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Sand - This usually refers to the filter medium used by a sand filter. The grade most often specified by filter manufacturers is grade No. 20 with a particle size of 45 to 55 mm (millimeters).

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Sand Filter - A filter using sand or sand and gravel as the filter medium.

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Sanitize - To render sanitary: to kill all living things, including bacteria and algae. Similar to sterilize.

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Scale - The precipitate that forms on surfaces in contact with water when the calcium hardness, pH or total alkalinity levels are too high. Results from chemically unbalanced pool and spa water. Scale may appear as grey, white or dark streaks on the plaster, fiberglass or vinyl. It may also appear as a hard crust around the tile.

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Scum - The extraneous or foreign matter which rises to the surface of the water and forms a layer or a film there. It can also be a residue deposited on the tile or walls of the pool or spa. Sources of scum are soap, oil, deodorant, hair spray, suntan lotions and others.

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Sediment - The solid material settled out from the water.

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Septum - That portion of the filter element consisting of cloth, wire screen or other porous material on which the filter medium or filter aid is deposited. The nylon grid on a D.E. filter is the septum.

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Sequestering Agent - Also called chelating agent - A chemical that will combine with dissolved metals in the water to prevent the metals from coming out of solution (precipitating or causing stains). May also be a chemical that removes dissolved metals from water.

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Shock Treat - The practice of adding significant amounts of an oxidizing chemical - (usually non- chlorine oxidizers, such as sodium persulfate or potassium peroxymonosulfate) - to water to destroy ammonia and nitrogen compounds or swimmer waste.

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Shotcrete - A mixture of sand and cement sprayed onto contoured and supported surfaces to build a pool or spa. Plaster is applied over the shotcrete. Shotcrete is premixed and pumped wet to the construction site.

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Silt - Soil particles having diameters between 0.004 and 0.062 mm (millimeters). Sometimes they may be too small to be trapped by the circulation system. In those cases, a clarifier or an alum product may be needed.

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Simazine - A chemical substance used in swimming pools and spas as an herbicide or algaecide. Mainly used for killing black algae.

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Skimmer - A device installed through the wall of a pool or spa that is connected to the suction line of the pump that draws water and floating debris in the water flow from the surface without causing much flow restriction.

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Skimmer Basket - A removable, slotted basket or strainer placed in the skimmer on the suction side of the pump, which is designed to trap floating debris in the water flow from the surface without causing much flow restriction.

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Skimmer Weir - Part of a skimmer that adjusts automatically to small changes in water level to assure a continuous flow of water to the skimmer. The small floating "door" on the side of the skimmer that faces the water over which water flows on its way to the skimmer. The weir also prevents debris from floating back into the pool when the pump shuts off.

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Slurry - Water or a liquid containing a high concentration of suspended solids. Diatomaceous earth (D.E.) is usually added to the filter as a slurry by mixing a small amount of D.E. in a bucket of water and then pouring the slurry into the skimmer with the filter on.

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Soda Ash (Sodium Carbonate) - A chemical used to raise total alkalinity in pool and spa water with only a slight affect on the pH.

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Sodium Bicarbonate (Baking Soda or Bicarb) - A chemical used to raise total alkalinity in pool and spa water with only a slight affect on the pH.

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Sodium Bisulfate (dry acid) - A chemical used to lower the pH and total alkalinity. 2 1/2 lbs. of dry acid are equal to 1 quart of muriatic acid.

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Sodium Bromide - A salt of bromine. It is used to establish a bromide "bank" in pool and spa water prior to beginning the use of bromine tablets.

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Sodium Carbonate (soda ash) - A chemical used to raise the pH and total alkalinity in pool and spa water.

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Sodium Dichlor - A fast-dissolving, granular, stabilized organic chlorine compound providing either 56% or 63% available chlorine. Used for regular as well as superchlorination. Contains an ingredient (cyanuric acid or stabilizer) that prevents the chlorine from being destroyed by the ultraviolet (UV) rays of the sun. Recommended for use in vinyl-liner, painted or fiberglass pools and acrylic or fiberglass spas.

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Sodium Hypochlorite - Liquid chlorine. Usually provides 10% to 12% available chlorine; has a pH of 13 and re- quires that small amounts of acid be added to the pool to neutralize the high pH. Good for regular chlorination and super chlorination. Not recommended for spas. Does not contain conditioner or stabilizer to protect it from sunlight, but it is protected if stabilizer or conditioner is already in the water.

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Sodium Persulfate - Active ingredient and chemical name of a non-chlorine shock treatment or non-chlorine oxidizer. Does not kill bacteria or algae but it will oxidize or destroy ammonia, nitrogen and swimmer waster. Does not increase chlorine or bromine levels the way that super chlorination does, so water may be entered 15 minutes after addition. It will not reactivate bromine.

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Sodium Sulfite - A chemical used to neutralize or de-chlorinate pool and spa water.

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Sodium Thiosulfate - A chemical used to neutralize or de-chlorinate pool and spa water.

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Soft Water - Water that has a very low calcium and magnesium content (water hardness) - usually means less than 100 ppm or 6 grains. Also water that has gone through a water softener. Pools and spas should never be filled with soft water from a softener. Water with less than 100 ppm of hardness should be increased to a minimum of 150 to 200 ppm using calcium chloride.

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Solar Cover - A cover that, when placed on the water's surface of a pool, spa or hot tub, increases the water temperature by absorption and transmission of solar radiation; reduces evaporation and prevents wind-borne debris from entering the water.

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Solar Heating System - It is usually panels or coils of plastic or metal through which water passes to increase the temperature from the sun's radiant heat.

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Source Water - Also called "tap" water - It is the water used to fill or refill the pool or spa.

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Spring Board - Also called "diving" board - A recreational mechanism for entering a swimming pool consisting of a semi-rigid board that derives its spring from a fulcrum mounted below the board and attached to the deck.

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Stabilized Chlorine - A family of chlorine pool sanitizers that contain conditioner (cyanuric acid or iso-cyanuric acid) to protect the chlorine from the degrading UV rays in sunlight. Most common types are sodium dichlor and trichlor. The granular form is dichlor which is fact- dissolving and can be used for regular chlorination or super chlorination by broad- casting into the pool or spa. Tablet or stick form is trichlor (which is usually used in a chlorine feeder - either the floating type or in-line erosion type) used for regular chlorination only.

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Stain - A discoloration or a colored deposit on the walls or bottom of a swimming pool or spa. Most often, stains are metals, such as iron, copper & manganese. They may appear as green, gray, brown or black. They may even discolor the water. Sometimes a sequestering agent or chelating agent will remove them. If not, usually an acid wash is necessary to remove them from the walls & bottom. The metals get in the water because the pH was too low or someone has added a low-pH chemical directly into the circulation system. The low-pH chemical dissolves a small amount of metal from the equipment. The metals begin to come out of solutions & deposit or stain the walls & bottom. Stains are sometimes confused with scale.

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Stain Inhibitor - Also called sequestering or chelating agent - A chemical that will combine with dissolved metals in the water to prevent the metals from coming out of solution (precipitating or causing stains). May also be a chemical that removes dissolved metals from water.

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Superchlorination - The practice of adding an extra large dose (5 to 10 ppm) of chlorine to the water to destroy ammonia, nitrogen and swimmer waste, which can build up in the water. This level of chlorine is required to destroy all of the combined chlorine in the water, which is called breakpoint chlorination.

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Surfactant - A soluble chemical compound that reduces the surface tension between two liquids. It is used in many detergents and soapy cleaning compounds.

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Suspended Solids - Insoluble solid particles that either float on the surface of or are in suspension in the water, causing turbidity. They may be held in suspension by agitation or flow. They may be removed by filtration, but if the particles are too small, they may not be trapped by the filter. In these cases, a clarifier or alum may be needed to remove them.