Glossary of Water Treatment Terms & Definitions - A


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 Alumina

A medium made by treating aluminum ore so that it becomes porous and highly adsorptive. Activated alumina will remove several contaminants including fluoride, arsenic, and selenium. It requires periodic cleaning with a regenerant such as alum, acid and/or caustic.

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, and is recognized by the US EPA as the best available technology for reduction of organic chemicals from drinking water.

Activated Silica

A negatively charged colloidal substance generally formed by combining a dilute sodium silicate solution with a dilute acidic solution (or other activant). Generally used as a coagulant aid.


Any substance that is or can be adsorbed. The liquid, gas or solid substance which is adsorbed as molecules, atoms, or ions.


A water treatment medium, usually solid, capable of the adsorption of liquids, gases, and/or suspended matter. Activated alumina and activated carbon are common adsorbents used in water processing.


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.


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.


An action or process conducted in the presence of air, such as aerobic digestion of organic matter by bacteria.

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.

Aggressive Water

A term usually applied to waters containing acid or oxygen which hasten corrosion (rusting).


Plant life (green scum) containing chlorophyll is usually found in stagnant surface water. Excessive growths may create taste and odor problems, and consume dissolved oxygen during decay. 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.


The quantitative capacity of water to neutralize an acid; that is, the measure of how much acid can be added to a liquid without causing a significant change in pH. Alkalinity is not the same as pH because water does not have to be strongly basic (high pH) to have high alkalinity. In the water industry, alkalinity is expressed in mg/l of equivalent calcium carbonate. There are three kinds of alkalinity: carbonate, bicarbonate, and hydroxide alkalinity. Total alkalinity is the sum of all three kinds of alkalinity. Different tests are used to determine the quantity of the different kinds of alkalinities present in water.

Alternating System

As 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.


The common name for aluminum sulfate [Al2 (SO4) x 14H2 O] which is often used as a coagulant in water treatment.


A single celled protozoan that is widely found in fresh and salt water. Some types of amoebas cause diseases such as amoebic dysentery.

Anaerobic organism

An organism that can thrive in the absence of oxygen (air), such as bacteria in a septic tank.

Angstrom unit

A unit of wavelength of light equal to .00001 millimeter or .0001 microns.


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.


The positive pole of an electrolytic system. The metal which goes into solution in a galvanic cell. Anodes of metals such as magnesium and 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.


Abbreviation for American National Standards Institute.


Containing water; watery.


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


Describes underground water trapped under pressure between layers of impermeable rock. An artesian well is one that taps artesian water.


Abbreviation for American Society of Mechanical Engineers.


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.