A unit of absorbed dose of ionizing radiation, corresponding to 100 ergs of energy per gram of absorbing material.     In the International System (SI) of Units, the rad is replaced by the gray (Gy); one gray (Gy) = 100 rads.

Radial Flow

The flow pattern in which water flows from the outside of a filter element to the center core. For example, a replaceable cartridge filter unit is often designed for radial flow.

Radial To Impeller

Perpendicular to the impeller shaft. Material being pumped flows at a right angle to the impeller.


1. A group of atoms acting as a single atom which go through chemical reactions without being changed.     2. Some examples are bicarbonate (HCO3-); hydroxide (OH-); sulfate (SO4--).   Free radicals contain one or more unpaired electrons and are usually short-lived and highly reactive.

Radioactive Fallout

Airborne particles of radioactive nuclei which result from nuclear explosions and settle out of the atmosphere, usually thousands of miles away from the place of detonation.

Radioactive Waste

Water or any other materials including spent nuclear reactor fuel, work clothes, or tools that contain radioisotopes.


Emissions of radiant atomic energy (alpha, beta, and/or gamma rays) from some elements (radium, radon, uranium, thorium, etc.) caused by the spontaneous disintegration of the nuclei of the atoms of these elements.


Any man-made or natural element which emits radiation in the form of alpha or beta particles, or as gamma rays.

Radium (Ra)

Naturally-occurring radioactive elements (radium 226 and radium 228) created in the decay of the uranium and thorium series. Radium can be removed from water by cation exchange softening.

Radon (Rn)

A colorless, odorless, 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.

Rainwater Catchment

AKA Rainwater Harvesting is the deliberate containment of rain or other natural precipitated water with the intention of controlled discharge or beneficial use in potable or non-potable applications. The relatively low mineral content of rainwater makes it highly desirable for reuse when properly processed for safety. Controlled discharge of harvested rainwater can be beneficial in storm water pollution control and energy harvesting. SEE ALSO potable water.


The spread from minimum to maximum values that an instrument is designed to measure.

Ranney Collector

This water collector is constructed as a dug well from 12 to 16 feet (3.5 to 5 m) in diameter that has been sunk as a caisson near the bank of a river or lake.     Screens are driven radially and approximately horizontally from this well into sand and the gravel deposits underlying the river.

Rated Capacity

As relates to softening or ion exchange, rated capacity, as stated by the manufacturer, may refer to any of several figures, all of which are given in relation to the period between regenerations:     1. the expected number of days the equipment will be in service;     2. the expected number of gallons of product water delivered; or     3. the grains of total hardness removed.     The capacity of an ion exchange system will vary, within limits, with the amount of regenerant used.     In filtration or adsorption applications, 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 delivered before it is expected that backwashing and rinse down should occur.

Rated Capacity

Rated capacity, as stated by the manufacturer, may refer to any of several figures, all of which are given in relation to the period between regenerations:  1. the expected number of days the equipment will be in service; 2. the expected number of gallons of product water delivered; or 3. the grains of total hardness removed.  The capacity of on ion exchange system will vary, within limits, with the amount of regenerant used.

Rated Capacity

The manufacturer's rated service cycle statement regarding the expected number of days the equipment will be in service or the expected number of gallons treated water delivered between servicing of the media (cleaning, regeneration, or replacement), as determined by testing or as specified by the manufacturer.

Rated In-Service Life

The total gallons (as specified by the manufacturer) of treated water delivered, or the length of time (based on water flow rates) the unit will be in operation before servicing (regenerating, cleaning, or replacement) of the treatment unit is expected to be necessary.

Rated Pressure Drop

In softening or filtration applications, rated pressure drop is the expected pressure drop in psi as stated by the equipment manufacturer or obtained under test conditions.     Rated pressure drop is based on rated service flow rate for clean water at 60 degrees Fahrenheit using a freshly regenerated softener or a backwashed filter.     The general limit is 15 psi at the specified service flow rate for the unit.

Rated Service Flow

The manufacturer-specified maximum and minimum flow rates at which a particular piece of water treatment equipment will continuously produce the desired quality of water.

Rated Softening Capacity

The statement by the water softener manufacturer about the expected number of grains per gallon of total hardness (as calcium carbonate equivalent) that will be removed between regenerations at the specified flow rate using the specified amount of regenerant (usually sodium chloride). The capacity of ion exchanger resin to remove hardness increases, within limits, with higher regeneration salt dosages; therefore, rated softener capacity must be related to the pounds of salt required for each regeneration.

Raw Water

1. Water, usually from wells or surface sources, which has had no previous treatment and is entering a water processing system or device.     2. Water at the inlet side of any water treatment system or device.

Reaction Tank

A tank or reservoir in which water treatment chemicals are allowed residence time to react with contaminants in the water; hydraulic or mechanical mixing to ensure thorough distribution, and internal piping or baffles to inhibit short-circuiting of the water flow may be provided.


The introduction of air through forced air diffusers into the lower layers of the reservoir.     As the air bubbles form and rise through the water, oxygen from the air dissolves into the water, and replenishes the dissolved oxygen.     The rising bubbles also cause the lower waters to rise to the surface where oxygen from the atmosphere is transferred to the water. This is sometimes called surface reaeration.


A pure chemical substance that is used to make new products or is used in chemical tests to measure, detect, or examine other substances.

Reagent-Grade Water (ASTM)

Very high purity water produced to meet the standards outlined in the American Society for Testing and Materials (ASTM), Designation D 1193-77.     Four grade levels, Types I through IV, are specified and three levels of maximum total bacterial count, Types A through C, are listed depending upon intended use.     Reagent grade water is used for chemical analysis and physical laboratory testing.


A process in which carbon dioxide is bubbled into the water being treated to lower the pH. The pH may also be lowered by the addition of acid. Recarbonation is the final stage in the lime-soda ash softening process. This process converts carbonate ions to bicarbonate ions and stabilizes the solution against the precipitation of carbonate compounds.


A device which indicates the result of a measurement. Most receivers in the water utility field use either a fixed scale and movable indicator (pointer) such as pressure gauge or a movable indicator like those used on a circular-flow recording chart. Also called an indicator.

Receiving Waters

All distinct bodies of water that receive runoff of waste water discharges, such as streams, rivers, ponds, lakes, and estuaries.


In biochemistry: a specialized molecule in a cell that binds a specific chemical with high specificity and high affinity.   In exposure assessment: an organism that receives, may receive, or has received environmental exposure to a chemical.


Process by which rainwater (precipitation) seeps into the groundwater system.

Recharge Area

Generally, an area that is connected with the underground aquifer(s) by a highly porous soil or rock layer. Water entering a recharge area may travel for miles underground.

Recharge Rate

The quantity of water per unit time that replenishes or refills an aquifer.


Inversely or oppositely related.     For example, 1,000 ohms of resistance is the reciprocal of 1/1,000 siemens (formerly mhos) of conductance.


1. In water treatment system design, the continuous operation of the transfer pump to keep water flowing through the system (especially through the disinfection component) at a rate above the water use rate in order to reduce the hazard of bacterial growth.     2. In cross flow membrane filtration systems, the recycling of a portion of the reject stream to maintain a desirable flow across the membrane while the system is in operation.

Reclaimed Brine

That portion of a previously-used brine solution used to regenerate a batch of cation resin for portable exchange softener tanks.     Brine which still measures at least 30 percent saturation and is low in total hardness can be reused in the first stages of the next cation batch regeneration.


A device that creates a permanent record, on a paper chart or magnetic tape, of the changes of some measured variable.


A shortened term for "oxidation-reduction". Used in terms such as "redox reactions" and "redox conditions".

Reducing Agent

Any substance, such as base metal (iron) or the sulfide ion (S2-), that will readily donate (give up) electrons.     The opposite is an oxidizing agent.


Reduction is the addition of hydrogen, removal of oxygen, or the addition of electrons to an element or compound. Under anaerobic conditions (no dissolved oxygen present), sulfur compounds are reduced to odor-producing hydrogen sulfide (H2S) and other compounds. The opposite of oxidation.


A physical or chemical quantity whose value is known exactly, and thus is used to calibrate or standardize instruments.


Resistant, under ordinary or various extraordinary conditions, to treatment or to change in chemical structure, such as resisting or being capable of enduring high temperatures without breaking down or decomposing.


In ion exchange or softening applications, a regenerant is the chemical solution used to restore an exhausted bed of ion exchange resin to the fully ionic (regenerated) form necessary for the desired ion exchange to again take place effectively. Regenerants are also used in a similar fashion to restore catalyst (e.g., iron and manganese removal) media for reuse.     Sodium chloride brine is normally used to regenerate cation exchange water softeners and for some dealkalizer systems. Hydrochloric and sulfuric acids are used for hydrogen-form cation deionizations units. Sodium hydroxide is normally used to regenerate OH- form anion deionizations systems. Potassium permangante regenerant is used to regenerate a manganese greensand iron and manganese removal filter.


In ion exchange applications, 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.     This process is also called recharging or rejuvenation. Catalyst media are recharged similarly.

Regeneration Cycle

The several steps including backwash, application of regenerant, dilution, and fresh or deionized water rinse necessary to accomplish regeneration of an ion exchange bed or oxidizing filter.

Regeneration Level

The quantity of regenerant, usually expressed in pounds per cubic foot of ion exchanger bed or pounds per regeneration, used in the regeneration cycle of an ion exchange system. Regeneration level may also be called salt dosage.

Regeneration Tanks

Large vessels, either gravity (open) or pressurized (closed), in which batches of ion exchange resin used in portable exchange tanks are recharged. Some filter media such as manganese zeolite are also reprocessed in this manner.

Regeneration Water

All of the water consumed in the regeneration steps: backwash, regeneration (brining), dilution, and rinse. Raw water or partially-treated water may be used for the rinse down.

Reject Staging

A method used to raise the production rates of a reverse osmosis system by using the reject water from the first stage as the feedwater for the second or succeeding stage in the RO process.

Reject Water

A term used in distillation, electrodialysis, reverse osmosis, and ultrafiltration to describe that portion of the incoming feedwater that has passed across the membrane but has not been converted to product water and is being sent to the drain. Also called brine, concentrate, or retentate.


In cross flow (membrane) filtration and distillation applications, the prevention by the membrane of the passage of total dissolved solids and other contaminants into the product water.

Rejection Rate

In a reverse osmosis or ultrafiltration system, rejection rate is     1. the quantity of the feedwater that does not pass through the membrane expressed as a percent of the total quantity of incoming feedwater; or     2. the concentration of contaminants that do not pass through the membrane as a percent of the total concentration of those particular contaminants in the feedwater.

Relative Density

The ratio of the density of a specific substance to the density of another substance which is used as a standard. The standard for comparison with liquids is pure water at 4 degrees Celsius. The standard for gases is air at normal temperature and pressure. Relative density was formerly called specific gravity.


The unit of dose equivalent from ionizing radiation to the total body or any internal organ or organ system. Equivalent dose (or the rem value) gives a measure of the biological harm caused by radiation exposure.     A millirem (mrem) is 1/1000 of a rem. In the International System (SI) of Units, the rem is replaced by the sievert (Sv); one sievert (Sv) equals 100 rems.


Capable of being taken away from a water treatment equipment unit using only simple tools such as a screwdriver, pliers, or open-end wrench.     Readily removable indicates capable of being taken away from a water treatment unit without the use of tools.


Pertaining to the kidney.


Commercially available hydrogen peroxide in a 24 to 1 dilution. Used as a disinfectant/sanitizer for reverse osmosis and ultrafiltration membranes and other water treatment equipment.

Repeat Compliance Period

For public water system regulation, any subsequent compliance period after the initial compliance period.

Representative Sample

1. A portion of water or material that is, as nearly as possible, identical in content and consistency to that in the larger body of water or material being sampled.     2. In water treatment equipment manufacturing and testing, a typical production line sample that exhibits the essential features corresponding to and equivalent to (within plus or minus 10 percent) the other production units of that model.

Reserve Capacity

The amount of further contaminant reduction or length of time a device can continue to operate at a high level of performance after a signal of upcoming exhaustion is triggered.


Any natural or artificial holding area used to store, regulate, or control water.

Residential Equipment

The term sometimes used to denote smaller-sized water processing equipment which has been designed primarily for home use and intermittent household water flow rates up to 12 gallons per minute.     Water Quality Association equipment performance standards define residential equipment as that having an inlet designed to accommodate pipe size of no greater than one-inch internal pipe size (IPS) diameter.     May also be referred to as domestic equipment or household equipment.


The amount of a specific material which remains in the water after the water has been through a water treatment step.     Residual may refer to undesirable material remaining as the result of incomplete removal (called leakage) or to material that is meant to remain in the treated water, such as "residual chlorine."

Residual Chlorine

Chlorine allowed to remain in a treated water after a specified period of contact time and to provide disinfection protection throughout the distribution system. The amount of residual chlorine is the difference between the total chlorine added and that consumed by the oxidizable matter.

Residual Disinfectant Concentration

C in CT calculations.    The concentration of disinfectant measured in mg/L in a representative sample of water.


The dry solids remaining after the evaporation of a sample of water or sludge.


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.

Resin Bead

In water processing, refers to the spherical shape of individual particles of ion exchange resin products, as compared to the irregular shaped particles of most other granular media products.

Resin Cleaner

One of several different chemical compounds used to cleanse ion exchange resin products of dissolved iron, aluminum, and various organics attracted to or bonded to the resin beads.


That property of a material that resists the flow of an electric current.     The standard unit of resistance is the ohm.


The process in which an organism uses oxygen for its life processes and gives off carbon dioxide.


In membrane filtration, retention describes the minimum particle or molecule size retained by the membrane under a given set of conditions, namely, pressure, flux recovery, and temperature.

Retrospective Study

An epidemiological study which compares diseased persons with nondiseased persons and works back in time to determine exposures.

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

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.     May be called hyperfiltration.

Reversible Effect

An effect which is not permanent, especially adverse effects which diminish when exposure to a toxic chemical is ceased.

Reversible Effect

An effect which is not permanent, especially adverse effects which diminish when exposure to a toxic chemical is ceased.

RfD (Reference dose)

The daily exposure level which, during an entire lifetime of a human, appears to be without appreciable risk on the basis of all facts know at the time.  Same as ADI.


A small channel eroded into the soil surface by runoff; rills easily can be smoothed out (obliterated) by normal tillage.


In softening or ion exchange applications, the step in the regeneration process in which fresh water is passed through the bed of resin to remove any excess or spent regenerant prior to placing the softener into service.

Riparian Rights

A doctrine of state water law under which a land owner is entitled to use the water on or bordering his property, including the right to prevent diversion or misuse of upstream waters.     Riparian land is land that borders on surface water.

Riser Pipe

As relates to well drilling, a steel pipe used in jet well drilling to carry water under pressure to the well point where unconsolidated material is loosened.    In downflow ion exchange systems, the riser is the central (internal) pipe which carries the processed water from the bottom of the resin bed into the service lines or directs backwash water to the bottom of the ion exchanger bed. In countercurrent systems, it distributes the regenerant to the bottom of the bed for the upflow regeneration process.

Riser Pipe (ion exchange)

In downflow systems, the riser is the central (internal)pipe which carriers the processed water from the bottom of the resin bed into the service lines or directs backwash water to the bottom of the ion exchanger bed.  In countercurrent systems, it distributes the regenerant to the bottom of the bed fro the upflow regeneration process.

Riser Pipe (well drilling)

A steel pipe used in jet well drilling to carry water under pressure to the well point where unconsolidated materials is loosened.


The potential for realization of unwanted adverse consequences or events.

Risk Assessment

A qualitative or quantitative evaluation of the environmental and/or health risk resulting from exposure to a chemical or physical agent (pollutant); combines exposure assessment results with toxicity assessment results to estimate risk.

Risk Characterization

Final component of risk assessment that involves integration of the data and analysis involved in hazard evaluation, dose-response evaluation, and human exposure evaluation to determine the likelihood that humans will experience any of the various forms of toxicity associated with a substance.

Risk Estimate

A description of the probability that organisms exposed to a specified dose of chemical will develop an adverse response (e.g., cancer).

Risk Factor

Characteristic (e.g., race, sex, age, obesity) or variable (e.g., smoking, occupational exposure level) associated with increased probability of a toxic effect.

Risk Management

Decisions about whether an assessed risk is sufficiently high to present a public health concern and about the appropriate means for control of a risk judged to be significant.

Risk Specific Dose

The dose associated with a specified risk level.


Reverse Osmosis


Any substance or chemical used to kill or control rodents.


A device used to measure the flow rate of gases and liquids.     The gas or liquid being measured flows vertically up a tapered, calibrated tube. Inside the tube is a small ball or bullet-shaped float (it may rotate) that rises or falls depending on the flow rate.     The flow rate may be read on a scale behind or on the tube by looking at the middle of the ball or at the widest part or top of the float.

Rotary Drilling

A common hydraulic well-drilling method which uses a rotating drill pipe with a hard-tooled drill bit attached at the bottom.     A fluid (drilling mud) is forced down through the drill pipe and then forced up again between the drill pipe and the well hole, carrying rock chippings (cuttings) up with the mud.


The rotating part of a machine. The rotor is surrounded by the stationary (nonmoving) parts (stator) of the machine.

Route of Exposure

The avenue by which a chemical comes into contact with an organism (e.g., inhalation, ingestion, dermal contact, injection).


That part of precipitation, snow melt, or irrigation water that runs off the land into streams or other surface water.     It can carry pollutants from the air and land into the receiving waters.

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.

Rzynar Index

A modification of the Langelier index used to calculate the degree of calcium carbonate saturation and to predict the likelihood of scale formation from a water supply.

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Updated on Mon, 22 Jun 2020 by Jonathan

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