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

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 pressure drop
The pressure drop of a water softener or filter at the rated service flow with clean water at a temperature of 60 ° F., 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. 
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.
A chemical process in which electrons are added to an atom, ion, or compound.
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 area used as regenerants for the cation and anion resins used in demineralization. 
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 treated water after a specified period of contact time to provide protection throughout a distribution system; the difference between the total chlorine added and that consumed by oxidizable matter. 
reverse osmosis (RO)
A process that reverses, by the application of pressure, the flow of water in a natural process of osmosis so that the water passes from the more concentrated to the more dilute solution through a semi-permeable membrane.
Following backwash in filters to resettle the media bed and purge any turbidity before returning to service mode.  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.