absorbent - A material, usually a porous solid, which takes another material into its interior. When rain soaks into soil, the soil is an absorbent.
absorption - The process in which one substance is taken into the body of an absorbent.
acid - A substance which increases the concentration of 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.
acidity - The quantitative capacity of water or a 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. (See mineral acidity, total acidity, carbon dioxide.)
acre-foot - The volume of water which would cover an area of one acre to a depth of one foot. It is equal to 43,560 cubic feet (1,233 cubic meters) or 325,851 gallons (1,233,L).
activated carbon - A granular material usually produced by roasting various grades of coal in the absence of air. It has a very porous structure and it is used in water conditioning as an adsorbent (see "adsorption") for organic matter and certain dissolved gases. Sometimes called "activated charcoal".
activated silica - A material usually formed from the reaction of a dilute silicate solution with a dilute acid. It is used as a coagulant aid.
adsorbent - 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.
adsorption - The process in which matter adheres to the surface of an adsorbent.
aeration - 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.
aerobic - An action or process conducted in the presence of air, such as aerobic digestion of organic matter by bacteria.
air gap - A clear vertical space between a water or drain line and the flood level of a receptacle to prevent back-flow or siphoning from the receptacle in the event of negative pressure or vacuum. Most plumbing codes require the air gap to be at least twice the diameter of the water or drain line, with a minimum of 1-1/2 inches (3.8 cm). (See vacuum breaker or back-flow presenter.)
algae - Small primitive plants containing chlorophyll, commonly found in surface water. Excessive growths may create taste and odor problems, and consume dissolved oxygen during decay.
alkali - 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 bicarbonate and carbonate compounds when they are present in the water or solution. (See alkali, base.)
alkalinity - 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 is expressed in terms of its calcium carbonate equivalent. (See alkali, base.)
alkyl benzene sulfonate - A term applied to a family of branched chain chemical compounds, formerly used as detergents,. Sometimes called "hard" detergents, because of their resistance to biological degradation, these compounds have been largely replaced with linear alkyl sulfonate (LAS) which are more readily degraded to simpler substances. (See detergent, linear alkyl sulfonate.)
alum - A common name for aluminum sulfate, used as a coagulant.
amoeba - A small, single-celled animal or protozoan.
anaerobic - An action or process conducted in the absence of air, such as the anaerobic digestion of organic matter by bacteria in a septic tank.
anion - A negatively charged ion in solution, such as bicarbonate, chloride, nitrate or sulfate.
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. (See ion exchange, demineralization.)
Angstrom unit - A unit of length equal to one ten-billionth of a meter.
anode - The positive pole of an electrolytic system meter when oxidation occurs. Anodes made of 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.
aquifer - A layer or zone below the surface of the earth which is capable of yielding a significant volume of water.
atom - The smallest particle of an element that can exist either alone or in combination.
attrition - In water treatment, the process in which solids are worn down or ground 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.