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A term used to describe visible sediment particles, used as both singular and plural.

Particle Size
As used in industry standards, the size of a particle suspended in water as determined by its smallest dimension, usually expressed in microns.

Parts Per Billion (ppb)
A basis for reporting the results of water and wastewater analysis, indicating the number of parts by weight of a dissolved or suspended constituent, per billion parts by weight of water or other solvent. One part per billion is equal to one microgram per liter, the preferred unit.

Parts Per Million (ppm)
A common basis for reporting the results of water and wastewater analysis, indicating the number of parts by weight of water or other solvent. In dilute water solutions, one part per million is practically equal to one milligram per liter, which is the preferred unit. 17.1 ppm equals one grain per US gallon. One ppm equals one pound per million pounds of water.

pH (potential of Hydrogen)
An expression of the acidity of a solution; the negative logarithm of the hydrogen ion concentration (pH 1 very acidic; pH 14, very basic; pH 7, neutral). e.g., pH 5 is 10 times the acidity of 6 and 100 times the acidity of 7. pH is a measure of intensity and not capacity. It is the logarithm of the reciprocal of the hydrogen ion concentration of a solution. The neutral point of 7 indicates the presence of equal concentrations of free hydrogen and free hydroxide ions.

An organism that may cause disease.

Pharmaceutical Grade Water
The definition of six grades of water by the U.S. Pharmacopoeia is as follows: 1.) Purified water 2.) Water for injection 3.) Bacteriostatic water for injection 4.) Sterile water for inhalation 5.) Sterile water for injection 6.) Sterile water for irrigation.

Point-of-use (POU)
A water treatment system designed to connect at the actual point-of-use for water; countertop or undersink treatment systems.

A treatment stage placed at the end of other treatment to bring the water to a more highly conditioned and more perfect state. For example, a mixed bed of ion exchange media installed as the final treatment step in the deionization process to remove last traces of undesirable ions.

A synthetic polymer of the nylon family used in the fabrication of reverse osmosis and ultra filtration membranes.

A sequestering agent used to tie up hardness and iron in solution. As a coating agent, it forms a thin passivating film on metal surfaces to control corrosion.

A synthetic polymer used in the fabrication of reverse osmosis and ultra filtration membranes that are characterized by extreme thermal stability and chemical resistance.

Polyvinyl Chloride (PVC)
A thermoplastic material produced by the polymerization of vinyl chloride. Used extensively in the U.S. for piping, food packaging, and injection molded plastic parts.

The complex network of channels in the interior of a particle of a sorbent.

Portable Exchange
Water softeners, deionizers, and filters that are designed for removal from its point of application for transport to a central station or plant for regeneration or servicing.

Positive Charge
The electrical potential acquired by an atom that has lost one or more electrons; a characteristic of a cation.

Potable Water
Water which is considered safe and fit for human consumption, culinary and domestic purposes and meets the requirements of the health authority having jurisdiction.

The abbreviation for “parts per billion”.

The abbreviation for “parts per million”.

To cause a dissolved substance to form a solid particle that can be removed by settling or filtering, such as in the removal of dissolved iron by oxidation, precipitation, and filtration. The term is also used to refer to the solid formed, and to the condensation of water in the atmosphere to form rain or snow.

Pressure Drop
A decrease in water pressure during its flow due to internal friction between molecules of water, and external friction due to irregularities or roughness in surfaces past which the water flows.

Any water treatment step performed prior to the primary treatment process, such as filtration prior to deionization.

The removal of undesirable matter from water or wastewater. It is the disinfection of water by the killing of microbial contaminants, such as coliform bacteria. A strict definition means the removal from water of all contaminants.

Quartz Sleeve
Also called a quartz jacket, it is a clear, pure quartz sleeve that is installed around the high intensity ultraviolet lamp in an ultraviolet system. It retards less than 10 percent of the radiation dosage in contrast to the poorer results offered by glass.

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.

Reactivation (Revivification)
Oxidation processes for restoring the adsorptive properties of a spent sorbent such as activated carbon.

Recovery (Percent Recovery)
A measurement applied to reverse osmosis and ultrafiltration equipment that characterizes the ratio of product water to feed water flow rates. The measurement is descriptive of reverse osmosis or ultra filtration equipment as a system and not of individual membrane elements. Expressed as a percentage, recovery is defined as: % Recovery = (Product flow rate/feed flow rate) x 100

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.

The process of returning the sodium ions to the mineral after it has exchanged all its sodium ions for calcium and magnesium from hard water. This is accomplished by first back washing the mineral bed to free it of all foreign matter, then passing salt brine through the mineral. The sodium ions attach themselves to the mineral, and the calcium and magnesium combine with the chloride from the brine to form calcium and magnesium chlorides, which are rinsed down the drain. All water softeners using the ion-exchange process are regenerated with these basic steps. In similar fashion cation and anion components of a demineralizer as well as manganese greensand are recharged with comparable sequences.

Rejection (Percent Rejection)
A measure of the ability of a reverse osmosis membrane to remove salts. Expressed as a percentage, rejection is defined as: % Rejection = (1-Product concentration/Feed concentration) x 100

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.

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

Resistivity is a measure of the current-resisting characteristics of a substance when an electrical charge is applied (the reciprocal of conductivity). The standard unit of resistance is the Ohm. Because of the variable nature of water, a distance between measuring probes must be maintained if accurate measurements are desired. The almost universal standard distance for this is the centimeter, hence the “Ohm-cm”. Because temperature affects resistivity of water, temperature-compensating devices are frequently used. These adjust the resistance meter to indicate what the water resistance would be at one temperature, usually 25C.

The ability of an adsorbent to resist desorption of an adsorbate.

The abbreviation for “reverse osmosis”.

Reverse Osmosis
A process for the removal of dissolved ions from water in which pressure is used to force the water through a semi-permeable membrane that 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.

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 that can be formed by the neutralization of an acid with a base.

Sand Filter
A treatment device or structure for removing solid or colloidal material of a type that cannot be removed by sedimentation. Such filters can be gravity rapid-rate or enclosed pressure type.

Saturated Solution
A solution containing the maximum amount of the dissolved substance that such a solution can hold at this temperature.

In reference to reverse osmosis equipment, scaling is the precipitation of sparingly soluble salts, such as calcium carbonate, onto the surface of a membrane. Scaling is associated with decreased flux and reduced reverse osmosis rejection rates.

The process by which solids are separated from water by gravity and deposited on the bottom of a container or basin.

Selective Ion Exchange
The use of a selective ion exchange medium with the property of removing specific ions from a solution.

Descriptive of a material, such as a reverse osmosis or ultra filtration membrane, which allows the passage of some molecules and prevents the passage of others.

Service Run
That portion of the operating cycle of a water conditioning unit during which treated water is being delivered, as opposed to the period when the unit is being backwashed, recharged or regenerated.

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.

Silt Density Index
The silt density Index (SDI) is a measure of the ability of water to foul a membrane or plug a filter. SDI is measured using an apparatus that typically consists of an inlet pressure regulator and pressure gauge followed by a filter holder containing a 0.45-micron microporous membrane filter. Commercial test kits, complete with instructions on how to calculate the index, are available.

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.

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.

Sodium Hydrosulfite
A strong reducing agent used as the main ingredient of several resin cleaners used to clean iron fouled in ion exchange resin beds.

Soft Water
Any water that contains less than 1.0 GPG (17.1 mg/L) of hardness minerals, expressed as calcium carbonate.

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 4C.

A physical or chemical process that reduces the number of organisms to a safe predetermined level.

Sulfate-reducing Bacteria
A group of bacteria that 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.

A yellowish solid element. The term is also used as a slang expression to refer to water containing hydrogen sulfide gas.

The expansion of an ion exchange bed that occurs when the reactive groups on the resin are converted from one form to another. This property is reversible and indeed, some resins shrink in the exhausted state.

The abbreviation for “total dissolved solids”.

Thin-film Composite Membrane (TFC)
Reverse osmosis membrane produced with polyamide-based polymer. It is resistant to bacteria and can withstand a wide pH range. However, it cannot tolerate chlorine.

Throughput Volume
The amount of solution passed through an exchange bed before exhaustion of the resin is reached.

Total Acidity
The total of all forms of acidity, including mineral acidity, carbon dioxide, and acid salts. Total acidity is usually determined by titration with a standard base solution to the phenolphthalein endpoint (pH 8.3).

Total Alkalinity
The alkalinity of a water a determined by titration with standard acid solution to the methyl orange endpoint (pH approximately 4.5); sometimes abbreviated as “M alkalinity”. Total alkalinity includes many alkalinity components, such as hydroxides, carbonates, and bicarbonates.

Total Chlorine
The total amount of chlorine in a solution, which includes the combined chlorine as well as the free available chlorine.

Total Dissolved Solids
The weight of solids per unit volume of water that are in true solution, usually determined by the evaporation of a measured volume of filtered water, and determination of the residue weight.

Total Hardness
The sum of all hardness constituents in water, expressed as their equivalent concentration of calcium carbonate. Primarily due to calcium and magnesium in solution, but may include small amounts of metals such as iron which can act like calcium and magnesium in certain reactions.

Total Organic Carbon
The measurement of carbon dioxide produced from organics when a water sample is atomized into a combustion chamber. The amount of carbon covalently bound in organic compounds in a water sample.

Total Solids
The weight of all solids dissolved and suspended organic and inorganic. Per unit volume of water, usually determined by the evaporation of a measured volume of water at 105C. in a pre weighed dish.

Trihalomethanes (THMs)
A group of organic chemicals to known to be carcinogenic in more than trace amounts that are produced from chlorination. They reduce the germicidal activity of chlorine in alkaline water.

A measure of the amount of finely divided suspended mater in water, which causes the scattering and adsorption of light rays.