Forms of Iron
Iron can occur in water in a number of different forms. The type of iron present is important when considering water treatment. Water that comes out of the faucet clear, but turns red or brown after standing is “ferrous” iron, commonly referred to as “clear-water” iron. Water which is red or yellow when first drawn is “ferric” iron, often referred to as “red- water” iron. Iron can form compounds with naturally occurring acids, and exist as “organic” iron. Organic iron is usually yellow or brown, but may be colorless. Water containing iron bacteria is said to contain “bacterial” iron.
Yellow or red colored water is often a good indication that iron is present. However, a testing the water can determine the exact amount of iron, which can be useful in determining the best type of treatment. In addition to testing for iron, it can be of value to also test for hardness, pH, alkalinity, and iron bacteria. The amount of a dissolved material in water is usually reported as the number of milligrams per liter (mg/L). This is the weight of material in 1 liter (approximately 1 quart) of water. A milligram per liter is approximately equal to 1 part per million (ppm). Iron in amounts above 0.3 mg/L is usually considered objectionable. Iron levels are usually less than 10 mg/L.
The most common method for controlling iron in water is water treatment. In some circumstances, another alternative is to use a different water source that is low in iron, such as a public water system or a well drawing water from a different water-bearing formation. In some cases, a new well may be an option, however, it is difficult to predict what the iron concentration will be. Neighboring wells may be an indicator, but the iron content of two nearby wells may be quite different.
Treatment of water containing iron depends on the form(s) of the iron present, the chemistry of the water, and the type of well and water system. Clear-water iron is often removed with a water softener. A water softener is actually designed to remove hardness minerals like calcium and magnesium. Iron will plug the softener, and must be periodically removed from the softener resin by backwashing.
Also, if the water hardness is low and the iron content high, or if the water system allows contact with air, such as occurs in an air-charged “galvanized” pressure tank, a softener will not work well. Ion exchange water softeners add sodium to the water which may be a concern for persons on a sodium restricted diet. Red-water iron can be removed in small quantities by a sediment filter, carbon filter, or water softener, but the treatment system will very quickly plug up. A more common treatment for red-water iron and clear-water iron in higher concentrations is a manganese greensand filter, often referred to as an “iron filter.”
Iron in Water
As rain falls or snow melts on the land surface, and water seeps through iron-bearing soil and rock, iron can be dissolved into the water. In some cases, iron can also result from corrosion of iron or steel well casing or water pipes.
Aeration (injecting air) or chemical oxidation (usually adding chlorine in the form of calcium or sodium hypochlorite) followed by filtration are options if iron levels are on the higher levels. Organic iron and tannins present special water treatment challenges. Tannins are natural organics produced by vegetation which stain water a tea-color. In fact, the tannins in coffee or tea produce the brown color. When tea or coffee is made with water containing iron, the tannins react with the iron forming a black residue. Organic iron is a compound formed from an organic acid and iron. Organic iron and tannins can occur in very shallow wells, or wells being affected by surface water. Organic iron and tannins can slow or prevent iron oxidation, so water softeners, aeration systems, and iron filters may not work well. Chemical oxidation followed by filtration may be an option.
Iron bacteria are organisms that consume iron to survive and, in the process, produce deposits of iron, and a red or brown slime called a “biofilm.” The organisms are not harmful to humans, but can significantly exacerbate an iron problem. The organisms naturally occur in shallow soils and groundwater, and they may be introduced into a well or water system when it is constructed or repaired. Treatment options for elimination or reduction of iron bacteria include physical removal, heat, and chemical treatment. The most common treatment is “shock” chlorination of the well and water system. After all, iron bacteria need iron to survive. Eliminating the bacteria will not eliminate the iron - both well treatment for the bacteria, and water treatment for the iron will be needed.
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