A small calendar disc attached to motors and equipment to indicate the year in which the last maintenance service was performed.


A small calendar disc attached to motors and equipment to indicate the year in which the last maintenance service was performed.

Day Tank

1. A tank used to hold diluted regenerant chemicals prior to being pumped into a batch regeneration of ion exchange beds. Day tanks are commonly used in exchange tank regeneration operations.   <p>  2. A tank used to store a chemical solution of known concentration for feed to a chemical feeder. A day tank usually stores sufficient chemical solution to properly treat the water being treated for at least one day.     Also called an age tank.

Dead End

The end of a water main which is not connected to other parts of the distribution system by means of a connecting loop of pipe.

Dead End Filtration

A flow pattern in which all water flows through the medium or membrane (as opposed to cross flow filtration) thus allowing a buildup of a particulate layer on or near the surface of the medium and requiring periodic backwashing, repeated cleaning, or cartridge replacement.


The dealkalization of water refers to the removal of alkalinity ions from the feed stream. This is readily done with chloride cycle anion exchange whereby the alkalinity (bicarbonate or carbonate) is removed in exchange for chloride. The resin can be regenerated with salt (NaCl). 

In a boiler, alkalinity decomposes under heat to produce CO2 so the steam that is produced contains carbonic acid (H2CO3) and can be highly corrosive. Dealkalization avoids this.  In addition, the removal of alkalinity avoids the formation of boiler scale (CaCO3) because the chloride salt of calcium is very soluble and does not form deposits. SEE ALSO anion softening.


To draw off the upper layer of liquid (water) after the heavier material (a solid or another liquid) has settled.


The process of drawing off a supernatant liquor without disturbing the underlying lower liquid layers and the precipitate.


The exchange of cations for hydrogen ions by a strong acid cation exchanger operated in the hydrogen form.


The deliberate removal of chlorine from water. The partial or complete reduction of residual chlorine by any chemical or physical process.


A unit for expressing the relative intensity of sounds on a scale from zero for the average least perceptible sound to about 130 for the average level at which sound causes pain to humans.


The process of removing color bodies such as tannins or humic acid from water by means of oxidation, coagulation/filtration, adsorption, or ion exchange.


The conversion of chemically unstable materials to more stable forms by chemical or biological action.     If organic matter decays when there is no oxygen present (anaerobic conditions or putrefaction), undesirable tastes and odors are produced. Decay of organic matter when oxygen is present (aerobic conditions) tends to produce much less objectionable tastes and odors.


The alteration of an ion exchange resin structure by destruction of the cross-link polymer (such as divinylbenzene) as the result of very aggressive chemical attack (by chlorine, ozone or hydrogen peroxide, for example) or heat.     Decross-linking causes increased moisture content in an ion exchange resin and the physical swelling of the beads.


The removal of excess fluoride in drinking water to prevent the mottling (brown stains) of teeth.

Degassing, Degasify, Degasification

The removal of dissolved gases such as carbon dioxide, methane, hydrogen sulfide, and oxygen by:   <p>  1. subjecting the water to a pressure below atmospheric pressure (vacuum degassing) or <p>  2. passing large amounts of air thoroughly through the water at atmospheric pressure (air stripping).


As relates to ion exchange, the loss of capacity, reduction of resin particle size, excessive swelling of resin particles, or any combination of these factors resulting in a lessening of the ion exchange capabilities of the resin.     This may occur as a result of the type of service for which the resin was used, the solution concentrations used, heat, or aggressive operating conditions.


As relates to ion exchange, the loss of capacity, reduction of resin particle size, excessive swelling of resin particles, or any combination of these factors resulting in a lessening of the ion exchange capabilities of the resin.     This may occur as a result of the type of service for which the resin was used, the solution concentrations used, heat, or aggressive operating conditions.


The removal of all ionized minerals and salts (both organic and inorganic) from a solution by a two-phase ion exchange procedure: First, positively-charged ions are removed by a cation exchange resin in exchange for a chemically equivalent amount of hydrogen ions. Second, negatively-charged ions are removed by an anion exchange resin for a chemically equivalent amount of hydroxide ions.     The hydrogen and hydroxide ions introduced in this process unite to form water molecules. This process is also called demineralization by ion exchange.

Delivered Water

The finished product water from a public or private utility water plant which is carried through a water main network of pipes and arrives at the point-of-use (homes, institutions, and business facilities).

Delta P

The pressure drop or loss (in psi) by flowing water in a pressurized system as the result of the velocity and turbulence of the flowing water, restrictions the water flows through, and roughness of surfaces the water flows past. The symbol for Delta P is P.

Demand Initiated Regeneration (DIR)

A method of automatically initiating regeneration or recycling in filters, deionizers, or softeners after a pre-determined metered volume of water has been processed.     In a softener or deionizer, regeneration may be triggered automatically based upon an electrical or mechanical signal. All operations including bypass (of hard or soft water depending upon design), backwashing, brining, rinsing, and returning the unit to service are initiated and performed automatically in response to the demand for treated water.

Demineralization by Ion Exchange

See deionization.


The biochemical conversion of nitrate and nitrite nitrogen in the soil dissolved in water to gaseous nitrogen.


The mass of a substance per specified unit of volume; for example, pounds per cubic foot. True density is the mass per unit volume excluding pores; apparent density is the mass per unit volume including pores.     The density of water is 1.0 gram per cubic centimeter or about 62.4 pounds per cubic foot.


See Water Spotting.

Depth Filter

Depth (deep bed) filters use a tortuous path to entrap particles deep within the body of the filtration media. A tortuous path is used in depth filters to effect rejection of solids smaller than the void spaces in the filtration media. The path enables electrostatic and intermolecular forces between the solids, causing them to clump together, adhere to the media, or form a bridge between voids that blocks the passage of additional particles. Particles are deposited in the upper part of the bed first, but once a layer of particles is deposited, additional particles simply flow past them to find a spot deeper in the bed. This is possible because the passage through the irregular maze of channels or paths in a granular bed is larger than the particles to be removed. Some of the particles are strained out; others are attracted to the medium because of electrostatic and intermolecular forces (Figure 1 C). Depth filtration can be single medium (sand) Figure 1C, or several different media Figure 1 D—either in stratified layers or mixed format.

Depth Filtration

A filtration process in which water flows through progressively smaller pore spaces in a filter media bed.     Depth filters are designed to entrap particles throughout the mass of filter media, as opposed to a surface filter where only the surface layer does the actual filtering.     Depth filtration can be accomplished with a multilayered bed or multimedia filtration. String-wound fiber cartridge elements can also function as depth filters.

Dermal Exposure

Contact between a chemical and the skin.


The removal of dissolved inorganic solids (salts) from a solution such as water to produce a liquid which is free of dissolved salts.     Desalination is typically accomplished by distillation, reverse osmosis, or electrodialysis.


[DES-uh-kant] A hygroscopic substance such as activated alumina, calcium chloride, silica gel, or zinc chloride that draws water vapor from the air.  Desiccants are used to maintain a dry environment for equipment and materials.


An ion exchange process designed for reduction of silica from a water supply.     Typically a strong base anion exchanger operated in OH- form is used.


The opposite of adsorption.     The process of removing an adsorbed material from the medium or resin on which it has been adsorbed. Desorption is usually accomplished by heating, a reduction of pressure, by the presence of another more strongly adsorbed substance, or a combination of these means.


A process used to thoroughly dry air; to remove virtually all moisture from air.


A closed container into which heated weighing or drying dishes are placed to cool in a dry environment.     The dishes may be empty or they may contain a sample. Desiccators contain a substance, such as anhydrous calcium chloride, which absorbs moisture and keeps the relative humidity near zero so that the dish or sample will not gain weight from absorbed moisture.


The development of vertical mining within a lake or reservoir to eliminate (either totally or partially) separate layers of temperature, plant, or animal life.     This vertical mixing can be caused by mechanical means (pump) or through the use of forced air diffusers which release air into the lower layers of the reservoir.


A sulfate-reducing bacteria in water which can convert sulfates and elemental sulfur to sulfide, thereby creating hydrogen sulfide gas and the concomitant "rotten egg" odor in water supplies.

Detention Lag

The time period between the moment a change is made and the moment when such a change is finally sensed by the associated measuring instrument.

Detention Time

1. The theoretical (calculated) time required for a small amount of water to pass through a tank at a given rate of flow.     2. The actual time in hours, minutes, or seconds that a small amount of water is in a settling basin, flocculating basin, or rapid-mix chamber. In storage reservoirs, detention time is the length of time entering water will be held before being drafted for use (several weeks to years, several months being typical).     Detention Time (hr) = Basin Volume (gal.)(24 hr/day)/Flow (gal/day)


Any material with cleansing powers: soaps, synthetic detergents, man-made alkaline materials, solvents, and abrasives. In common domestic usage, the term is often used to refer to synthetic detergents.

Dew Point

The temperature to which air must be cooled to cause condensation of the water vapor it contains.


1. To remove or separate a portion of the water present in a sludge or slurry. To dry sludge so it can be handled and disposed.     2. To remove or drain the water from a tank or a trench.


A corrosion process that removes zinc from brass but leaves the copper in place; the brass (valve or fitting) retains its original dimensions but is severely weakened and is prone to structural failure, leaks, or seepage through the body walls.     Occurs most readily in waters with high chlorides and pH greater than eight.


The separation of components of a solution by diffusion through a semipermeable membrane which is capable of passing certain ions or molecules while rejecting others.

Diaphragm Pump

A positive-displacement pump in which the reciprocating piston is separated from the solution by a flexible diaphragm, thus protecting the piston from corrosion and erosion and avoiding related problems with packing and seals.


A type of microscopic algae with cell walls that contain silica.

Diatomaceous Earth

A fine, siliceous (made of silica) "earth" composed mainly of the skeletal remains of diatoms, a type of free-floating, microscopic plant found in the ocean.     Diatomaceous earth is used as a water filtration medium.

Diatomaceous Earth Filtration

A filtration method resulting in substantial particulate removal, that uses a process in which:     1. A "precoat" cake of diatomaceous earth filter media is deposited on a support membrane (septum); and     2. While the water if filtered by passing through the cake of the septum, additional filter media, known as "body feed," is continuously added to the feedwater to maintain the permeability of the filter cake.

Diatomic Molecule

A molecule containing only two atoms, such as hydrogen as H2 or oxygen as O2.


Dissolved Inorganic Carbon.

Dielectric Fitting

A plumbing fitting made of an electrical nonconductor (such as plastic) used to control galvanic corrosion when joining pipes of dissimilar metals (such as copper and galvanized steel).     If the dielectric fitting is used in a main water line, a bypass strap may be necessary to maintain the continuity of existing electrical grounding since the household water pipes may have been used to ground the household electrical lines.

Differential Pressure

The difference in pressures at two points in a water system. Differences may be due to variations in elevation, or to friction losses, or to pressure drops caused by resistance to water flow through pipes, softeners, filters, or other devices.


See Distributor.


The movement of suspended or dissolved particles from a more concentrated to a less concentrated region as a result of the random movement of individual particles; the process tends to distribute them uniformly throughout the available volume.

Diffusion Feeder

A chemical feed system in which chemicals are added to a water stream in controlled quantities for pipe line or metal surface protection and/or disinfection methods.     A diffusion feeder is designed in such a way that a small stream of water is diverted through a tank so that the water flows over the chemical material (or solution), a small amount of which is diffused (dissolved) into the water and carried back to the main water line.     A diffusion feeder is also called a bypass feeder.


The process by which complex organic materials are broken down and decomposed into simpler substances as a result of a chemical or biological reaction or a combination of reactions.     Aerobic digestion takes place in the presence of air; anaerobic digestion takes place in the absence of air.

Digital Readout

Use of numbers to indicate the value or measurement of a variable.     The readout of an instrument by a direct, numerical reading or the measured value.

Dihydrogen Oxide

Water (H2O).

Dilute Solution

A solution that has been made weaker, usually by the addition of water.


The act of adding more solvent or water to a given solution to make it less concentrated.     Sometimes this is done to attain the proper concentration; sometimes to make the solution easier to handle.


Lakes and reservoirs which freeze over and normally go through two stratification and two mixing cycles within a year.


Demand Initiated Regeneration.

Direct Current (DC)

Electrical current flowing in one direction only and essentially free from pulsation.

Direct Filtration

A filtration method of treating water which consists of the addition of coagulant chemicals, flash mixing, coagulation, minimal flocculation, and filtration.     The flocculation facilities may be omitted, but the physical/chemical reactions will occur to some extent. The sedimentation process is omitted.

Direct Runoff

Water that flows over the ground surface or through the ground directly into streams, rivers, or lakes.

Discharge Head

The pressure (in pounds per square inch or psi) measured at the center line of a pump discharge and very closely to the discharge flange, converted into feet.


Disinfect    --------------------------------------------------------------------------------    To free from infection by either a chemical or physical means; causing the absence of pathogenic or indicator coliform bacteria in drinking water.     Disinfectants kill or inactivate 99.9 to 99.9999 percent (but not 100%) of microorganisms under controlled conditions. Some common disinfectants are the halogens: chlorine, iodine, bromine, and hypochlorites; ozone, potassium permanganate, hydrogen peroxide, peracetic acid; formaldehyde, glutaraldehyde, phenol (carbolic acid), organic acids, benzoic and salicylic acids and their sodium salts, high pH, heat, ionizing radiation, and electromagnetic waves such as those of ultraviolet light.     The USEPA requires that a disinfection claim must show killing or inactivation of all vegetative microbes in 10 minutes.


The treatment of water to inactivate, destroy, and/or remove pathogenic (disease-producing) bacteria, viruses, cysts, and other microorganisms (but not completely eliminating all microorganisms) for the purpose of making the water microbiologically safe for human consumption.     A disinfection operation should reduce the total viable microorganism population by 99.9 to 99.9999 percent. Disinfection may involve the use of disinfecting chemicals such as chlorine, iodine, ozone, or hydrogen peroxide; or it may involve physical processes such as distillation, microfiltration, ultrafiltration, boiling, or ultraviolet radiation.

Disinfection Byproduct

A compound formed by the reaction of a disinfectant such as chlorine with organic material in the water supply.


A chemical (such as a polyphosphate) added to water which causes particulates in water to remain in suspension.

Dispersing Agent

A material that increases the stability of particles in a liquid.

Disposable Component

Any component of a piece of water treatment equipment or water treatment system which is manufactured to be disposed of instead of repaired or reused.     Example: A cartridge filter element.


The process by which a chemical combination breaks up into simpler constituents such as atoms, groups of atoms, ions, or multiple different molecules.     Often this breakdown is reversible, as in the case of ionization.

Dissolved Inorganic Carbon

1. The fraction of inorganic carbon (the carbonate, bicarbonate, and dissolved CO2) in water that passes through a 0.45 micron pore-diameter filter.

2. A calculated estimate of the concentrations of carbonate alkalinity present in water as carbon dioxide gas, (CO2 ), bicarbonate (HCO3- ), and carbonate (CO32-). The calculation is based on the pH and alkalinity measurements. The result is expressed as mg of carbon per liter, mg C/L

Dissolved Matter

That portion of matter or solids, exclusive of gases, which is dispersed in water to produce a homogenous liquid.     According to the definition used in the water treatment industry, "dissolved matter" is that portion of the total matter that will pass through a 0.45-micron pore-diameter membrane filter.

Dissolved Organic Carbon

The fraction of total organic carbon (all carbon atoms covalently bonded in organic molecules) in water that passes through a 0.45-micron pore-diameter filter.

Dissolved Oxygen

Measure of water quality indicating free oxygen dissolved in water.

Dissolved Solids

Dissolved Solids    --------------------------------------------------------------------------------    The weight of matter, including both organic and inorganic matter, in true solution in a stated volume of water.     The amount of dissolved solids is usually determined by filtering water through a 0.45 pore-diameter micron filter and weighing the filtrate residue left after the evaporation of the water at 180 degrees Celsius.


The product water or condensate, which is mineral-free and potable, from a distiller unit.


The process of separating the water from the organic and inorganic contaminants through a combination of evaporation (or vaporization), cooling, and condensation.

Distilled Water

Water which has been cleansed by passing through one or more evaporation-condensation cycles until it contains a very low amount of dissolved solids (usually less than 5.0 ppm TDS).


Distributor    --------------------------------------------------------------------------------    A fitting, usually installed at the top and bottom of the tank in a loose media system, which is designed to produce even flow through all sections of an ion exchanger or filter media bed and to function as a retainer of the media in the tank.     May also be called a diffuser.


[die-VAY-lent] Having a valence of two, such as the ferrous ion, Fe<sup>2+</sup>.     Also called <i>bivalent</i>.


1. Use of part of a stream flow as a water supply.     2. A structural conveyance (or ditch) constructed across a slope to intercept runoff flowing down a hillside, and diverting it to some convenient discharge point.

Divinylbenzene (DVB)

A polymerization monomer used as a cross-linking agent by polymerization with styrene in the manufacture of many synthetic ion exchange resin products.  The degree of DVB cross-linkage is a factor in exchanger resistance to chemical oxidation.  Standard cation resin usually contains about 8% DVB.  Macroporous resins contain over 12% DVB cross-linking.


Dissolved Organic Carbon.


[CaMg(CO3)2] A carbonate mineral of calcium and magnesium found in nature in extensive beds of compact limestone and marble that are rich in magnesium carbonate.

Domestic or Other Nondistribution System Plumbing Problem

A coliform contamination problem in a public water system with more than one service connection that is limited to the specific service connection from which the coliform positive sample was taken.


The quantity of a chemical administered to an organism.


The actual quantity of a chemical to which an organism is exposed.

Dose Equivalent

The product of the absorbed dose from ionizing radiation and such factors as account for differences in biological effectiveness due to the type of radiation and its distribution in the body as specified by the International Commission on Radiological Units and Measurements (ICRU).


A quantitative relationship between the dose of a chemical and an effect caused by the chemical.

Dose-Response Curve

A graphical presentation of the relationship between degree of exposure to a chemical (dose) and observed biological effect or response.

Dose-Response Evaluation

A component of risk assessment that describes the quantitative relationship between the amount of exposure to a substance and the extent of toxic injury or disease.

Dose-Response Relationship

The quantitative relationship between the amount of exposure to a substance and the extent of toxic injury produced.

Down Gradients

The direction that groundwater flows; similar in concept to downstream for surface water, such as down river.


The term used to designate the direction (downward) in which water or a regenerant flows through an ion exchange or filter media bed during any phase of the operating cycle.     This is the flow pattern found in conventional column operation: in at the top, out at the bottom of the column.     The pattern may also be called cocurrent flow in ion exchange systems.

Downflow Softening

The softening process in which raw water enters at the top of the softener bed column and passes downward through the cation resin and out the bottom.     In this process, the brining would also be in this same cocurrent direction.


A method of measuring the chlorine residual in water. The residual may be determined by either titrating or comparing a developed color with color standards.     DPD stands for N,N-diethyl-p-phenylene-diamine.


1. The act of drawing or removing water from a tank or reservoir.     2. The water which is drawn or removed from a tank or reservoir.


A pipe, conduit, or receptacle in a building which carries liquids by gravity to waste.     The term is sometimes limited to refer to disposal of liquids other than sewage.

Drain Line

A pipeline which is used to carry backwash water, regeneration wastes, and/or rinse water from a water treatment system to a drainage receptacle or waste system.


A technique to improve the productivity of some agricultural land by removing excess water from the soil; surface drainage is accomplished with open ditches; subsurface drainage uses porous conduits (drain tile) buried beneath the soil surface.

Drainage Basin

The area of land that drains water, sediment, and dissolved materials to a common outlet at some point along a stream channel.


1. The drop in the water table or level of water in the ground when water is being pumped from a well.     2. The amount of water used from a tank or reservoir.     3. The drop in the water level of a tank or reservoir.

Drift (cooling water treatment)

The loss of water that is entrained in the stack discharge.

Drilled Well

A well constructed by either cable tool or rotary methods which operates by cutting or abrasion; materials are brought to the surface by means of a hollow drill tool, a boiler, a sand pump, or by another hydraulic and/or self-cleaning method.

Drinking Water

1. A water, treated or untreated, which is intended for human use and consumption and considered to be free of harmful chemicals and disease-causing bacteria, cysts, viruses, or other microorganisms.     2. Safe water that has been further treated to enhance aesthetic quality and/or reduce mineral content by one or more point-of-use processing devices.

Drinking Water Standards

Standards that define allowable water quality limits for potable and domestic water supplies.     The U.S. Environmental Protection Agency (EPA) is responsible for the National Primary Drinking Water Regulations, which are health-related standards that establish the Maximum Contaminant Levels (MCLs) for regulated substances in drinking water.     An MCL is the highest permissible level of a contaminant allowed in water delivered to the consumer's tap. MCLs are enforceable at public water systems under the Safe Drinking Water Act.     The EPA also has set Maximum Contaminant Level Goals (MCLGs) at levels which no known or anticipated adverse effects on the health of persons occur and which allow an adequate margin of safety. The enforceable MCL is set as close to the MCLG as reasonable, taking into consideration the costs and treatment techniques available to public water systems.     National Secondary Drinking Water Regulations, also issued by the EPA, pertain to aesthetic characteristics of water and are advised, but not enforceable, by the Federal government.

Driven Well

A shallow, usually small well (having a diameter of 1.5 inches to 3 inches or 4 to 10 cm) constructed without the aid of any drilling, boring, or jetting device, by driving a series of connected pipe lengths into unconsolidated material to a water-bearing stratum.

Dry-Salt Saturator Tank

A brine tank, usually full of undissolved salt and with saturated brine below the undissolved salt.     This is the type of brine tank used with most automatically-regenerated home softeners because it reduces the frequency of refilling the tank with salt.

Dual Chamber

A water processing system composed of two separate tanks (or compartments) vertically connected, one above the other and operated by one common set of (master) valve controls.     Example: filter (top) and softening unit below (bottom).

Dual-Function Media

Any filter or ion exchange media which is used to perform two treatment steps in an application.     Example: activated carbon filtration and adsorption; or cation resin softening and dissolved iron removal.

Dug Well

A shallow, large diameter well constructed by excavating with power machinery or hand tools instead of drilling or driving.     Typically a dug well is constructed for an individual residential water supply and yields considerably less than 100 U.S. gallons per minute (380 L/min.)




Estimated exposure (in mg/L) which is interpreted to be protective for noncarcinogenic endpoints of toxicity over a lifetime of exposure.  DWEL was developed for chamicals that have a significant carcinogenic potential (Group B).  Provides risk manager with evaluation on noncancer endpoints, but infers that carcinogenicity should be considered the toxic effect of greatest concern.

Dynamic Pressure

When a pump is operating, the vertical distance (in feet) from a reference point (such as a pump center line) to the hydraulic grade line is the dynamic head.

Dynamic Reaction

An ion exchange reaction which takes place as the water moves past the exchange resin or resins.

Dynamic System

A process or system in which motion occurs as compared to static conditions with no motion.     For example: an ion exchange system is considered a dynamic system because the continuous flow of the water to be treated creates continuous motion, as opposed to a static batch system in which the water does not move during the reaction process.

Dynamic Water Pressure

The water pressure at the inlet to a dynamic water processing system.

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Updated on Mon, 22 Jun 2020 by Jonathan

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