Packed Bed

A bed of filter or ion exchange medium which is completely retained so that no bed expansion can occur, and no backwash step is used to reclassify the filter or resin. Packed beds are usually part of the design features in ion exchange water softeners used to obtain high capacity and increased regeneration efficiency.

Packed Tower Aeration

A method of treating water to remove volatile organic chemical (VOCs) contaminants. As water is mixed with air, VOCs move from water to air which then passes through carbon filters to trap the contaminants.


Water at a desirable temperature that is free from objectionable tastes, odors, colors, and turbidity. Pleasing to the senses.

Parshall Flume

A device used to measure the flow in an open channel. The flume narrows to a throat of fixed dimensions and then expands again. The rate of flow can be calculated by measuring the difference in head (pressure) before and at the throat of the flume.


A very tiny, separate subdivision of matter.

Particle Count

The results of a microscopic examination of treated water with a special "particle counter" which classifies suspended particles by number and size.

Particle Filtration

Filtration of particles in the size range of two microns or larger in diameter. Particle filtration is typically handled by cartridge filters and media filters.

Particle Size

As used in water industry standards, this term refers to the size, expressed in microns, of a particle suspended in water as determined by the smallest dimension.


A very small solid suspended in water which can vary widely in size, shape, density, and electrical charge. Colloidal and dispersed particulates are artificially gathered together by the processes of coagulation and flocculation.

Partition Coefficient

A measure of the extent to which a pesticide is divided between the soil and water phases.

Parts Per Billion (ppb)

A measure of proportion by weight which is equivalent to one-unit weight of solute (dissolved substance) per billion unit weights of the solution.     This measurement is often used as a measure of concentration when analyzing water for contaminants. Since one liter of water weighs one billion micrograms, one ppb is the equivalent of one microgram per liter when used in water analysis.

Parts Per Million (ppm)

A measure of proportion by weight which is equivalent to one unit of weight of solute (dissolved substance) per million weights of solution.     Since one liter of water weighs one million milligrams, one ppm is equal to one milligram per liter (mg/L). Milligram per liter is the preferred unit of measure in water or waste water analysis.

Pascal (Pa)

A unit of pressure equal to one newton of force per square meter.     One thousand pascals equal one kilopascal (KPa); a kilopascal equals 0.145 pounds per square inch.     1 psi = 6895 Pa = 6.895 kN/sq.m = 0.0703 kg/sq.cm


A process for the partial sterilization (disinfection) of a substance, usually a liquid, by heating it to a critical temperature for a specified period of time. Pasteurization does not greatly change the chemical composition of the sterilized substance.


Capable of causing disease.

Pathogenic Organisms

Organisms, including bacteria, viruses, or cysts, capable of causing diseases (typhoid, cholera, dysentery) in a host (such as a person).     There are many types of organisms which do NOT cause disease. These organisms are called nonpathogenic.


Microorganisms that can cause disease in other organisms or in humans, animals, and plants.     They may be bacteria, viruses, or parasites and are found in sewage, in runoff from animal farms or rural areas populated with domestic and/or wild animals, and in water used for swimming.     Fish and shellfish contaminated by pathogens, or the contaminated water itself, can cause serious illnesses.


The study of disease.

Peak Operating Flow

The maximum rate of flow under which a treatment unit is designed to properly function and produce a certain quality product water.

Peracetic Acid

(CH3COOOH)  A strong oxidizing liquid used in a proprietary one percent solution with hydrogen peroxide as an effective sanitizer and disinfectant for both cellulosic and thin-film composite reverse osmosis membranes.     Can be made by reaction of acetic acid (vinegar, CH3COOH) and hydrogen peroxide (H2O2-) with a sulfuric acid catalyst.

Percent Recovery

The percentage of the feedwater which becomes product water.     Determined by the number of gallons (or liters) of product water divided by the total gallons (or liters) of feedwater and multiplied by 100. The percent recovery is called recovery rate in reverse osmosis and ultrafiltration.

Percent Rejection

(reverse osmosis/ultrafiltration) The percentage of TDS in the feedwater that is prevented from passing the membrane with the permeate.     The formula used is: the difference obtained from the TDS in feedwater minus TDS in permeate divided by TDS in feedwater; then multiply the answer obtained by 100 to obtain a percent.

Percent Saturation

The amount of a substance that is dissolved in a solution compared with the amount that could be dissolved in the solution, expressed as a percent.     Amount of substance that is dissolved divided by the amount that could be dissolved in solution X 100 percent.


Water that passes through soil or rocks under the force of gravity.


1. The slow seepage of water into and through the ground.    2. The slow passage of water through a filter medium.

Performance Evaluation Sample

A reference sample provided to a laboratory for the purpose of demonstrating that the laboratory can successfully analyze the sample within limits of performance specified by the Agency. The true value of the concentration of the reference material is unknown to the laboratory at the time of the analysis.


Microscopic plants and animals that are firmly attached to solid surfaces under water such as rocks, logs, pilings, and other structures.

Peristaltic Pump

A self-priming pump that achieves pumping action by moving a system of rollers against a flexible tube. The pumped fluids (e.g., chemical feeds) are never exposed to the air or to the mechanical moving parts. The roller design prevents siphoning by providing a constant seal on the pumping tube.


A volcanic glass metallic alloy that is composed of a carbide of iron, cementite (Fe3C), and a ferric oxide compound, fernite (e.g., NaFeO2), that are combined in proportions that yield the lowest melting point of this metal combination. Forms a lightweight and porous aggregate when expanded by heat which, in powdered form, can be used as the thin uniform coating on the septum in precoat filtration.


1. The ability of a body, such as a reverse osmosis or ultrafiltration membrane, to pass a liquid under pressure or to pass ions under the influence of an electric current as an ion exchange membrane in electrodialysis.     2. The ability of rock or soil to transmit water.


1. To penetrate and pass through, as water penetrates and passes through soil and other porous materials.    2. That portion of the feedwater which passes through a membrane to become product water.

Permissible Dose

The dose of a chemical that may be received by an individual without the expectation of a significantly harmful result.


The ability of a semipermeable membrane to also be an ion exchanger and to allow selective passage of anions or cations under the influence of an electric current.     Permselectivity is a term often used in relation to electrodialysis.

Permutit Process

An older term for the cation exchange method of water softening.


1. Any compound containing the two oxygen atoms united together into a bivalent -O-O- group. Readily releases abnormally active atomic oxygen, and therefore any peroxide is a strong oxidizing agent.     2. Hydrogen peroxide.

Peroxone Process

A water treatment process in which ozone is exposed to ultraviolet light or hydrogen peroxide as it is being applied to the water. This advanced oxidation process (AOP) initiates the formation of highly reactive hydroxy radicals to achieve a higher level and range of oxidations.


The resistance to degradation as measured by the period of time required for complete decomposition of a material.


Any substance or chemical designed or formulated to kill or control weeds or animal pests.

Petroleum Derivatives

Chemicals formed when gasoline breaks down in contact with groundwater.


cross-linked polyethylene.

pH (pronounce as separate letters)

The degree of the acidity or basicity of a solution as measured on a scale ("pH scale") of 0 to 14. The midpoint of 7.0 on the pH scale represents neutrality--that is, a "neutral" solution is neither acidic nor basic. Numbers below 7.0 indicate acidity; numbers above 7.0 indicate basicity. Substances that when dissolved in water cause the pH to be less than 7.0 often create a sour taste and may sting when in contact with skin. The resulting solution will turn litmus paper red. Substances that when dissolved in water cause pH to be greater than 7.0 create a bitter taste and a slippery feel and will turn litmus paper blue. Water with a high pH tends to cause drying of the skin.

More specifically, pH is the negative of the base 10 logarithm of the hydrogen ion concentration of a solution, -log10[H+], where “p” is the short hand notation for “-log10”.  The hydrogen ion concentration, [H+], is the moles of hydrogen ions per liter of solution. In neutral water, for example, the hydrogen ion concentration is 1x10-7 mol/L; the pH is therefore 7. Because both letters in the term pH are representative notations, the term must be written with a lower case “p” and an upper case “H”.

pH is an exponential function. A pH of 10 is ten times as basic as a pH of 9 and one hundred times as basic as a pH of 8. Similarly, a pH of 2 is one hundred times as acidic as a pH of 4 and one thousand times as acidic as a pH of 5.

In the field, pH is measured with a pH meter, which must be properly calibrated for accuracy. As pH in a water sample can change with time due to fluctuation of carbon dioxide concentration, pH should be measured quickly after sample collection.

Pharmaceutical Grade Water

The collective term for six types of water as defined by the U.S. Pharmacopeia:     1. purified water;   2. water for injection;   3. bacteriostatic water for injection;   4. sterile water for inhalation;   5. sterile water for injection; and   6. sterile water for irrigation.


The dynamic behavior of chemicals inside biological systems; it includes the processes of uptake, distribution, metabolism, and excretion.


A term used in general chemistry to refer to a particular homogeneous form (solid, liquid, or gaseous) of a substance which might exist in other forms or phases.    For example, ice is the solid phase of water.

Phenolic Compounds

Organic compounds that are derivatives of benzene.


An acid-base indicator which has no color in acid solution but turns pink to red as the solution becomes alkaline.

Phenolphthalein Alkalinity

The alkalinity in a water sample measured by the amount of standard acid required to lower the pH to a level of 8.3, as indicated by the change in color of phenolphthalein from pink to clear.     Phenolphthalein alkalinity is expressed as milligrams per liter equivalent calcium carbonate.


A salt of phosphoric acid.  In the water industry, polyphosphates are used as sequestering agents to control iron and hardness, and as coating agents to control corrosion by formation of a thin passivating film on metal surfaces.     The complex phosphates also are a group of sequestering agents widely used in detergent formulations (except where phosphates are banned by law) because of their superiority in chemical water softening, sequestering, and other builder functions.     Sodium tripolyphosphate was the original builder upon which modern laundry detergent technology developed, and is used in laundry granules, automatic dishwasher detergents, and cleansers. It is adaptable to the spray drying process by which granules are made.    Tetrasodium pyrophosphate is also used in detergent granules, but since it does not rank as high in overall performance as sodium tripolyphosphate, its application is more limited.     Highly soluble tetrapotassium pyrophosphate is used in liquid laundry detergents and in hard surface cleaners, where it serves as a builder, water softener, and source of alkalinity.     Another complex phosphate, sodium metaphosphate, is marketed as a packaged water softener. The most widely used sodium metaphosphate is sodium hexametaphosphate (SHMP), which softens by sequestering.     The orthophosphate form of phosphates, trisodium phosphate (also called sodium orthophosphate), is a water softener that inactivates hardness minerals by precipitation. It is used to a limited extent in soap and detergent formulations as a builder, as a source of alkalinity, and for its water-softening properties. It is also used in powdered hard surface cleaners and cleansers to supply alkaline cleaning power.     Chlorinated trisodium phosphate is a dry chlorine bleach that, in water, acts much like sodium hypochlorite (liquid chlorine bleach). It provides a means of incorporating chlorine bleach effectively in dry products, and for this reason is used in cleansers and automatic dishwasher detergents. It also provides alkalinity that aids in cleaning.    Waters containing concentrations of iron, manganese, calcium, or magnesium sometimes can be treated with a sequestrant such as polyphosphate and kept from depositing these mineral precipitates or scales for a period of time.  However, polyphosphate sequestering is not permanent, and therefore may not be as effective as actually removing the iron, manganese, and hardness minerals, as is done with iron filters and ion exchange water softening, for example.  The sequestering value of polyphosphates is destroyed when they revert (hydrate) to orthophosphate.  Polyphosphate reversion or hydration to orthophosphate occurs naturally in water with time.  Intentions would be for this reversion not to happen and not to drop the sequestered water hardness, iron, and manganese out until after it reaches the wastewater.  But, the polyphosphate reversion process can be accelerated by various uncontrolled conditions, such as low pH, high temperature, and the presence of the oxides of certain heavy metals, including iron, calcium, copper, and zinc in water.  It is important in phosphate feed water treatment operations to: 1)maintain a stable pH within the phosphate product's performance rage. 2) determine the polyphosphate composition or blend that is most compatible with the specific water quality objectives and conditions, and 3) apply the appropriate dosage of phosphate to accomodate the system demand.  Because of the difficulty in maintaining phosphate stabilities in the presence of varying pH, time, temperature, and metal oxides in most natural water supplies, the actual removal of iron, manganese, and water hardness is generally a more assuredly effective water treatment method.    Municipal applications of polyphosphates to water supplies can interfere with home water treatment technologies.  A portion of any water hardness, iron,

Phosphorus (P)

A nonmetallic element which is essential to life.     However, too much phosphorus in a body of water can cause excessive growth of plant life, which can create a lack of oxygen as the plants die and decay.


The basic unit (quantum) of electromagnetic radiation.  Light waves, gamma rays, x-rays, and so on consist of photons.  Photons are discrete concentrations of energy that seem to have no rest mass and move at the speed of light.  Their nature can be described only in mathematical terms.  Photons are emitted when electrons move from one energy state to another, as in an excited atom.  SEE ALSO gamma decay; radiation.


The chemical process by which green plants make carbohydrates (which the plants use as food) from carbon dioxide and water in the presence of sunlight and chlorophyll. Oxygen is released as a byproduct of photosynthesis.


A type of plant with very long roots and extensive root systems which draws its water from the water table or other permanent groundwater supplies.     Examples of phreatophytes are willow and salt cedar. Excessive growths of phreatophytes are undesirable in areas where water is scarce since they can consume large quantities of water.

Physical Stability

A measure of the ability of an ion exchanger or a filter medium to resist breakdown caused by the physical forces such as crushing, attrition, or high temperatures to which it is subjected during use.

Physical Water Treatment Device

A device that modifies the properties of water by physical means as opposed to chemical or mechanical means. Physical water treatment devices do not use, nor do they impart significant levels of chemical substance(s) to the water stream being treated. This class of device includes those that reduce or prevent certain scale deposits that are normal to hard water behavior. 

They may provide a measurable reduction in hardness depending on the measurement method applied by conversion of calcium and magnesium ions to a precipitate form, but do not provide a reduction of total calcium or magnesium content of the treated stream nor have they been proven to achieve soft water. Physical water treatment devices use a range of processes, including media-induced precipitation, magnetic, electronic, electrostatic, and electromagnetic technologies. 


Small, usually microscopic plants (such as algae), found in lakes, reservoirs, and other bodies of water.

Pico (p)

A prefix used in the metric system to mean one-trillionth or 10-12 or 0.000000000001.

Picocurie (pCi)

A measure of radioactivity.     One picocurie of radioactivity is equivalent to 0.037 nuclear disintegrations per second, or to 0.037 becquerels (Bqs).

Pilot Testing

Pilot testing usually refers to a pilot plant which is a trial intermediate between laboratory bench testing and full scale-operation in the field. Pilot testing is often conducted on-site in the field with a scaled down replica of the full-scale treatment plant.


A measure of the completeness of an incomplete chemical reaction, using a logarithmic scale. Also used to express the extent of dissociation of weak acids and complex ions. The weaker the electrolyte, the larger is its pK. The strengths of different acids may be compared by using pK values. Mathematically speaking, pK is the negative of the logarithm of the ionization (dissociation) constant (pKeq) of a chemical compound.

Plan View

A diagram or photo showing a facility as it would appear when looking down on top of it.


1. Small, usually microscopic, free-floating plants (phytoplankton) and animals (zooplankton) in aquatic systems.    2. All of the smaller free-floating, suspended, or self-propelled organisms in a body of water.


Any high polymer, usually synthetic, material that during its manufacture or processing can be extruded, molded, cast, drawn, or laminated into objects of all sizes and shapes by application of heat or pressure, by chemical condensation, or by casting during polymerization of monomers, and that can retain the new shape under conditions of use.

Plastic Pipe

Tubing or pipe made from unreinforced thermoplastic polymers such as polyethylene, polybutylene, polyvinyl chloride, chlorinated polyvinyl chloride, and polypropylenes, and from reinforced thermosetting polymers such as epoxides and polyesters with glass fibers as reinforcing to increase strength.     Plastic pipe is characteristically flexible to rigid, lightweight, and strong; it resists attack by chemicals, corrosion, and weathering.

Plug Flow

A flow pattern in which the water being processed passes through the medium (such as a granular filter or an ion exchange bed) in a "piston-like" fashion instead of in turbulent or mixed flow patterns such as are found in other processes like ultraviolet light disinfection and electrodialysis.


The way polluted water extends downstream from the pollution source (analogous to smoke from a smokestack as it drifts downwind in the atmosphere).


Powered or moved by air pressure or compressed air.

Pneumatic Tank

A pressurized holding tank which is part of a closed water system (such as for a household well system) and is used to create a steady flow of water and avoid water surges created by the pump kicking on and off.


Point of entry.

Point Source

A stationary location or fixed facility from which pollutants are discharged or emitted.     Also, any single identifiable source of pollution, e.g., a pipe, ditch, ship, ore pit, factory smokestack.

Point Source

A stationery location or fixed facility from which pollutants are discharged or emitted.  Also, any single identifiable source of pollution, e.g., a pipe, ditch, ship, ore pit, factory smokestack.

Point-of-Entry (POE) Treatment

Full service water treatment applied to the water entering a house or building for the purpose of reducing contaminants in the water distributed throughout the house or building (outside faucets may be excepted from treatment).

Point-of-Use(POU) Treatment

Water treatment applied to a single tap used for the purpose of reducing contaminants in water at that one outlet.     POU treatment is often used to treat water for drinking and cooking only.


As relates to electricity and corrosion control, to disrupt the corrosion process by developing a barrier on an anodic or cathodic surface.

Pole Shader

A copper bar circling the laminated iron core inside the coil of a magnetic starter.


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.

Polishing Filter

A filter installed for use after the primary water treatment stage to remove any traces of undesirable matter or to polish the water.


A contaminant existing at a concentration high enough to endanger the environment or the public health or to be otherwise objectionable.


Generally, the presence of matter or energy whose nature, location, or quantity produces undesired environmental effects. Under the Clean Water Act, for example, the term is defined as the man-made or man-induced alteration of the physical, biological, and radiological integrity of water.


A molecular chain polymer made of amide   (-CONH-) linkages.     Proteins of plants such as soybeans, peanuts, and corn are natural polyamides. Nylon is example of a synthetic polyamide.     Thin-film composite reverse osmosis membranes are constructed of a thin layer of an aromatic polyamide extruded onto a less dense polysufone substrate.

Polybutylene (PB)

A thermoplastic polymer, such as butyl rubber, of a butylene liquefied petroleum gas -- isobutene, butene-1, or butene-2; manufactured in various degrees of elasticity, strength, and stability. Used for films, coatings, pipes, tubing, fittings, and many other services.


A thermoplastic polymer resin that is a linear polyester of carbonic acid. Polycarbonate is a transparent, nontoxic, noncorrosive, heat resistant, high impact strength plastic; it is generally stable, but may be subject to attack by strong alkalies and some organic hydrocarbons. It can be molded, extruded, or thermoformed, and is commonly used for numerous services, such as non-breakable windows, household appliances, tubing, piping, and cartridge filter sumps.


A polymeric electrolyte (natural or synthetic) with a long chain-like structure and a high molecular weight which may be used as a cationic, anionic, or nonionic flocculent (or coagulant aid) in the treatment of potable water. Often called a polymer.


A tough thermoplastic polymer (-CH2CH2-) of ethylene that resists chemicals and absorbs very little moisture.     Polyethylene can vary from soft and flexible to hard and rigid depending on the pressures and catalysts used during manufacturing.     Low density polyethylene has its melting point at about 240 degrees Fahrenheit and tensile strength of 1500 psi; high density polyethylene melts at 275 degrees Fahrenheit and has tensile strength of 4000 psi.     Among services, polyethylene is commonly used for tubing and piping, food packaging, garment bags, and molded plastic products.


A chemical formed by the union of many monomers (a molecule of low molecular weight).     Polymers are used with other chemical coagulants to aid in binding small suspended particles to larger chemical flocs for their removal from water.     All polyelectrolytes are polymers, but not all polymers are polyelectrolytes.


Any of a broad familiy of inorganic phosphorus compounds that are commonly referred to as molecularly dehydrated phosphates or condensed phosphates, including hexametphosphate, tripolyphosphate (P3O105-), and pyrophosphate (P2O75-), among others.  Unlike orthophosphate (PO43-), for which four oxygen atoms always surround a single central phosphate, polyphosphates are arranged as polymeric chains or occasionally rings that vary in their phosphorus-to-oxygen ratio and chain length with different commercial formulations.  Polyphosphate is used as a sequestering agent to control iron and hardness, and as a coating agent that forms a thin passivating film on metal surfaces to control corrosion.  Polyphosphates in solution are anionic and may be removed from water with anion exchange resins.  However, polyphosphates that have reacted with a metal (e.g., iron) can form a sticky colloidal precipitate that must be filtered to be removed from water.  Inorganic polyphosphates may tend to breakdown (hydrolyze) into orthophosphate groups plus molecules of shortened chain length over time and under extreme temperature and pH conditions.  Upon breakdown these phosphate compounds can drop out any sequestered metal ions, such as iron and hardness, allowing them to precipitate from the water.


A thermoplastic polymer of propylene resembling polyethylene and used for making molded and extruded plastic products such as water pipe, tubing, and fittings.


Polymerized styrene.     Polystyrene forms the skeletal structure of most common ion exchange resin beads.


A synthetic thermoplastic polymer. Used in the manufacture of ultrafiltration membranes and in thin-film composite and charged polysulfone reverse osmosis membranes.

Polyvinyl Chloride (PVC)

A thermoplastic polymer resin material (-CH2CHCl-) that is rigid and practically chemically inert.     Commonly used for water pipes and fittings, as well as numerous other services such as siding, gutters, raincoats, chemical containers, flooring, toys, tennis court surfaces, and films and package coatings for food containers.

Polyvinylidene Fluoride (PVDF)

A thermoplastic fluorocarbon polymer that can be used for injection molded or extruded products.     More highly inert and resistant to oxidative degradation than polypropylene plastic, for example, but not as inert as the tetrafluoroethylene (TFE) fluorocarbon polymers (Teflon).

Population at Risk

A population subgroup that is more likely to be exposed to a chemical, or is more sensitive to a chemical, than is the general population.

Population Equivalent

A unit of measure used to express the strength of waste water from any source. (That is, not from household waste water only.) In making such calculations, 0.17 pounds of BOD (biological oxygen demand) per capita per day is often used as the standard figure.     Thus, waste water with 17 pounds of BOD per day would have a population equivalent of 17 divided by 0.17 or 100 people.

Pore Volume

The volume of pores in a unit weight of media particles or filter bed.


Very small open spaces in a rock or granular material, including the complex network of channels in the interior of a particle, such as those within an ion exchange resin bead or an activated carbon granule.


A measure of the volume of pores in a material.     Porosity is calculated as a ratio of the interstices of a material (e.g., the volume of spaces between the media particles in a filter bed) to the volume of its mass, and is expressed as a percentage.


Full of pores through which water, light, etc. may pass.

Portable Exchange (PE) Tanks

Tanks containing up to two cubic feet of ion exchanger products or filter media are rented to homeowners or business clients with the beds fully-regenerated and ready for use.     Portable exchange tanks do not have the valving controls required for regeneration. Upon exhaustion (determined by predetermined calendar days, meter, or monitoring device), the tanks are returned to a central regeneration plant where the resin or other media in each tank is reprocessed and restored for reuse.     Portable exchange tanks may be available with water softening or deionization resins, mixed ion exchange media, manganese zeolite, activated alumina, and activated carbon.     Portable exchange tanks are used for both household and commercial applications.

Positive Charge

The electrical potential of an atom which has lost one or more electrons, therefore leaving it with more protons than electrons.     Cations carry positive charges.

Positive Displacement Pump

A type of piston, diaphragm, gear, or screw pump that delivers a constant volume with each stroke.     Positive displacement pumps are used as chemical solution feeders.


An antiparticle whose mass and spin are the same as those of an electron, but with an opposite (positive) electric charge of equal magnitude.  In the nucleus of an atom, a proton can be transformed to a neutron via emission of a positron-a process known as beta-plus radioactive decay.  SEE ALSO antiparticle; beta decay.


The application of chlorine to a water following other water treatment processes.

Potable (Drinking) Water

A water supply which meets USEPA and/or state water quality standards and that is considered safe and fit for human consumption.

Potassium Chloride (KCl)

A colorless potassium salt which can be used as a regenerant in cation exchange water softeners and dealkalizers.

Potassium Cycle

The use of potassium chloride salt instead of sodium chloride salt in the regeneration of cation ion exchange water softeners. The potassium ion (K+) becomes the exchangeable ion rather than the sodium ion (Na+) in the sodium cycle system.

Potassium Permanganate

(KMnO4) Dark purple, odorless crystals (with a blue metallic sheen) that dissolve in water to produce a purple-red color. Potassium permanganate is a powerful oxidizing agent that is used in water treatment as both an oxidizer and a disinfectant. It is also an effective regenerant for manganese oxidizing filters.


Amount of material necessary to produce a given level of a deleterious effect.


The effect of one chemical to increase the effect of another chemical.

Potentiometric Surface

The level to which water will rise in cased wells or other cased excavations into aquifers, measured as feet above mean sea level.



Pounds Per Square Inch (psi)

Unit of measure for expressing pressure.

Pounds Per Square Inch Gauge (psig)

Pressure measured with respect to that of the atmosphere. This is a pressure gauge reading in which the gauge is adjusted to read zero at the surrounding atmospheric pressure. It is commonly called gauge pressure.

Power Factor

The ratio of the true power passing through an electric circuit to the product of the voltage and amperage in the circuit. This is a measure of the lag or load of the current with respect to the voltage.


Parts per billion.


Parts per million.


Parts per trillion.


The application of chlorine to a water supply prior to other water treatment processes which may follow.


1. As a verb: to cause a dissolved substance to form a solid which comes out of solution and can be removed by settling or filtering. For example, the reduction of dissolved iron by oxidation, precipitation and filtration.     2. As a noun: the solid formed when a dissolved substance comes out of solution in such a way that it can be settled or filtered out.     3. As a verb: to cause moisture to condense and be deposited as rain, sleet, snow, etc.


1. The process by which atmospheric moisture falls onto a land or water surface as rain, snow, hail, or other forms of moisture.     2. The chemical transformation of a substance in solution into an insoluble form (precipitate).


The ability of an instrument to measure a process variable and to repeatedly obtain the same result. The ability of an instrument to reproduce the same results.


The application, usually by slurry, of a very fine granular filter medium such as diatomaceous earth to a retaining membrane or fabric surface prior to a service run. Precoating makes the medium unfit for further use. At the end of each service run, the precoat medium is rinsed off and disposed of prior to application of a new precoat to the filter septum.


Compounds which lead to other compounds. For example, natural humic and fulvic acids which, upon combination with chlorine, lead to trihalomethanes.

Preplumbed Installation

An installation which allows domestic water treatment equipment to be easily installed because the necessary bypass and valves are already in place. An example would be a new home that already has all of the plumbing needed for installing a water treatment device.


Water rights which are acquired by diverting water and putting it to use in accordance with specified procedures. These procedures include filing a request to use unused water in a stream, river, or lake with a state agency.


An agent that prevents the deterioration of materials; usually associated with the prevention of biological deterioration.

Pressure Control

A switch which operates on changes in pressure. Usually this is a diaphragm pressing against a spring. When the force on the diaphragm overcomes the spring pressure, the switch is actuated (activated).

Pressure Differential

The difference in the pressure between two points in a water system.     The difference may be due to the difference in elevation and/or to pressure drop resulting from water flow.

Pressure Drop

1. A decrease in the water pressure (in psi) which occurs as the water flows. Pressure drop may occur for several reasons: internal friction between the molecules of water, external friction between the water and the walls of the piping system, or rough areas in the channel through which the water flows.    2. The difference between the inlet and outlet water pressure during water flow through a water treatment device such as a water conditioner. Abbreviated P and measured in pounds per square inch gauge pressure.

Pressure Head

The vertical distance (in feet) equal to the pressure (in psi) at a specific point. The pressure head is equal to the pressure in psi times 2.31 ft/psi.

Pressure Swing Adsorption (PSA)

A process using a molecular sieve material, when pressurized to just overy 30 psig, to adsorb nitrogen and moisture from air, and to concentrate oxygen in air to approximately 80 to 95 percent oxygen, as in the air feed to an ozone generator.


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

Prevalence Study

An epidemiological study which examines the relationship between diseases and exposures as they exist in a defined population at a particular point in time.


The responsibility for ensuring that a law is implemented, and the authority to enforce a law and related regulations.     A primacy agency has the primary responsibility for administrating and enforcing regulations such as "primary enforcement responsibility" as defined in the Safe Drinking Water Act.

Primary Element

An instrument that measures (senses) a physical condition or variable of interest.     Floats and thermocouples are examples of primary elements.     Also called a sensor.

Primary Treatment

1.n The first major treatment in a wastewater treatment works, consisting usually of sedimentation.  2. The removal of a substantial amount of suspended matter but little or no colloidal and dissolved matter.


The action of filling a pump casing with water to remove the air.     Most pumps must be primed before startup or they will not pump any water.

Prior Appropriation

A doctrine of water law that allocates the right to use water on a first-come, first-serve basis.

Process Variable

A physical or chemical quantity which is usually measured and controlled in the operation of a water treatment plant or an industrial plant.

Process Water

Water used in a manufacturing or treatment process or in the actual product manufactured.     Examples would be water used for washing, rinsing, direct contact, cooling, solution makeup, chemical reactions, and gas scrubbing in industrial and food processing operations.     In many cases, water is specifically treated to produce the quality of water needed for the process.

Product Staging (reverse osmosis)

The practice of using some of the product water from the first stage of RO treatment as feedwater for the second stage.

Product Water

Water that has been through the total treatment process and meets the quality standards required for the use to which the water will be put.     Product water is called by different names, depending upon which treatment process it has gone through:     "Centrate" from a centrifuge   "Distillate" from a distiller   "Filtrate" from a filter unit   "Finished" from a municipal treatment plant   "Deionized" from a cation and anion exchange system   "Softened" from a softener unit   "Permeate" from a reverse osmosis or ultrafiltration unit.

Product Water Dispensing Rate

That amount of product water available from the full-open dispensing outlet of a water treatment unit for a specified period of time.

Production Rate

The amount (gallons or liters) of product water the system produces per minute or (especially for reverse osmosis) per 24-hour period.


A drawing showing elevation plotted against distance, such as the vertical section or side view of a pipeline.

Prospective Study

An epidemiological study which examines the development of disease in a group of persons determined to be presently free of the disease.


A group of enzymes that is effective in breaking down proteins into smaller, less complex molecules.


An original water treatment equipment unit on which a specific equipment line is modeled.


Microscopic, usually single-celled microorganisms which live in water and are relatively larger in comparison to other microbes.     Protozoa are higher on the food chain than the bacteria that they eat. Many protozoa are parasitic.     Singular form: protozoan or protozoon.

Prussian Blue

A blue paste or liquid (often on a paper-like carbon paper) used to show a contact area.     Used to determine if gate valve seats fit properly.


Pressure swing adsorption.


Pounds per square inch.


Pounds per square inch gauge.

Public Water System

A system for the provision to the public of piped water for human consumption, if such system has at least 15 service connections or regularly serves an average of at least 25 individuals daily at least 60 days out of the year.     Such term includes: 1. any collection, treatment, storage, and distribution facilities under control of the operator of such system and used primarily in connection with such system; and 2. any collection or pretreatment storage facilities not under such control which are used primarily in connection with such system. A public water system is either a "community water system" or a "noncommunity water system."


Porous volcanic rock.


A stable, natural, glassy aluminum silicate mineral from volcanic ash which is used as a water treatment filtration medium.

Pumping Station

Mechanical devices installed in sewer or water systems or other liquid-carrying pipelines that move the liquids to a higher level.

Pumping Water Level

The vertical distance in feet from the center line of the pump discharge to the level of the free pool while water is being drawn from the pool.

Pure Water

This term has no real meaning unless the word "pure" is defined by some standard. The Water Quality Association Promotion Guidelines recommend against the use of the words "Pure Water" in advertising unless the meaning of "pure" is very clearly explained for the consumer. The capacity of the words "pure", "purification," "purifier" and other derivatives of the word "pure" to mislead consumers is considerable.     In an absolute sense, all available water contains an amount of some substances in addition to H2O. The context that is meant by use of these words should always be clearly and accurately defined.

Purified Water

Water produced from water meeting the USEPA standards for safe drinking water through treatment by distillation, reverse osmosis, deionization, or other processes and which meets United States Pharmacopoeia (USP) purity standards for "purified water" can be labeled as "purified."     These standards regulate pH, chloride, sulfate, ammonia, calcium, carbon dioxide, heavy metals, oxidizable substances, total solids, and bacteria.     If water meeting the USP standard has been distilled, it can alternatively be labeled "distilled water."


An agency or person that supplies water (usually potable water).


Biological decomposition of organic matter, with the production of ill-smelling and tasting products, associated with anaerobic (no oxygen present) conditions.


Polyvinyl chloride.


Iron disulfide (FeS2). A common mineral that has a bright metallic luster and a brass-yellow color.     Also called iron pyrite or fool's gold.


Substances (often of unknown origin) that produce fever when introduced into the human body. Being chemically and physically stable, pyrogens are not necessarily destroyed by conditions that kill bacteria.


(MnO2) Manganese dioxide mineral ore which is sold under several trade names. It is the most important ore of manganese and is usually of an iron-black or dark steel-gray color with a metallic luster. Pyrolusite has very effective capacity as an iron, manganese, and/or hydrogen sulfide sacrificial filter medium but is very heavy, requiring high backwash rates of 20 gallons or more per square foot of media surface area.

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Updated on Mon, 09 Aug 2021 by Tanya

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