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 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.
raw water – Untreated water, or any water before it reaches a specific water treatment device or process.
red water – Water which has a reddish or brownish appearance due to the presence of precipitated iron and/or iron bacteria.
regenerant – 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 are used as regenerants for the cation and anion resins used in demineralization.
residual – 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 or to material meant to remain in the treated water.
resin – Synthetic organic ion exchange material, such as the high capacity cation exchange resin widely used in water softeners.
reverse osmosis – A process for the removal of dissolved ions from water in which pressure is used to force the water through a semipermeable membrane which will transmit the water but reject most other dissolved materials.
saline water – Water containing an excessive amount of dissolved salts, usually over 10,000 mg/L.
salt – The common name for the specific chemical compound sodium chloride used in the regeneration of ion exchange water softeners. In chemistry, the term is applied to a class of chemical compounds which can be formed by the neutralization of an acid with a base.
sequester – 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 reaction with soap.
soap – 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.
soda ash – The common name for sodium carbonate, a chemical compound used as an alkaline builder in some soap and detergent formulations, to neutralize acid water, and in the lime-soda ash water treatment process.
sodium – An ion found in natural water supplies, and introduced to water in the ion exchange water softening process. Sodium compounds are highly soluble, and do not react with soaps or detergents.
sodium chloride – The chemical name for common salt, widely used in the regeneration of ion exchange water softeners.
soft water – Any water which contains less than 1.0 gpg (17.1 mg/L) of hardness minerals, expressed as calcium carbonate.
solvent – The liquid, such as water, in which other materials (solutes) are dissolved.
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.
sulfate-reducting bacteria – A group of bacteria which are capable of reducing sulfates in water to hydrogen sulfide gas, thus producing obnoxious tastes and odors. These bacteria have no sanitary significance, and are classed as nuisance organisms.
sulfur – A yellowish solid element. The term is also used as a slang expression to refer to water containing hydrogen sulfide gas.