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Common Water Related Terms

A simple glossary of terms related to water conditions, water contaminants and water treatment solutions.



AB 1953

On January 1, 2010, the California bill AB 1953 from the 2006 legislative session that became law and pertains to potable water filtration products, went into effect. This law limits the weighted average lead content of products to not exceed more than 0.25% for pipes and pipe fittings, plumbing fittings, and fixtures sold in California intended to dispense water for human consumption.


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.


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.


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


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.

American Water Works Association (AWWA)

The AWWA is the authoritative resource on safe water, providing knowledge, information and advocacy to improve the quality and supply of water in North America and beyond. The AWWA advances public health, safety and welfare by uniting the efforts of the full spectrum of the water community. The AWWA website is


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


Backflow preventer

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

CA Prop 65

Proposition 65 (formally titled "The Safe Drinking Water and Toxic Enforcement Act of 1986") is a California law that has been in effect since 1986 to promote clean drinking water and keep toxic substances that cause cancer and birth defects out of consumer products. It is administered by Cal/EPA's California Office of Environmental Health Hazard Assessment (OEHHA). Proposition 65 regulates substances listed by California as causing cancer or birth defects or other reproductive harm in two ways. The first regulatory arm of Proposition 65 prohibits businesses from knowingly discharging listed substances into drinking water sources, or onto land where the substances can pass into drinking water sources. The second regulatory arm of Proposition 65 prohibits businesses from knowingly exposing individuals to listed substances without providing a clear and reasonable warning.


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.

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.

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.


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


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.


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 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).

Coliform bacteria

A group of organisms primarily found in human and animal intestines and wastes, and thus widely used as indicator organisms to show the presence of such wastes in water and the possible presence of pathogenic (disease producing) bacteria.


The destructive disintegration of a metal by electrochemical means.


(see Parasitic protozoan cyst)



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.

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.


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

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.


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.


E. coli

Escherichia coli, one of the members of the coli-form group of bacteria indicating fecal contamination. (See fecal, coliform.)


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

Escherichia Coli

One of the members of the coliform group of bacteria indicating fecal contamination.



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


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.


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.

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.)


Gallionella ferruginea

A genus of stalked, ribbon-like bacteria which utilize iron in their metabolism and cause staining, plugging, and odor problems in water systems. (See Iron bacteria.)


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.


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



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.

Hard water

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


The chemical combination of water into a substance.

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.


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



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.


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.



Kilogram (Kg.)

One thousand grams.



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.


Abbreviation for “Maximum Contaminant Level,” the maximum allowable concentration of a contaminant in water as established in the U.S. EPA Drinking Water Regulations.

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.


Mercury (chemical symbol Hg) is a heavy metal that occurs in several forms, all of which can produce toxic effects in high enough doses.


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.

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 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.


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.

MTBE (methyl tertiary-butyl ether)

A volatile organic chemical compound used as a fuel additive in motor gasoline. MTBE is easily dissolved in water and has been found in public and private drinking water supplies. Low levels of MTBE can make water undrinkable due to its offensive taste and odor and health risks.


National Sanitation Foundation (NSF)

NSF International, The Public Health and Safety Company™, a not-for-profit, non-governmental organization, is the world leader in standards development, product certification, education, and risk-management for public health and safety. For 65 years, NSF has been committed to public health, safety, and protection of the environment. While focusing on food, water, indoor air, and the environment, NSF develops national standards, provides learning opportunities, and provides third-party conformity assessment services while representing the interests of all stakeholders. The primary stakeholder groups include industry, the regulatory community, and the public at large. NSF is widely recognized for its scientific and technical expertise in the health and environmental sciences. Its professional staff includes engineers, chemists, toxicologists, and environmental health professionals with broad experience both in public and private organizations.

Negative charge

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

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.

NSF Std. 42

Drinking Water Treatment Units - Aesthetic Effects

This standard covers point-of-use (POU) and point-of-entry (POE) systems designed to reduce specific aesthetic or non-health-related contaminants (chlorine, taste and odor, and particulates) that may be present in public or private drinking water.

NSF Std. 44

Cation Exchange Water Softeners.

This standard covers residential cation exchange water softeners designed to reduce hardness from public or private water supplies. Additionally, this standard can verify the system's ability to reduce radium and barium.

NSF Std. 53

Drinking Water Treatment Units - Health Effects

This standard addresses point-of-use (POU) and point-of-entry (POE) systems designed to reduce specific health-related contaminants, such as Cryptosporidium, Giardia, lead, volatile organic chemicals (VOCs), MTBE (methyl tertiary-butyl ether), that may be present in public or private drinking water.

NSF Std. 58

Reverse Osmosis Drinking Water Treatment Systems

This standard was developed for point-of-use (POU) reverse osmosis (RO) treatment systems. These systems typically consist of a pre-filter, RO membrane, and post-filter. Standard 58 includes contaminant reduction claims commonly treated using RO, including fluoride, hexavalent and trivalent chromium, total dissolved solids, nitrates, etc. that may be present in public or private drinking water.

NSF Std. 55

Ultraviolet Microbiological Water Treatment Systems

This standard establishes requirements for point-of-use (POU) and point-of-entry (POE) non-public water supply (non-PWS) ultraviolet systems and includes two optional classifications. Class A systems (40,000 uwsec/cm2) are designed to disinfect and/or remove microorganisms from contaminated water, including bacteria and viruses, to a safe level. Class B systems (16,000 uw-sec/cm2) are designed for supplemental bactericidal treatment of public drinking water or other drinking water, which has been deemed acceptable by a local health agency.


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.


Parasitic protozoan cysts

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.


Tiny subdivisions of solid or liquid matter suspended in a gas or liquid.

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.


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.

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.

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.



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.


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

Red water

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


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 I 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.

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


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.

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.



The abbreviation for “total dissolved solids”.

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”.

Throughput volume

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

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 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.


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 NTU.


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 (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 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.


The abbreviation for “United States Environmental Protection Agency”.


The abbreviation for “United States Public Health Service”.



The smallest form of like 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.


Capable of vaporization at a relatively low temperature.

Volatile organic compounds (VOCs)

VOCs are ground-water contaminants of concern because of very large environmental releases, human toxicity, and a tendency for some compounds to persist in and migrate with ground-water to drinking-water supply wells….In general, VOCs have high vapor pressures, low-to-medium water solubilities, and low molecular weights. Some VOCs may occur naturally in the environment, other compounds occur only as a result of manmade activities, and some compounds have both origins.

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.


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 Quality Association (WQA)

The Water Quality Association is a not-for-profit international trade association representing the residential, commercial, and industrial water treatment industry. Its membership consists of both manufacturers and dealers/distributors of equipment. WQA is a resource and information source, a voice for the industry, an educator of professionals, a laboratory for product testing, and a communicator with the public. The WQA’s website is

Water softening

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




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