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

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.