inch.


The prefix used with units of measure to indicate one thousandth of the unit.  Example: a milliliter is one thousandth of a liter.

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 is 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 unit of length equal to one thousandth of a micron often used to express the wavelength of colors of visible light in colorimetric analytical procedures.  The symbol for the millimicron is “mμ.”


A term applied to inorganic substances, such as rocks and similar matter found in the earth 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.

mineral acidity

Acidity due to the presence of inorganic acid such as hydrochloric, sulfuric, and nitric acid, as opposed to acidity due to carbonic acid or organic acids. 


The molecular weight of a chemical compound expressed in grams.


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.

most probable number (MPN)

The term used to indicate the number of organisms which, according to statistical theory, would be most likely to produce the results observed in certain bacteriological tests; usually expressed as a number per 100 ml of water.


negative charge

The electrical charge on an electrode or ion in solution due to the presence of an excess of electrons.

negative head

A condition of negative pressure or partial vacuum.

negative pressure

A pressure below that of the surrounding atmospheric pressure at a specific point; a partial vacuum.

nephelometric turbidity unit (NTU)

An arbitrary unit of measuring the turbidity in water by the light scattering effects of fine suspended particles in light beam.


In electrical systems, the term used to indicate neither an excess nor a lack of electrons; a condition of balance between positive and negative charges.  In chemistry, the term used to indicate a balance between acids and bases; the neutral point on the pH scale is 7.0, indicating the presence of equal numbers of free hydrogen (acidic) and hydroxide (basic) ions.


In general, the addition of either an acid or a base to a solution as required to produce a neutral solution. The use of alkaline or basic materials to neutralize the acidity of some waters is common practice in water conditioning.


A common designation for alkaline materials such as calcite (calcium carbonate) or magnesia (magnesium oxide) used in the neutralization of acid waters.

noncarbonated hardness

Water hardness due to the presence of compounds such as calcium and magnesium chlorides, sulfates, or nitrates; the excess of total hardness over total alkalinity.

normal solution

A solution containing a gram equivalent weight of a substance in a liter of 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.

organic iron

(See iron bacteria.)

organic matter

Substances of or 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.


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 diluted solution to the concentrated solution.  (See reverse osmosis.)


A chemical substance capable of promoting oxidation, for example O2, O3, Cl2.


A 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. 


An unstable form of oxygen (O3), which can be generated by an electrical discharge through air or regular oxygen.  It is a strong oxidizing agent and has been used in water conditioning as a disinfectant.


particle size

As used in industry standards, the size of a particle suspended in water as determined by its smallest dimension, usually expressed in microns.

parts per million (ppm)

A common basis for reporting the results of water and wastewater analyses, indicating the number of parts by weight of a dissolved or suspended constituent, per million 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.  17.12 ppm equals one grain per U.S. Gallon.


An organism which may cause disease.

permanent hardness

Water hardness due to the presence of the chlorides and sulfates of calcium and magnesium which will not be precipitated by boiling.  This term is largely replaced by “noncarbonated hardness.”


The reciprocal of the logarithm of the hydrogen ion concentration.  The pH scale is from zero to 14, and 7.0 is the neutral point, indicating the presence of equal concentrations of free hydrogen and hydroxide ions.  pH values below 7.0 indicate increasing acidity, and pH values above 7.0 indicate increasing base concentrations.


The reciprocal of the logarithm of the hydroxide ion concentration.  The pOH is related to pH by the expressions: pH + pOH = 14.


“Pollution is an impairment of quality such that it interferes with the intended usages.” (House Report 2021.)


A measure of the volume of internal pores in filter media and ion exchangers, sometimes expressed as a ration to the total volume of the medium.

positive charge

The electrical charge on an electrode or ion in solution due to the removal of electrons.


The application of chlorine to a water following other water treatment processes.

potable water

Water which is safe and suitable for human consumption.


The abbreviation for “parts per million.”


The application of chlorine to a water prior to other water treatment processes. 


To cause a dissolved substance to form a solid particle which can be removed by settling or filtering, such as in the removal of dissolved iron by oxidation, precipitation, and filtration.  The term is also used to refer to the solid formed and to the condensation of water in the atmosphere to form rain or snow. 


The phenomenon that occurs when a substance in solution is chemically transformed into an insoluble form.  The conversion of dissolved solids into suspended solids which may be concentrated subsequently by flocculation and sedimentation.


The application of a granular filter medium, such as diatomaceous earth, to a membrane prior to the service cycle of a filter.

preliminary filter

A filter used in a water treatment plant for the partial removal of turbidity before final filtration.  Such filters are usually of the rapid type, and their use allows final filtration at a more rapid rate or reduces or removes the necessity of other preliminary treatment of the water.  Also called contact filter, contact roughing filter, roughing filter.

pressure differential

The difference in pressure between two points in a system due to differences in elevation and/or pressure drop due to flow.

pressure drop

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

pressure tank

A tank used in connection with a water distribution system, for a single household, for several houses, or for a portion of a larger water system, which is airtight and holds both air and water, and in which the air is compressed and the pressure so created is transmitted to the water.


There are currently no glossary terms for this letter.


rated capacity

The basis for calculating the period of time, or number of gallons delivered by a water softener or filter, between regenerations or servicing as determined under specific test conditions.

rated pressure drop

The pressure drop of a water softener or filter at the rated service flow with clean water at a temperature of 60 ° F., with a freshly regenerated and/or backwashed softener or filter as determined under standard test conditions.

rated service flow

The manufacturer’s specified maximum flow rate at which a water softener will deliver soft water, or a filter will deliver quality water as specified for its type, as determined under standard test conditions.  A manufacturer may also specify a minimum flow rate or a range of service flows. 

raw water

Untreated water, or any water, before it reaches a specific water treatment device or process.

red water

Water which has a reddish or brownish appearance due to the presence of precipitated iron and/or iron bacteria.


A chemical process in which electrons are added to an atom, ion, or compound.


A solution of a chemical compound used to restore the capacity of an ion exchange system.  Sodium chloride brine is used as a regenerant for ion exchange water softeners, and acids and bases area used as regenerants for the cation and anion resins used in demineralization. 


The amount of a specific material remaining in the water following a water treatment process; may refer to material remaining as a result of incomplete removal (see leakage) or to material meant to remain in the treated water (see residual chlorine.)

residual chlorine

Chlorine remaining in treated water after a specified period of contact time to provide protection throughout a distribution system; the difference between the total chlorine added and that consumed by oxidizable matter. 

reverse osmosis (RO)

A process that reverses, by the application of pressure, the flow of water in a natural process of osmosis so that the water passes from the more concentrated to the more dilute solution through a semi-permeable membrane.


Following backwash in filters to resettle the media bed and purge any turbidity before returning to service mode.  That portion of the regeneration cycle of an ion exchanger in which fresh water is passed through the column to remove spent and excess regenerant prior to placing the system in service.



The process in which solid suspended particulates settle out of a liquid (water).  Usually the water or liquid is subjected to little or no movement.  The process may be accelerated by feeding a coagulant such as alum.  Also referred to as “settling”.

semi permeable membrane

Usually a thin, organic film which will allow the passage of some ions or materials while preventing the passage of others.  Some membranes will only allow the passage of anions; others will allow the passage of cations.  Some membranes reject most dissolved substances but allow the passage of water.


A chemical reaction in which certain ions are bound into a stable, water soluble compound, thus preventing undesirable action by the ions.

sequestering agent

A chemical compound sometimes fed into water to tie up undesirable ions, keep them in solution, and eliminate or reduce the normal effects of the ions.  For example, polyphosphates can sequester hardness and prevent reactions with soap.

service run

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

service unit

A term sometimes applied to softeners or filters which are regenerated or backwashed at a central point and transported to the point of use for connection to the water system.  Also known as portable exchange units.

siliceous gel

A synthetic hydrated sodium alumino silicate with ion exchange properties once widely used in ion exchange water softeners.


The semi-fluid solid matter collected at the bottom of a system tank or watercourse as a result of the sedimentation or settling of suspended solids or precipitates.


An abnormally high concentration of an undesirable substance which passes through a water system, usually brief or intermittent in nature, and often related to an upset of a system.  For example, a slug of iron may occur during high flow which disturbs and suspends previously deposited iron precipitates.


One of a class of chemical compounds which possesses cleaning properties formed by the reaction of a fatty acid with a base or alkali.  Sodium and potassium soaps are soluble and useful, but can be converted to insoluble calcium and magnesium soaps (curd) by the presence of these hardness ions in water.

soft water

Any water which contains less than 1.0 gpg (17.1 mg/L) of hardness minerals expressed as calcium carbonate. 

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 substance which is dissolved in a solvent.  Dissolved solids, such as the minerals found in water, are solutes.

solution feeder

A mechanical device, such as a power driven pump or an eductor system, designed to feed a solution of a water treatment chemical into the water system usually in proportion to flow. 


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


(See adsorbent.)


The concentration of dissolved solids on the surface (absorption) of suspended solids or solids contained in a fixed bed. 

specific gravity

The ratio of the weight of a specific volume of a substance compared to the weight of the same volume of pure water at 4 ° C.


In general, the reproductive body of an organism capable of reproducing the organism under favorable conditions.  In water, most spores resist adverse conditions which would readily destroy the parent organism.  The spore is sometimes considered the resting state of the organism.


Fixed in position, resting, or without motion, as opposed to dynamic or moving. 

static system

A system or process in which the reactants are not flowing or moving.


A process in which all living organisms are destroyed and residual removed from liquid.


A yellowish, solid element (S).  The term is also used as a slang expression to refer to water containing hydrogen sulfide gas.


The addition of excess amounts of chlorine to a water supply to speed chemical reactions or insure disinfection with short contact time.  The chlorine residual following superchlorination is high enough to be unpalatable, and thus dechlorination is commonly employed before the water is used.


The clear liquid lying above sediment or precipitate.

surface area

(See filter area.)

surface tension

The result of attraction between molecules of a liquid which causes the surface of the liquid to act as a thin elastic film under tension.  Surface tension causes water to form spherical drops and to reduce penetration into fabrics. Soaps, detergents, and wetting agents reduce surface tension and increase penetration by water.


A contraction of the term “surface-active agent”.

suspended solids

Solid particles in the water which are not in solution.


temporary hardness

Water hardness due to the presence of calcium and magnesium carbonates and bicarbonates which can be precipitated by heating the water.  Now largely replaced by the term “carbonate hardness”.


A very low concentration of a substance in water.  The term is sometimes used to indicate the concentration which can just be detected.

throughput volume

The amount of solution passed through an ion exchange bed before the ion exchanger is exhausted.


An analytical process in which a standard solution in a calibrated vessel is added to a measured volume of sample until an endpoint, such as a color change, is reached.  From the volume of the sample and the volume of standard solution used, the concentration of specific material may be calculated.

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 concentration of chlorine in a water, including combined and free chlorine.

total dissolved solids

The weight of solids per unit volume of water which are in true solution, usually determined by the evaporation of a measured volume of filtered water, and determination of the residue weight.

total hardness

The sum of all hardness constituents 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.

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 150 ° C. in a pre-weighed dish.


Having an adverse physiological effect on man.


A very small concentration of a material, high enough to be detected but too low to be measured by standard analytical methods.


The process in which blister-like growths of metal oxides develop in pipes as a result of the corrosion of the pipe metal. Iron oxide tubercles often develop over pits in iron or steel pipe and can seriously restrict the flow of water.


A measure of the amount of finely divided suspended matter in water which causes the scattering and adsorption of light rays.  Turbidity is usually reported in arbitrary units determined by measurements of light scattering.  Usually expressed as JTU.


A type of flow characterized by cross currents and eddys as opposed to laminar flow.  Turbulence may be caused by curves, bends, and changes in channel size, obstructions, or excessive flow rates and will significantly increase pressure drop.

turbulent velocity

The velocity of water flowing in a conduit above which the flow will always be turbulent and below which the flow may be either turbulent or laminar depending upon circumstances.


ultrafiltration (UF)

The process of removing colloidal and dispersed particles from a liquid by passing the liquid through a membrane under high pressure.  Separation or removal of particulates of more than 10Ǻ and less than 200 angstroms.

ultraviolet radiation (UV)

Light waves shorter than visible blue-violet waves of the spectrum having wave lengths of less than 3,900 D Angstroms.

ultraviolet ray

Light rays beyond the violet of the spectrum invisible to humans.


A drain that carries away groundwater or the drainage from prepared beds to which water or wastewater has been applied. 

uniform flow

A flow in which the velocities are the same in both magnitude and direction from point to point along the conduit.

uniformity coefficient

The degree of variation in the size of the grains that constitute a granular material; the ration of (a) the diameter of a grain of a size that is barely too large to pass through a sieve that allows 60 percent of the material (by weight) to pass through, to (b) the diameter of a grain of a size that is barely too large to pass through a sieve that allows 10 percent of the material (by weight) to pass through.  The coefficient is unity for any material having grains all the same size, and it increases above unity with variation in size of grain.


A term used to indicate the direction (up) in which water or regenerant flows through an ion exchanger or filter media bed during any phase of the operating cycle.  Also referred to as counter-current flow.


vacuum breaker

A mechanical device which automatically vents a water line to the atmosphere when subjected to a partial vacuum, thus preventing backflow. (See backflow, air gap, backflow preventer).


A small positive or negative whole number, which indicates the net number of electrons gained or lost in the formation of an ion, and thus the numbers of each kind of ion necessary for a balanced chemical reaction.  For example, two hydrogen ions (each with a valence of +1) must be present for each ion of oxygen (-2) to form a molecule of water (H2O).


Alive and capable of continued like.


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 by cell division, as do most bacteria.


The resistance of fluids to flow, due to internal forces and friction between molecules.

void volume

The volume of the pores or spaces between particles of ion exchanger, filter media, or other granular material, often expressed as a percentage of the total volume occupied by the material.


Capable of vaporization at a relatively low temperature.

volatile solids

Matter which remains as a residue after evaporation at 105 or 180 ° C., but which is lost after ignition at 600 ° C.  Includes most forms of organic matter.


Referring to measurement by volume rather than weight.


water conditioning

Virtually any form of water treatment designed to improve the aesthetic quality of water by the neutralization, inhibition, or removal of undesirable substances.  (Not health related).

water hammer

A shock wave or series of waves produced by the abrupt acceleration of deceleration of water flow due to inertia.  Water hammer may produce instantaneous pressures many times the normal pressure.

water softening

The removal of calcium and magnesium, the ions which are the principal cause of hardness, from water.

water table

The level of the top of the zone of saturation in which free water exists in the pores and crevices of rocks and other earth strata. 


There are currently no glossary terms for this letter.


There are currently no glossary terms for this letter.



A group of hydrated sodium alumino silicates, either natural or synthetic, with ion exchange properties. 

zeolite softening

The removal of calcium and magnesium by ion exchange using natural or synthetic zeolite.  The term is sometimes used to refer to all ion exchange softening processes, even though organic ion exchange resins, not inorganic zeolites, are in most common use today.

zero soft

Water with a total hardness less than 1.0 grain per U.S. Gallon, as calcium carbonate.

zone of saturation

The layer in the ground in which all of the available voids are filled with water.

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