Glossary of Water Treatment Terms & Definitions - F


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.

Ferric iron is sometimes known as red water iron because of its rusty red appearance when drawn. It is actually clear water iron, which has been oxidized, usually from dissolved oxygen or other factors in the water. This type of iron is not dissolved in the water but rather is suspended in solution

Ferric iron can also be formed from ferrous iron simply by letting it stand for a while. This sometimes can be witnessed in toilet bowls. Homeowners are sometimes surprised to find that upon returning from vacation, the water in their toilets has turned color. This somewhat unpleasant example shows a transformation from ferrous to ferric iron due the addition of oxygen from the air.

The same transformation can be witnessed at sinks or bathtubs with drippy faucets. Clear water drips from the faucets, but after remaining a while on the surface of the sink or tub the water turns red or yellow indicating the presence of iron. Once again, ferrous iron has changed to ferric iron by the addition of oxygen from the air.

Even though particles of ferric iron are suspended in the water, there is still a big problem when trying to remove them with simple filtration. The iron particles will rapidly clog filters causing low flow rates and making frequent filter changes necessary. Additionally, ferric iron is almost always accompanied by ferrous iron, which will simply slip through the filter causing staining when it is finally exposed to oxygen from the air.

The secret to removing ferric iron is to use a large capacity, automatically backwashing filter.


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.

Water that is clear when drawn but changes to a yellow or rusty color upon standing is known as ferrous or clear water iron. This iron has not yet been exposed to oxygen and therefore has not "rusted" or oxidized. This iron is totally dissolved in water. This clear iron can easily pass through standard home store sediment filters, thwarting the best efforts of homeowners, then later change to a staining color on the surface of sinks, toilets or showers where air oxidizes it.

No amount of sediment filters or even carbon filters can stop this type of iron. The clear iron simply passes right through the filter. After time, the water remaining in the filter does itself oxidize, clogging the filter, dramatically reducing the water flow rate, forcing the homeowner to continuously replace the filter inserts.

The secret to removing this type of iron is to get it to oxidize completely so it can be filtered effectively before it reaches the house.



A naturally occurring ore which serves as a catalytic filter media in the removal of iron, hydrogen sulfide and manganese. It normally requires only backwashing, but the use of oxidizers such as chlorine or potassium permanganate enhances its action.


A device used to clean water by removing iron, silt, taste, odor, color, etc., before it is fed into the softener or supply lines of the consumer. Includes mechanical, adsorptive, oxidizing and neutralizing filters. Available as media beds in tanks or as cartridge type devices


The tradename for aluminum silicate (pumicite) granular product used as a general purpose filter medium. Lighter in weight, it requires a lower backwash rate. Typically removed suspended solids down to the 20-40 micron range.


The process of passing water through a porous substance to remove solids in suspension. Available as media beds in tanks or as cartridge type devices


Smaller than the specified size or particles of ion exchange or filtration materials. An excess of fines can create undesirable pressure drop in the system.

Fixture Count

A count of the total number of plumbing fixtures in a building to estimate peak flow rates and the sizing of equipment, especially for commercial buildings.

Fixture Unit

An arbitrary unit assigned to different type of plumbing fixtures, and used to estimate flow rate and drain capacity requirements.

Flash Distillation

A distillation process in which hot water is introduced into a low pressure chamber causing some of the water to flash or quickly turn to steam.


Materials added to water which can cause gelatinous clouds of precipitate to enclose fine particles of foreign material in order to settle or filter them from the water.

Flow Controller

An in-line self pressure adjusting or orifice to regulate the flow of water or regenerant through a water conditioner.

Flow Rate

The volume of solution which passes through a given quantity of resin within a given time. Flow rate is usually expressed in terms of gallons per minute per cubic foot of resin, or as milliliters per minute per milliliter of resin. If the flow rate is greater than it should be, the water will not be completely softened or filtered.

Flush Valve (Flushometer)

A self closing valve used for flushing urinals and toilets. This type of valve allows flow rates of 15-20 gpm for up to 10 seconds.


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.


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



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