Iron Removal

Iron Removal
The type of iron in the water will need to be determined first from tests before the water can be treated as there are five main types of iron in water and they are treated differently. As little as 0.3 ppm of iron in the water is all that is required to cause staining and the clogging up of appliances.

The different types of iron in water are as follows:

Oxidized iron

Soluble iron

Colloidal iron

Bacteria iron

Organic-bound iron

Oxidized iron
This form of iron appears as red particles in fresh water drawn from surface water bodies such as dams. The most efficient way of removing oxidized iron is with a fine mechanical filter such as Diatomaceous earth, a naturally occurring, soft, chalk-like sedimentary material that has formed from hard algal-like diatoms and consists mostly of silica (86%) with smaller amounts of sodium, magnesium and iron. Diatomaceous earth is very light due to its high porosity and particle sizes are typically 2-10 microns. This type of filter material is most commonly used by brewers to remove yeast from beer but is also used in swimming pool filters and aquarium filters among many other uses – such as in the production of dynamite. Membrane filters are generally not used for oxidized iron removal as the filter clogs to readily. Chemical dosing of a coagulant can also be used successfully with this type of iron to settle out the material after a sufficient amount of contact time to allow for coagulation to occur with the settled sludge then sent to waste.

Soluble iron
This is the most common form of iron found in underground water supplies, especially boreholes and well points that penetrate a ferricrete layer. Ferricrete (or “koffieklip”) consists of sand and gravel that has been cemented into a hard mineral conglomerate by iron oxide derived from the oxidation of percolating solutions of iron salts. On exposure to oxygen in the air the iron oxidizes (or rusts) and precipitates out of the water, causing staining. Household walls that turn yellow, or in extreme cases an orange brown colour, from garden sprinkler systems indicate that iron exists in the soluble form in the irrigation water as the sprinklers providing ideal aeration of the water. Sometimes the iron in this form is present at such high concentrations that plants and grass leaves get blinded by the oxidized layers of iron and wilt. The best and least expensive way to remove sufficient quantities of this type of iron for irrigation purposes is to replicate what the garden sprinkler is achieving by aerating the water in a fine sprinkler device such as a “pigs tail” nozzle inside a large storage tank linked to a second large storage tank. The tanks need to be sized to allow at least 24 hours of settling time before the water is drawn off and the outflow from the first tank need to be separated from the down-flow of the water being aerated by means of a down-draught device. The settled iron sludge will need to be drawn off every 6 months to a year or so. If the water is intended for domestic consumption or industrial use it may also be necessary to oxidize the water further with chlorine using a dose feeder and then removing the particles formed with a membrane filter followed by a granulated activated carbon filter for removing the chlorine. Instead of dosing chlorine it is also possible to make use of a water conditioner, such as a softener, or a combination of a softener and a filter for removing additional amounts of soluble iron. In this case the softener would use a resin that recharges with chlorine or potassium permanganate. Sometimes specific chemicals that coat the iron in the water and prevent it from reaching oxygen and oxidizing are also used.

Colloidal iron
This type of iron consists of very small particles of oxidized iron suspended in the water and it is usually bound together with other substances. This iron looks more like simple colour in the water than particles as they are so small and the particles typically resist agglomeration, i.e., the combining together of like substances forming larger, heavier, more filterable ones, due to the static electrical charges that they carry. Removing this type of iron is usually achieved in one of two ways: dosing chlorine into the water by means of a feeder to oxidize the organic material off from the iron and allowing agglomeration to occur, or by dosing polymers that attract the static charges on the particles so that larger clumps of material are formed that can then be filtered out mechanically. If chlorine dosing is used the chlorine will need to be removed afterwards using granulated activated carbon filters.

Bacterial iron
Bacterial iron appears as a reddish or green slime build-up in water tanks and fittings and is caused by bacteria that feed on the iron in the water. The slime can build up all along the water flow path and when it break free from the surface it cause the water to become very discoloured and can clog up appliances. A filter its own won’t remove this type of bacterial contamination and the only way to get rid of this type of iron is to mechanically remove the slime as much as possible first and then chlorinate the water to kill the bacteria. It may be necessary to use fairly high concentrations of chlorine throughout the water reticulation system to kill off all the organisms and to repeat the procedure fairly regularly to discourage re-growth, flushing the system thoroughly to waste after chlorination to remove the chlorine before using the water.

Organic bound iron
Iron-containing complexes are formed when iron combines with tannins and other organics and this form of iron cannot be removed by ion exchange or oxidizing filters. Often mistaken for colloidal iron, a water test to indicate the presence of tannins will confirm, if they are present, the likelihood of this form of iron. Low concentrations of organically bound iron can be removed using a granulated activated carbon filter which will absorb the complex out of the water, however the activated carbon will need to be closely monitored to determine when it has become saturated. High concentrations of this form of iron may require feed dosing of chlorine to oxidize the organics so as to break them off from the iron so that the latter can be precipitated and filtered out in the same manner as soluble iron above.

Copper
Blue or green stains caused by water indicate the presence of copper in the water supply or the deterioration of copper pipes due to highly corrosive water. Copper in the water can be removed by ion exchange using a water softener with the removal rate being about the same as it is for iron. In the case of a highly corrosive water, where the pH is 5-7 it can be raised by passing the water through sacrificial media such as calcium carbonate so that the corrosiveness is reduced. In cases where the pH is less than 5 chemicals will need to be dosed into the water. If the corrosivity is caused by excess oxygen, the hot water will be much more corrosive than the cold in which case the water is treated by feeding polyphosphate or silicates into the water to coat and protect the plumbing, or to aerate the water to release the excess oxygen.