The chemical formula for water (dihydrogen oxide).


The time required for half of the substance present at the beginning to dissipate or disintegrate.


A geological term for rock salt, a mineral which is more than 95 percent sodium chloride (NaCl). Also called native salt or fossil salt.


A family group of elements, including bromine, chlorine, fluorine, astatine, and iodine, which are extremely active chemically. These elements exist in the free state normally as diatomic     molecules, but more commonly are found as the ionic component in compounds with various other elements.


Thriving in a salt environment.

Halophilic Bacteria

Salt-tolerant bacteria often found in solar salt which has not been fully kiln dried or in salt which may have been exposed to unsanitary conditions over a long period of time.

Hard Water

Water containing a total mineral hardness (typically from calcium and magnesium ions) of one grain per U.S. gallon (1gpg = 17.1 ppm) or more measured as calcium carbonate equivalent. SEE ALSO hardness.


Hardness is a measurable property of water brought about by the presence of calcium and magnesium ions (and other multivalent metallic ions) that can cause scale formation, soap curd, water spotting, and harshness in fabrics laundered in such water. The term was colloquially used to describe certain waters that caused difficulty in using soap; i.e. “this water makes soap hard to lather”.

Hardness is expressed in grains per US gallon (gpg) where 1 grain equals 17.1 ppm as calcium carbonate equivalent. The degree of hardness expressed in gpg that is generally accepted:


Less than 1 gpg is termed as soft water

1.0 to 3.5 gpg is slightly hard

3.5 to 7.0 gpg is moderately hard

7.0 to 10.5 gpg is termed as hard

Above 10.5 gpg is very hard.

While many divalent and trivalent metallic cations can potentially contribute to “hard water” symptoms, the most commonly referenced “hardness” contaminants are calcium and magnesium ions in water, expressed as calcium carbonate (CaCO3).

In practice, when sizing a water conditioner for the removal of hardness in residential applications, it is common to count only the calcium and magnesium content in calculating the hardness load. “Total” hardness would include iron, manganese, strontium, barium and radium plus the common hardness. In certain applications requiring very low hardness residuals, if the total hardness exceeds the common hardness by more than 1 percent, specific design treatment or pretreatment and/or regeneration schemes are employed to prevent fouling of the system as a result of the marginal solubility of the heavier elements.


SEE ALSO carbonate hardness; noncarbonate hardness; total hardness.

Hardness as Calcium Carbonate

The value obtained when the hardness-forming salts are calculated in terms of equivalent quantities of calcium carbonate.     This method of water analysis provides a common basis for comparison of different salts and compounds.

Hazard Evaluation

A component of risk assessment that involves gathering and evaluating data on the types of health injury or disease (e.g., cancer) that may be produced by a chemical and on the conditions of exposure under which injury or disease is produced.


The pressure at any given point in a water system, generally expressed in pounds per square inch (psi).     May also be called pressure head or velocity head. Head is calculated as the pressure exerted by a hypothetical column of water standing at the height to which the free surface of water would rise above any point in a hydraulic system.     For this reason, head is sometimes expressed as the height of a column of water which would produce a given pressure; this measurement may be called hydrostatic head.     The pressure head in feet of water is equal to the pressure in psi times 2.31 ft/psi.

Head Loss

The reduction of water pressure (head), measured in psi, in a hydraulic or plumbing system.     Head loss is a measure of the resistance of the medium bed (or other water treatment system) and/or plumbing system to the flow of the water through it. In water treatment technology, head loss is basically the same as pressure drop.


A large central pipe with two or more side outlets which is located at the bottom of a vessel containing water that has passed through a filter bed or ion exchange media bed.     One purpose of the header is to collect the processed product water, but the header may also be used to distribute backwash water and regenerants across the bottom of the bed area.

Health Contaminant

Any substance or condition that may have any adverse effect on human health.     Health contaminants in water are regulated as part of the National Primary Drinking Water Regulations enforced by the U.S. Environmental Protection Agency and most states.

Health Effects Concern

Exposure to contaminant at a toxicologically significant level as defined by the maximum contaminant levels (MCLs) for regulated contaminants or action for nonregulated contaminants.

Heat Exchanger

An apparatus used to transfer heat from one medium to another or used to remove heat from or add heat to a fluid.

Heat of Vaporization

The amount of additional (latent) heat needed to change a given amount of liquid existing at its boiling point temperature into a vapor.

Heat Sensor

A device that opens and closes a switch in response to changes in the temperature. This device might be a metal contact, or a thermocouple which generates a minute electrical current proportional to the difference in heat, or a variable resistor whose value changes in response to changes in temperature. Also called a temperature sensor.

Heavy Metals

Metallic elements with high atomic weights, e.g., mercury, chromium, cadmium, arsenic, and lead. They can damage living things at low concentrations and tend to accumulate in the food chain.


A measure of area in the metric system similar to an acre.     One hectare is equal to 10,000 square meters and 2.4711 acres.


In ion exchange applications, the lower zone of the ion exchange bed that is "passed by" in either the softening, deionization, or dealkalization mode or during the application of regenerants. This condition is usually due to the configuration of the vessel or the lack of a good underdrain distribution system.


The production of blood and blood cells: hemopoiesis.


A deep red pigment (C34H32N4O4Fe) which contains reduced (ferrous) iron.     Heme is found in red blood cells (hemoglobin). It is also found outside the body in the nonprotein portions of some organic molecules called hemoproteins. In water quality treatment, it may be referred to as heme iron, which is organically-bound iron that can cause water to have a pinkish cast to it.


The process of purifying a kidney patient's blood by means of a dialysis membrane.

Hemodialysis Grade Water

Water which meets the requirements set forth by the American National Standards for Hemodialysis Systems and covered in the Association for the Advancement of Medical Instrumentation (AAMI) standards.


The rupturing of red blood cells which sometimes occurs during hemodialysis.    Hemolysis may be caused by the presence of chloramines in dialysis water.

Henrys Law

A law of chemistry that states that the weight of a gas dissolved (at a given temperature) in a liquid is proportional to the pressure of the gas above the liquid.


Pertaining to the liver.


Hepatitis is an inflammation of the liver usually caused by an acute viral infection. Yellow jaundice is one symptom of hepatitis.


A malignant tumor occurring in the liver.


A compound, usually a man-made organic chemical, used to kill or control plant growth.

Hertz (Hz)

The number of complete electromagnetic cycles or waves in one second of an electrical or electronic circuit. Also called the frequency of the current. Abbreviated Hz.


A type of organic compound in which the characteristic chemical groups are linked in a closed ring structure and in which one or more atoms in the ring is an element other than carbon, e.g., silica, sulfur, nitrogen, etc.


heteros, other + trophe, nourishment.     Obtaining food or nourishment from other organisms.     Heterotrophic organisms or heterotrophs are consumers such as man or decomposers such as bacteria that obtain their nourishment and energy from the organic molecules manufactured by the autotrophic organisms (the producers). All animals, for example, are consumers and heterotrophs that depend on complex organic molecules produced by autotrophs for their food and energy.

Heterotrophic Microorganisms

Bacteria and other microorganisms that use organic matter synthesized by other organisms for energy and growth.

Heterotrophic Plate Count (HPC)

A procedure for estimating the total number of live nonphotosynthetic bacteria in water.     Colony-forming units (CFU) are counted after spreading an aliquot portion of a sample over a membrane or pour plate and incubating in an amiable growth medium (agar) and at an amiable temperature.     (These generally are not considered to be disease-causing bacteria.)


A chemical that is used as a sequestering agent.  Normally, a metal (M) hexametaphosphate would have the chemical formula (MPO3)6, in which the cyclic metal metaphosphate is based on rings of alternating phosphorus and oxygen atoms.  The so-called sodium hexametaphosphate, however, is probably a polymer with the formula (NaPO3)n, in which n is between 10 and 20.  Hexametaphosphate is added to water to increase the solubility of certain ions and to deter precipitation of certain chemicals.

High Frequency Ozonation

Operation of an ozone generator at frequencies equal to or greater than 1,000 cycles per second or 1,000 hertz.

High Line Jumpers

Pipes or hoses connected to fire hydrants and laid on top of the ground to provide emergency water service for an isolated portion of a municipal distribution system.

High Performance Liquid Chromatography (HPLC)

A general term applied to modern instrumental techniques adopted to greatly increase the scope and precision of the liquid chromatography analytical method.     Among the varied HPLC techniques are:     Reversed phase chromatography, which uses a polar liquid phase for elution of a column containing a nonpolar phase. This technique is used, for example, to analyze the polynuclear aromatic hydrocarbon category (pyrenes, naphthalenes, etc.) of synthetic organic compounds;   Ion exchange chromatography, which makes it possible to separate large numbers of cations and anions;   Exclusion chromatography, which separates, on a porous gel, compounds according to their size and makes it possible to determine their apparent molecular weights and to separate fractions of different molecular weights for further analyses.

High Salting

The use of 15 pounds or more of salt (NaCl or KCl) to regenerate each cubic foot of cation resin.     High salting is generally recommended for high total hardness water and water containing high concentrations of dissolved heavy metals.

High Voltage Electrode

As relates to ozonation, the outlet post on the voltage transformer which produces more than 1,000 volts.

High-Frequency Ozonation

Operation of an ozone generator at frequencies equal to or greater than 1,000 cycles per second or 1,000 hertz.

High-to-Low Dose Extrapolation

The process of prediction of low exposure risks to rodents from the measured high exposure-high risk data.


The study of the structure of cells and tissues; usually involves microscopic examination of tissue slices.


Having the same composition throughout.     Homogenized milk is homogeneous because the fat globules have been broken into smaller colloid-sized particles that remain distributed throughout the liquid milk. In nonhomogenized milk, the fat globules rise to the top of the container as cream.

Hose Bib

Faucet.     A location in a water line where a hose is connected.

Hot Lime-Soda Softening

A method of partially softening water by adding lime and soda ash to chemically precipitate the calcium, magnesium, iron, and silica at a water temperature of about 212 degrees Fahrenheit. This process also drives off carbon dioxide.

Hot Process Softening

A term used to encompass several softening/clarifying processes using lime, lime and soda ash, or lime and cation softening to treat water which is at or near the boiling point.     Hot process softening can remove carbon dioxide, silica, and precipitated magnesium and is used mainly for boiler feedwater preparation and sulfur mining.

Hot-Lime Softening

A partial softening method which requires adding a lime slurry to water which is at about 212 degrees Fahrenheit and then chemically precipitating and removing the calcium and magnesium hardness via sedimentation and filtering.


Heterotrophic Plate Count


High Performance Liquid Chromatography (same as high pressure liquid chromatography)


(pronounce as separate letters):  High Test Hypochlorite. Calcium hypochlorite or Ca(OCl)<sub>2</sub>. "HTH" is a trademark (Olin) for a high-test calcium hypochlorite product containing 70 percent available chlorine that is commercially available in both water-soluble granular and tablet form.

Human Equivalent Dose

A dose which, when administered to humans, produces an effect equal to that produced by a dose in animals.

Human Exposure Evaluation

A component of risk assessment that involves describing the nature and size of the population exposed to a substance and the magnitude and duration of their exposure.     The evaluation could concern past exposures, current exposures, or anticipated exposures.

Human Health Risk

The likelihood (or probability) that a given exposure or series of exposures may have or will damage the health of individuals experiencing the exposures.

Humic Acid

Humic substances that are soluble in strong base solutions but insoluble in acidified (to pH < 2) water, and that affect water quality through exchange of species, such as cations or organic materials.

Humic Substances

The organic portion of soil that remains after prolonged microbial decomposition, and that is formed by the decay of leaves, wood, and other vegetable matter. Humic substances can impart a yellowish-brown to brownish-black color to water; detectable to 0.1 ppm in water.     Humic substances are commonly classified on the basis of solubility.     If a material containing humic substances or humus is extracted with a strong base and the resulting solution is then acidified, the products are a) a nonextractable plant residue called humin, b) a material called humic acid that precipitates from the acidified (pH < 2) solution, and c) an organic material called fulvic acid that remains dissolved in the acidified solution.     The high molecular weight and polyelectrolytic humic substance macromolecules range from a molecular weight of a few hundred for fulvic acid to tens of thousands for the humic acid and humin fractions.     Humic substances form suspected-carcinogenic trihalomethanes (THMs, such as chloroform, dibromochloromethane, bromodichloromethane, and bromoform) by reaction with chlorine. Humic substances are excellent chelating agents that bind with and hold metal ions in water, and they also effectively exchange cations with water.


The process of increasing the water vapor or moisture content.


Humic substances that remain insoluble in both strong base solutions and in water.     Humins can effectively exchange cations with water and may accumulate quantities of metals. Lignite coal, for example, is largely a humin and humic-acid material, which, through ion exchange, tends to remove some metal ions from water.


A substance formed by combining water with a compound.

Hydrated Lime

Ca(OH)2 A strong alkali chemical, calcium hydroxide, obtained by treating (slaking) lime with water in a heat-producing reaction until the calcium oxide has been converted to calcium hydroxide.     Hydrated lime is used in lime softening water treatment. Also called calcium hydrate; calcium hydroxide; caustic lime; slaked lime.


The chemical combination of water into another substance.


Referring to water or other fluids in motion.

Hydraulic Classification

The rearrangement, during backwashing, of ion exchange or other media particles according to size.     As the result of classification, the smallest particles tend to rise to the top of the bed while the largest tend to sink to the bottom because of weight or surface area ratio.

Hydraulic Conductivity

The capacity of rock or soil formations to transmit water to a pumping well. Capacity is related to the amount and size of interconnecting pore spaces in the rock.

Hydraulic Grade Line

The surface or profile of water flowing in an open channel or a pipe flowing partially full. If a pipe is under pressure, the hydraulic grade line is at the level water would rise to in a small vertical tube connected to the pipe.

Hydraulic Gradient

The slope of the hydraulic grade line.     This is the slope of the water surface in an open channel, the slope of the water surface of the groundwater table, or the slope of the water pressure for pipes under pressure.

Hydraulic Staging

Multiple passes of water between electrodes used in an electrodialysis or through a sequence of subsequent membranes or filters used in a reverse osmosis or filtration system to achieve further treatment.


A liquid compound (H2NNH2) used as a strong reducing agent for transition metals and as an oxidation inhibitor for boiler feedwater and cooling water.


An organic compound containing only carbon and hydrogen.     Hydrocarbons often occur in petroleum products, natural gas, and coals.

Hydrochloric Acid (HCl)

A water-based solution of hydrogen chloride which is a strong, highly corrosive acid. HCl may be used as a regenerant for cation resin deionization systems operated in the hydrogen (H<sup>+</sup>) cycle.    HCl may be used as a regenerant for cation resin deionization systems operated in the hydrogen (H+) cycle.


The basic form of most separators which act on the principle of centrifugal force and are used to remove sand and abrasives from well water.

Hydrogen Bond

The weak attraction between a hydrogen atom carrying a partial positive charge and some other atom with a partial negative charge.     Hydrogen bonds occur in polar compounds such as water by the attraction of a hydrogen atom of one molecule to two unshared electrons of another molecule.     Hydrogen bonds are less than one-tenth as strong as covalent bonds where electrons are actually shared by a pair of atoms, but they significantly affect properties such as the melting point, boiling point, and crystalline structure of substances.

Hydrogen Cycle

A cation exchange cycle (H+ form) in which the cation medium is regenerated with acid and all cations in the water are removed by exchange with hydrogen ions.

Hydrogen Ion Concentration

The concentration of hydrogen ions, reported in moles per liter of solution, used as a measure of the acidity of the solution. Also known as pH.

Hydrogen Peroxide

H2O2 A strong disinfectant and oxidizing agent used mostly in dilute water-based solutions.     Hydrogen peroxide can be formed in water with a 1948-angstrom mercury-vapor ultraviolet lamp.     Hydrogen peroxide may be used in advanced oxidation processes in combination with ozone to encourage the formation of highly reactive hydroxyl radicals; this process is then called the Peroxone process. Also called peroxide.

Hydrogen Sulfide (H2S)

A corrosive and flammable gas often found dissolved in well water and often accompanied by iron and low pH values.   Hydrogen sulfide develops from decaying organic matter, from sulfate-reducing bacteria, and from petroleum refining. H2S formation can be catalyzed by a magnesium anode rod in a hot water heater; anaerobic sulfate-reducing bacteria can live in a hot water heater so long as water temperatures are below 140° Fahrenheit, and to produce H<sub>2</sub>S the SRB need only a ready source of sulfate ions in the incoming water supply and a ready source of electrons as provided by the water heater's anode rod.    The odor of water with as little as 0.5 milligrams per liter (mg/L) of hydrogen sulfide concentration is detectable by most people.  Concentrations less than one mg/L give the water a "musty" or "swampy" odor.  Over one mg/L hydrogen sulfide concentration gives water a very disagreeable "rotten egg" odor and makes the water corrosive to plumbing.  Generally, hydrogen sulfide levels are less than 10 mg/L, but occasionally amounts of 50 to 75 mg/L are found.  At higher pH levels hydrogen sulfides present in the ionized alkaline sulfide (HS<sub>-</sub> and S<sup>2-</sup>) forms, and at low pH levels it is present in the H<sub>2</sub>S gaseous form.  At pH 5.0 about 90 percent is present as gaseous H<sub>2</sub>S and 10 percent as sulfide.  At pH 8 only six percent is present as a gas.    Hydrogen sulfide concentrations up to about six mg/L can be removed from water using an oxidizing filter (same as an iron filter).  Hydrogen sulfide concentrations exceeding six mg/L can be removed by injecting an oxidizing chemical such as household bleach or potassium permanganate and using a filter.  The oxidizing chemical should be thoroughly mixed into the water upstream from a retention tank to provide at least 20 minutes of contact time between the chemical and water.  Sulfur particles can then be removed by activated carbon filtration.  When potassium permanganate is used a manganese greensand filter is recommended.  Other oxidants such as ozone and hydrogen peroxide are also excellent for oxidizing hydrogen sulfide to precipitate sulfur and even to innocuous sulfate.

Hydrogeologic Conditions

Conditions stemming from the interaction of groundwater and the surrounding soil and rock.


A person who studies and works with groundwater.


The geology of groundwater, with particular emphasis on the chemistry and movement of water.


A graph of the rate of runoff plotted against time for a point on a channel.

Hydrologic Cycle

The complete circuit pursued by water in nature, including 1. falling of precipitation (rain, hail, sleet, snow, dew); 2. the journal of fallen water over and through the earth's surface formations; and 3. eventual evaporation of the water and its return to the atmosphere to again fall as precipitation.


The study of the occurrence, distribution, and circulation of the natural waters of the earth.


The product of hydrolysis.


A chemical reaction in which a substance reacts with water and becomes a different substance. This involves the ionization of the water molecule as well as splitting of the compound hydrolyzed.     An example is the chemical reaction of salt with water which then forms an acid and a base: (NaCl + H2O hydrolyzes to HCl + NaOH).


The treatment of ores by wet processes as in leaching and accompanying operations, and the technology of separation or recovery of heavy or noble metals from liquid solutions by ion exchange methods.


A device for measuring the density or specific gravity of liquids.     Hydrometers commonly consist of a thin glass or metal tube graduated to indicate either specific gravities or percentages of solution constituents and weighted so that they float upright.


Having a strong affinity (liking) for water, and thereby exhibiting the characteristic of absorbing water.     Example: Cotton is a hydrophilic fiber. The opposite of hydrophobic.


Having a strong aversion (dislike) for water, and thereby exhibiting the characteristic of repelling water.     Example: Nylon is a hydrophobic fiber. The opposite of hydrophilic.

Hydropneumatic System

A system that uses both air and water in its operation.     An example is the pressure tank which uses an air chamber to maintain pressure in a well system even when the pump is not operating.


The water of the earth, including surface lakes, streams and oceans, underground water, and water in the atmosphere.

Hydrostatic Pressure

The pressure at a specific elevation exerted by a body of water at rest or,  In the case of groundwater, the pressure at a specific elevation due to the weight of water at higher levels in the same zone of saturation.

Hydrostatic Test

A pressure test procedure in which a vessel or system is filled with water, purged of air, sealed, subjected to water pressure, and then observed and/or tested for leaks, distortions, and/or mechanical failure.


The negatively charged ion composed of an oxygen and a hydrogen atom (OH-).

Hydroxide Alkalinity

Alkalinity caused by hydroxide ions. SEE ALSO alkalinity.


1. A diatomic functional group in organic molecules, consisting of an atom of hydrogen and an atom of oxygen and indicating an alcohol. The hydroxyl group is covalently bonded to the organic molecule.

2. A highly reactive, diatomic radical consisting of a single atom of hydrogen and a single atom of oxygen, not to be confused with the negatively charged hydroxide anion.


Having the characteristic of drawing moisture in from the atmosphere such as silica gel, calcium chloride, or zinc chloride. Sodium chloride (NaCl) is also a hygroscopic substance.


A water treatment process in which desalination of water is achieved by forcing salt solutions, under pressure, through a membrane which generally passes water more readily than salts. An early term for reverse osmosis technology.


The application of hypochlorite compounds to water for the purpose of disinfection.


Chlorine pumps, chemical feed pumps, or devices used to dispense chlorine solutions made from hypochlorites such as bleach (sodium hypochlorite) or calcium hypochlorite into the water being treated.


The Ocl<sup>-</sup> anion, which is the dominant reaction product of chlorine in water at pH greater than 8.     Calcium and sodium hypochlorites are commonly used as disinfecting and bleaching agents.     High-test hypochlorite is a dry solid, largely calcium hypochlorite, which has excellent stability when kept in dry storage and is used as a disinfectant.

Hypochlorous Acid (HOCl)

The dominant reaction product of chlorine in water at pH less than 7.


The cold stagnant deep-water layer of a lake.

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Updated on Mon, 22 Jun 2020 by Jonathan

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