RADICAL: A group of atoms acting as a single atom which go through chemical reactions without being changed.
RADON (Rn): A colourless, odourless, short lived radioactive gas which is produced by decay of the uranium/radium series and is soluble in water. Radon is considered carcinogenic when inhaled by humans. Radon can be removed from water by aeration or activated carbon.
RATED CAPACITY (filtration or adsorption): The manufacturer’s statement regarding the expected number of days the equipment will be in service or the expected number of gallons of product water is delivered before backwash, rinse or replacement is needed.
RATED FLOW RATE: The specified maximum and minimum flow rate at which a particular piece of water treatment equipment will continuously produce the desired quality of water.
RATED PRESSURE DROP: The expected pressure drop in psi as stated by the equipment manufacturer or obtained under test conditions.
RAW WATER: Water, usually from wells or surface sources, which has had no previous treatment and is entering the water processing system or device. The water at the inlet side of any water treatment device.
RECOVERY RATE: The percentage of water being processed by reverse osmosis that is produced as permeate.
REDOX: A shortened term for “oxidation-reduction”. Used in terms such as redox reactions and redox conditions.
REGENERATION (ion exchange, softening): The use of a chemical solution (regenerant) to displace the contaminant ions deposited on the ion exchange resin during the service run and replace them with the kind of ions necessary to restore the capacity of the exchange medium for reuse.
REJECT WATER: A term used in distillation, reverse osmosis and ultra filtration to describe that portion of the incoming feed water that has passed across the membrane but has not been converted to product water and is being sent to drain.
REJECTION RATE: In a reverse osmosis or ultra filtration system, rejection rate is the quantity of feed water that does not pass through the membrane expressed as a percent of the total quantity of incoming fed water.
REMOVABLE: capable of being taken away from a water treatment equipment unit using only simple tools such as a screw driver, pliers, or open ended wrench. Readily removable indicates capable of being taken away from a water treatment unit without the use of tools.
RESERVOIR TANK: Same as storage tank on a reverse osmosis system.
RESIDENTIAL EQUIPMENT: The term sometimes used to denote smaller sized water processing equipment which has been designed primarily for home use.
RESIDUAL: The amount of a specific material which remains in the water after the water has been through water treatment step.
RESIDUAL CHLORINE: Chlorine allowed to remain in a treated water after a specified period of contact time and allowed to provide disinfection protection through out the distribution system. The amount of residual chlorine is the difference between the total chlorine added and that consumed by the oxidizable matter.
RESIN: As used in the water processing industry, this term refers to ion exchange resin products which are usually specifically-manufactured organic polymer beads used in softening and other ion exchange processes to remove dissolved salts from water.
RESISTIVITY: Resistivity is a measure of the ability of a substance, such as water, to carry an electric current. It is expressed in units of ohm-cm and is the reciprocal of conductivity. Resistivity measurements are commonly used to asses the quality of water produced by deionizers. Like conductivity, resistivity is temperature dependent and should be measured with a temperature-compensated meter. The usual reference temperature is 25ºC.
RETENTION: Contact time.
REVERSE OSMOSIS (RO): A water treatment process that removes undesirable materials from water by using pressure to force the water molecules through a semipermeable membrane. This process is called “reverse” osmosis because the pressure forces the water to flow in the reverse direction (from the concentrated solution to the dilute solution) to the flow direction (from the dilute to the concentrated) in the process of natural osmosis. RO removes ionized salts, colloids and organic molecules down to a molecular weight of 100.
RUST (ferric oxide): A reddish corrosion product occasionally found in water. Rust is formed as a result of electrochemical interaction between iron and atmospheric oxygen in the presence of moisture.