A system made up of the community of living things (animals, plants, and microorganisms) which are interrelated to each other and the physical and chemical environment in which they live.


A circular movement or whirlpool occurring in flowing water due to currents created by various obstructions or dynamic forces in the water.


See Ethylene Diamine Tetraacetic Acid.


The process of bringing something out, sucking it out, or separating it out from something else, as in drawing soda pop out of a can with a straw.


A venturi with an opening at the throat which will educt (suck in) air or liquid.     Eduction is accomplished by joining the fluids just after the constriction in the tube where a vacuum is created, thus sucking the incoming fluid into the flowing water stream. This device is commonly used to introduce (through both eduction and ejection) a gas or liquid into a flowing stream.     For example, when feeding a chemical to a water supply, the venturi serves to educt the chemical from its solution container and to eject it into the water stream.     Also called an ejector.

Effective Corrosion Inhibitor Residual

A concentration of corrosion inhibitor sufficient to form a protective coating on the interior walls of a pipe, reducing its corrosion.

Effective Range

That portion of the design range (usually upper 90 percent) in which an instrument has acceptable accuracy.

Effective Size

A measure of the diameter of particles in a media bed or resin bed. Effective size is that mesh size which will permit 10 percent of the bed's particles to pass and will retain the remaining 90 percent; in other words, that size for which 10 percent of the media grains or particles are smaller and 90 percent are larger.


As relates to ion exchange, a measure of the effectiveness of the operational performance of an ion exchanger, usually based on the ratio of output per unit of input. This ratio is often expressed as the amount of regenerant required to produce a unit of contaminant reduction capacity. For example: pounds of salt per kilograins of hardness removed or pounds of acid per kilogram of salt removed. SEE A:LSO salt efficiency.     In media filtration, efficiency is the percent of contaminant reduction which occurs with a specified medium volume and specified water contact time.     In membrane filtration, the figure obtained (expressed as a percent) by dividing the volume (gallons or liters) of product water produced by the total volume (gallons or liters) of feedwater fed to the particular unit or system.


The outflow from any water processing system or device. Sometimes used to mean the product water of a given device or system.


The process of forcing something out, expelling it.


A device used to disperse a chemical solution into water being treated.

Ejector Pump

A shallow or deep well pump operating on the venturi principle.     Commonly referred to as a jet pump.

Electric Timer (Time Clock)

An electric control device which initiates regeneration of an automatic water softener or the recycle phase of a filter unit.

Electrical Conductivity

The property of a substance to conduct (carry) heat or electricity; the unit of measure is the siemens (formerly called mho), which is the reciprocal of resistivity (1 divided by resistivity).

Electrochemical Reaction

Chemical changes produced by electricity (electrolysis) or the production of electricity by chemical changes (galvanic action).     In corrosion, a chemical reaction is accompanied by the flow of electrons through a metallic path. The electron flow may come from an external force and cause the reaction, such as electrolysis caused by a DC (direct current) electric railway or the electron flow may be caused by a chemical reaction as in the galvanic action of a flashlight dry cell.

Electrochemical Series

A list of metals with the standard electrode potentials given in volts.     The size and sign of the electrode potential indicates how easily these elements will take on or give up electrons, or corrode. Hydrogen is conventionally assigned a value of zero.


A conductive, usually metallic, substance used to establish electrical contact with nonmetallic parts of a circuit.


A dialysis process using semipermeable membranes in which ions migrate through the membranes from a less concentrated to a more concentrated solution as a result of the ions' respective attractions to a positive electrode (anode) and a negative electrode (cathode) created by direct electric current.

Electrokinetic Potential

See Zeta Potential.


The decomposition of material by an outside elecrical current.


1.  A substance which when dissolved in water separates into two or more ions which can carry an electric current.   <p>  2.  A nonmetallic substance which can carry an electric current by movement of ions instead of electrons.

Electrolytic Cell

A device in which the chemical decomposition of material causes an electric current to flow. Also, a device in which a chemical reaction occurs as a result of the flow of electric current.     Chlorine and caustic (NaOH) are made from salt (NaCl) in electrolytic cells.

Electromotive Force (EMF)

The electrical pressure available to cause a flow of current (amperage) when an electrical circuit is closed.

Electromotive Series

A list of metals and alloys presented in the order of their tendency to corrode (or go into solution).     Also called the Galvanic Series.     This is a practical application of the theoretical electrochemical series.


A negatively-charged particle that revolves around the nucleus of an atom. In a neutral (without electrical charge) atom, the negative charges of the electrons are balanced by the positive charges of the protons in the nucleus. When an atom loses or gains one or more electrons, resulting in the atom carrying a negative or positive charge, the atom has become an ion.


A substance that consists of only one kind of atom and which cannot be decomposed into simpler substances by ordinary chemical means.     Examples: iron, calcium, carbon, magnesium. There are more than 100 elements, some found in nature, some man-made. Several elements may combine into more chemically complex substances such as compounds.


The process of separating or washing out adsorbed material, especially by use of a solvent.     In ion exchange, the stripping of ions from the medium by passing a more highly concentrated ionized solution through the ion exchanger bed.

Emission Spectroscopy

A chemical analytical technique used to determine metal elements in water by measuring the well-defined characteristic radiation given off by each respective element as the thermally-excited element returns from an atomic vapor state to its fundamental state.


An ingredient for making skin soft or supple, or soothing the skin.     Materials such as fatty acids and lanolin are included in some toilet bars and skin preparation products to provide emollient properties.


The dispersion or suspension of fine particles or globules of one or more liquids in another liquid.    The emulsification process is important in all types of cleaning where oily or fatty soils are encountered. The principal agent in emulsification is the surfactant, with aid from a builder that ties up hardness minerals.

End Bells

Devices used to hold the rotor and stator of a motor in position.

Endangerment Assessment

A site-specific risk assessment of the actual or potential danger to human health or welfare and the environment from the release of hazardous substances or waste.     The endangerment assessment document is prepared in support of enforcement actions under U.S. environmental laws such as CERCLA or RCRA.


Something peculiar to a particular people or locality, such as a disease which is always present in the population.


Infection of the heart valves.


A term used to describe a process or change in which heat is absorbed and that requires high temperature for the initiation and maintenance. For example, melting ice absorbs heat and is, therefore, an endothermic process.


A heat-resistant pyrogen (specifically a lipopolysaccharide) found in the cell walls of certain disease-producing bacteria.


The point at which a process or cycle is completed because a predetermined measurable value has been reached.     Example: When the hardness in the product water reaches a set endpoint showing excessive hardness leakage, the resin bed is considered to be exhausted and in need of regeneration; when a certain predetermined color endpoint is reached during chemical titration, the process is complete.


A pesticide toxic to freshwater and marine aquatic life that produces adverse health effects in domestic water supplies.

Energy Grade Line (EGL)

Energy Grade Line (EGL)    --------------------------------------------------------------------------------    A line that represents the elevation of energy head of water flowing in a pipe, conduit, or channel. The line is drawn above the hydraulic grade line (gradient) a distance equal to the velocity head of the water flowing at each section or point along the pipe or channel.


Of intestinal origin, especially applied to wastes or bacteria.


To trap bubbles in water either mechanically through turbulence or chemically through a reaction.


The process of fine droplets of liquid vapor being physically carried along or carried over by steam during distillation or evaporation. In boiler operation this is called carryover; in cooling water processes it is called drift.


The capacity of a system or a body to hold energy that is not available for changing the temperature of the system (or body) or for doing work.


A large class of complex proteinaceous molecules, which act as catalysts in biochemical reactions, and as produced by living cells can bring about digestion (breakdown) of organic molecules into smaller units that can be used by living cells.     Selected types of enzymes are useful in laundering, particularly in presoaking, where they break down certain soils and stains to simpler forms, which are then more readily and completely removed by the laundry soap or detergent. Reasons for their primary use in presoaking are: they need more than the usual wash period of 10-15 minutes to be effective, especially on stubborn stains and soil; also, their effectiveness is deactivated by liquid chlorine bleach, so the two must be used separately to obtain the full benefit of each.




Widespread outbreak of a disease, or a large number of cases of a disease, in a single community or relatively small area.     Disease may spread from person to person, and/or by the exposure of many persons to a single source, such as a water supply.

Epidemiologic Study

Study of human populations to identify causes of disease. Such studies often compare the health status of a group of persons who have been exposed to a suspect agent with that of a comparable non-exposed group.


A branch of medicine which studies epidemics (diseases which affect significant number of people during the same time period in the same locality).     The objective of epidemiology is to determine the factors that cause epidemic diseases and how to prevent them.


The topmost and warmest layer of water in an unfrozen lake. The epilimnion also contains the most oxygen of any part of the lake.


A means of providing more uniform flow rate and composition of a water supply by use of a reservoir that receives water from a pump or treatment system, evens out the incoming flow variation, and permits temporary water withdrawal in excess of the pump or treatment system capacity.


In chemistry, a condition in which reversible chemical reactions are taking place at the same time in such a manner (equivalent rate) that there is no change in the net concentration of the substances involved in the chemical processes. As a result of these simultaneous reactions, the substances involved in the reactions can be shown to be in a constant ratio to each other.     In physics, the state in which the action of multiple forces produces a steady balance or seeming lack of change. This may be due to a true stop in action or to continuing actions which neutralize each other, resulting in no net change.

Equilibrium Shift

A change in the relative concentrations of reacting substances such that a different reaction or reaction rate is caused.     For example, a change in the relative concentrations of sodium and calcium ions will dictate both the exchange rate and the selection of which ions will be adsorbed to and released from the ion exchange resin beads.

Equivalent Per Million

A unit of concentration used in chemical calculations and calculated by dividing the concentration in ppm (or mg/L) by the equivalent weight.

Equivalent Weight

<p>The weight of a material (an element, compound, or ion) that yields (or per) one ionic valence charge.  Equivalent weight is computed by dividing the atomic or molecular weight by the valence or number of reactive protons of the substance.</p>    <p>For an element, the equivalent weight is the atomic weight divided by the valence or number of reactive protons.  For example, calcium (Ca) has an atomic weight of 40 and has two reactive protons (valence +2), so its equivalent weight is 40/2 = 20.  For a compound, the equivalent weight is the molecular weight divided by the number of reactive protons (or positive valence in the compound).  For example, sodium chloride (NaCl) has a molecular weight of 58.5 and one reactive proton (positive valence +1), so the equivalent weight is 58.5/1 = 58.5.  Calcium carbonate (CaCO<sub>3</sub>) has a molecular weight of 100 and two reactive protons (positive valence of +2 in the Ca<sup>2+</sup> ion), so the equivalent weight is 100/2 = 50.</p>    <p>Equivalent weight is also called combining weight.  Elements, compounds, and ions entering into combinations always do so in quantities proportional to their equivalent weights.</p>


The process in which a material is worn away by a stream of liquid (water) or air, often due to the presence of abrasive particles in the stream.     Erosion is a physical or mechanical wearing away process rather than a chemical or electrochemical wearing away process (corrosion).


A compound formed by the reaction between an acid and an alcohol with the elimination of a molecule of water.


The deterioration by chemical change on the surface of glassware caused by the action of high temperatures and detergents, and that is more prevalent or intensified in soft or softened water supplies.     Very high water temperatures in automatic dishwashers can cause detergent phosphate compounds to change into even more aggressive forms. If enough dish soil or water hardness is available, it will react with the most aggressive of these sequestering phosphates. Otherwise, however, the excessive detergent agents can actually extract elements directly from the glassware composition.     In early stages, incipient etching appears as a rainbow-colored film similar to an oil-on-water film. As etching progresses, this changes to opaqueness, which appears similar to filming except that it cannot be removed or repaired since etching is an actual eating away of the glass. It is sometimes called "soft water filming."     The solution to chemical etching is to use less detergent, water temperatures below 140 degrees F, and sufficient amounts of water during the rinse cycle. (Poor rinsing can also be caused by overloading the dishwasher.)     Mechanical etching can occur when two glasses rub against each other in the dishwasher.


See Ethyl Alcohol

Ethyl Alcohol

The most common variety of alcohol; also called grain alcohol and ethanol. Ethyl alcohol has good solvent and antifreeze properties and is soluble at all concentrations in water.


The cause of a disease.


Reservoirs and lakes which are rich in nutrients and very productive in terms of aquatic animal and plant life.


The increase in the nutrient levels of a lake or other body of water; this usually causes an increase in the growth of aquatic animal and plant life.

Evaporation Chamber

That part of a distillation system in which water is changed into vapor.


A mineral precipitated as a result of evaporation, such as the solids left behind in the distillation process.


The combined processes of evaporation and transpiration. It can be defined as the sum of water used by vegetation and water lost by evaporation.

Exchange Capacity

See Rated Capacity

Exchange Sites

Locations on each bead of ion exchange resin which hold mobile ions that are available for exchange with other ions in the solution that passes through the resin bed.     In cation water softening, for example, mobile sodium ions located at the various exchange sites are exchanged for calcium, magnesium, iron, or other polyvalvent cations in the water being softened.     Exchange sites are also called functional groups.

Exchange Tanks

See Portable Exchange Tanks

Exchange Velocity

The rate at which one ion is displaced in favor of another in an ion exchanger.


See Ion Exchanger

Exclusion Chromatography

See High Performance Liquid Chromatography


A state with primary enforcement responsibility under the U.S. Safe Drinking Water Act may relieve a public water system from a requirement respecting an MCL, treatment technique, or both, by granting an exemption if certain conditions exist.     These are:     1.  The system cannot comply with an MCL or treatment technique due to compelling factors which may include economic factors;   2.  The system was in operation on the effective date of the MCL or treatment technique requirement;  3.  The exemption will not result in an unreasonable public health risk.


The state of an ion exchanger or other adsorbent that is no longer capable of useful ion exchange due to the depletion of the initial supply of available exchangeable ions.     A unit that is "exhausted" requires regeneration to restore its capacity to treat water.

Exhaustion Wave Front

The boundary between the absence and presence of a contaminant as it passes through a media bed.


A term used to describe a chemical process in which heat is released. For example, combustion is an exothermic process because heat is released.


Contact with a chemical or physical agent.

Exposure Assessment

The determination or estimation (qualitative or quantitative) of the magnitude, frequency, duration, route, and extent (number of people) of exposure to a chemical.

Exposure Coefficient

Term which combines information on the frequency, mode, and magnitude of contact with contaminated medium to yield a quantitative value of the amount of contaminated medium contacted per day.

Exposure Level

The amount (concentration) of a chemical at the absorptive surfaces of an organism.

Exposure Scenario

A set of conditioners or assumptions about sources, exposure pathways, concentrations of toxic chemicals and populations (numbers, characteristics, and habits) which aid the investigator in evaluating and quantifying exposure in a given situation.

External Water Treatment

A term used in boiler water treatment referring to the "outside" (that is, not inside the boiler) preparation of the source water to be used for boiler feedwater or boiler makeup water.     This preparation may include such steps as cation exchange softening, pH modification, or dealkalization.


Estimation of unknown values by extending or projecting from known value.

Glad we could be helpful. Thanks for the feedback.

Sorry we couldn't be helpful. Your feedback will help us improve this article.

How helpful was this page?

Updated on Mon, 22 Jun 2020 by Jonathan

In this section