rated capacity- The basis for calculating the period of time, or number of gallons delivered by a water softener, filter, or deionizer, between regenerations or servicing, as determined under specific test conditions. (See rated in-service life, rated softener capacity.)

rated in-service life - The length of time or total gallons delivered between servicing of the media in a filter as determined under standard test conditions.

rated pressure drop - The pressure drop of a water softener or filter at the rated service flow, with clean water at a temperature of 60^oF, with a freshly regenerated and/or backwashed softener or filter, as determined under standard 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.

rated softener capacity - A water softener capacity rating based on grains of hardness removed while producing soft water between successive regenerations, and related to the pounds of salt required for each regeneration, as determined under standard test conditions.

raw water- Untreated water, or any water before it reaches a specific water treatment device or process.

recovery- In reverse osmosis processes, indicates the amount of product water taken from the feed water stream; expressed as a percentage of product water flow rate to feed water flow rate. (See concentration factor.)

reduction - A chemical process in which electrons are added to an atom, ion or compound, causing the substance's valence to decrease. Whenever reduction occurs, an off-setting oxidation reaction must occur. (See oxidation.)

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.

regeneration - The process of restoring an ion exchange medium to a usable state after exhaustion. In general, it includes the backwash, regenerant introduction and fresh water rinse steps necessary to prepare a water softener exchange bed for service. Specifically, the term may be applied to the step in which the regenerant solution is passed through the exchanger bed (salt brine for softeners, acid and bases for deionizers.

regeneration level - The quantity of regenerant used in the regeneration of an ion exchange unit or system, usually expressed in pounds per regeneration and/or pounds per regeneration per cubic foot of ion exchanger.

rejection - In reverse osmosis processes, the degree of removal of dissolved salts from the feed water as it passed through a semipermeable membrane (also called "salt rejection"); expressed as a percentage of the feed water TDS. (See total dissolved solids.)

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 (see leakage), or to material meant to remain in the treated water. (See residual chlorine.)

residual chlorine - Chlorine remaining in a treated water after a specified period of contact time to provide continuing protection throughout a distribution system; the difference between the total chlorine added, and that consumed by oxidizable matter. (See combined available chlorine, free available chlorine.)

resin- Synthetic organic ion exchange material, such as the high capacity cation exchange resin widely used in water softeners.

resistance - In water conditioning, the opposition offered by water to the flow of electricity through it; the reciprocal of electrical conductance. The unit of measurement for electrical resistance is the Ohm. Electrical resistance can be used to approximate the mineral content, or lack of it, in high quality water. (See conductance.)

resistivity- A capacity for resisting the flow of electricity. (See resistance.)

reverse deionization - The use of the anion exchange resin ahead of the cation exchange resin (the reverse of the usual order), in a deionization system.

reverse osmosis - (R.O.) A process that reverses, by the application of pressure, the natural process of osmosis so that water passed from the more concentrated to the more dilute solution through a semipermeable membrane, thus producing a stream of water up to 98% free of dissolved solids.

rinse - That portion of the regeneration cycle of an ion exchanger in which fresh water is passed through the column to remove spent and excess regenerant, prior to placing the system in service.

R. O. - The abbreviation for "reverse osmosis".