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Ferric Iron
Small solid iron particles containing trivalent iron, usually as gelatinous ferric hydroxide or ferric oxide (Fe₂O₃), which are suspended in water and visible as “rusty” water. Ferrous (iron in solution) is readily converted to ferric iron by exposure to oxygen found both in water and air. Ferric iron can by removed by filtration, but not by ion exchange.

Ferrous Iron
Usually ferrous hydroxide which when dissolved in water produces a clear solution. Often called clear water iron, it can be removed by ion exchange.

Filter
Specifically, a device or system for the removal of solid particles (suspended solids); in general, includes mechanical, adsorptive, oxidizing and neutralizing filters.

Flocculation
The agglomeration or gathering of finely divided suspended solids into larger, usually gelatinous, particles; the development of a “floc” after treatment with a coagulant by gentle stirring or mixing.

Flow Control
A device designed to limit the flow of water or regenerant to a predetermined value over a broad range of inlet water pressures.

Flow Rate
The quantity of water or regenerant which passes a given point in a specified unit of time, often expressed in gallons per minute.

Flux
In crossflow filtration, it is the product flow rate through a reverse osmosis, electrodialysis or ultrafiltration membrane. It is usually given in terms of volume unit per time per membrane area.

Fouling
The process in which undesirable foreign matter accumulates in a bed of filter media or ion exchanger, clogging pores and coating surfaces and thus inhibiting or retarding the proper operation of the bed.

Freeboard
The vertical distance between a bed of filter media or ion exchange material and the over flow or collector for backwash water; the height above the bed of granular media available for bed expansion during backwashing; may be expressed either as a linear distance or a percentage of bed depth.

Gallon
A common unit of liquid volume; the US gallon has a volume of 231 cubic inches or 3.78533 liters; the British (Imperial) gallon has a volume of 277.418 cubic inches or 4.54596 liters.

GAC
Granular activated carbon.

GPG
Abbreviation for “grain per gallon”.

Grain (gr)
A unit of weight equal to 1/7000th of a pound, or 0.0648 gram.

Grain Per Gallon (GPG)
A common basis for reporting water analyses in the United States and Canada; one grain per U.S. gallon equals 17.12 milligrams per liter (mg/L) or parts per million (ppm).

Gram (g)
The basic unit of weight (mass) of the metric system, originally intended to be the weight of 1 cubic centimeter of water at 4C.

Gravel Support Bed
A layer or layers of graded gravel and course sand placed around and above the underdrain metalwork of a water treatment system. It facilitates even distribution and collection of both product water and backwash flow.

Greensand
A natural mineral, primarily composed of complex silicates, which possess ion exchange properties.

Groundwater
The term describing all subsurface water and the source of well water. It can be found in aquifers as deep as several miles.

Halogens
A family of elements that includes bromine, chlorine, fluorine, astatine, and iodine. They are very active chemically. They are commonly found as the ionic component in compounds with various other elements.

Hardness
A characteristic of natural water due to the presence of dissolved calcium and magnesium. Hardness is usually expressed in grains per gallon, parts per million, or milligrams per liter, all as calcium carbonate equivalent.

Hard Water
Water with a total hardness of one grain per gallon or more, as calcium carbonate equivalent.

Heme Iron
Organically bound iron that can give water a pinkish cast. It is found only in groundwater supplies and cannot be removed by filtration. Like soluble iron, heme iron stains fixtures with a rust or orange coloring. It may draw clear and turn yellow or pink when exposed to oxygen.

Hexametaphosphate
A chemical, such as sodium hexametaphosphate, added to water to increase the solubility of certain ions and to inhibit precipitation of certain chemicals. Known as a sequestering agent, it forms a thin film that protects metals from corrosion.

Hydrogen Cycle
A complete course of cation exchange operation in which the cation medium is regenerated with acid and then all cations in the water are removed by exchange with hydrogen ions.

Hydrogen Sulfide
A corrosive and flammable gas produced from decaying organic matter, commonly known as “sulfur”.

Hydraulic
Referring to water or other fluids in motion.

Hydraulic Classification
A process in which particles of the same specific gravity may be graded according to size by backwashing or other relative upward flow of water, with the smallest particles tending to rise to the top, because of variations in weight to surface area ratios.

Hydrologic Cycle
The water cycle, including precipitation of water from the atmosphere as rain or snow, flow of water over or through the earth, and evaporation or transpiration to water vapor in the atmosphere.

Hydrophilic
Pertaining to a substance which readily absorbs water. (“water-loving”)

Hydrophobic
Pertaining to a substance that does not readily absorbs water. (“water-hating”)

Hydroxyl
The term used to describe the anionic hydroxide radical (OH-) that is responsible for the alkalinity of a solution.

Influent
The water entering a water treatment devise.

Inorganic Matter
Matter that is not derived from living organisms and contains no organically produced carbon; includes rocks, minerals and metals.

In-parallel Flow
A piping arrangement that directs separate streams through two or more water treatment units in a balanced manner, providing equal flow to each device. The inlets of two or more units are connected together and the outlets are connected together such that water will flow through the units simultaneously.

In-series Flow
A piping system in which all of the effluent flow of one unit in a water treatment system is fed to a second and succeeding unit. This arrangement achieves a greater reduction of contaminants than can be obtained by the passage through a single unit.

Iodine Number
A measure of the ability of activated carbon to adsorb substances with low molecular weights. It is the milligrams of iodine that can be adsorbed on one gram of activated carbon.

Ion
An atom, or group of atoms in a solution which function as a unit, and has a positive or negative electrical charge, due to the gain or loss of one or more electrons. It is smaller than a colloid.

Ion Exchange
A reversible process in which ions are released from an insoluble permanent material in exchange for other ions in a surrounding solution; the direction of the exchange depends upon the affinities of the ion exchanger for the ions present, and the concentrations of the ions in the solution.

Ion Exchanger
A permanent, insoluble material that contains ions that will exchange reversibly with other ions in a surrounding solution. Both cation and anion exchangers are used in water conditioning.

ionization
The process in which atoms gain or lose electrons and thus become ions with positive or negative charges; sometimes used as synonymous with dissociation, the separation of molecules into charged ions in solution.

Iron
An element often found dissolved in ground water (in the form of ferrous iron) in concentrations usually ranging from zero to 10 ppm (mg/L). It is objectionable in water supplies because of the staining caused after oxidation and precipitation (as ferric hydroxide).

Iron Bacteria
Organisms that are capable of utilizing ferrous iron, either from the water or from steel pipe, in their metabolism, and precipitating ferric hydroxide in their sheaths and gelatinous deposits. These organisms tend to collect in pipe lines and tanks during periods of low flow, and to break loose in slugs of turbid water to create staining, taste and odor problems.