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Iron and Manganese Removal

If the amount of iron and manganese in water is not very significant, it can be removed by most water softeners along with water hardness. When the water softener is regenerated, iron and manganese ions will be flushed out the same way as calcium and magnesium ions. However, with larger amounts of iron in the water (more than 0.1 mg/l), precipitated iron residue may build up on the softening material regardless of backflushing and slowly decrease the efficiency of the softener. This can sometimes be controlled by special cleaning products mixed with the salt used for regeneration of the softeners.

If the iron and manganese concentrations are above 0.1 mg/l (combination of both ions) an iron filter should be used. The medium in this type of filter oxidizes iron and manganese and removes precipitated matter. The most common type is called a green sand filter. These filters also must be flushed periodically and regenerated with potassium permanganate to restore oxidizing power.

The softener and iron filter are effective only if the iron or manganese is not bound to organic matter and there are no iron or manganese bacteria in the water. The oxidizing media of the iron filters are not strong enough to break these materials down. Where iron and manganese are bound to organic matter, or concentrations of these two metals are very high, or iron or manganese bacteria are present, a strong oxidizing substance must be applied before filtration. The most commonly used chemical in these systems is household bleach (hypochloride) injected ahead of the pressure tank. This procedure disinfects the water and at the same time oxidizes iron, manganese, and organic matter, which will then precipitate. Sedimentation and/or filtration is then needed to remove the precipitants. Chlorine solutions tend to lose their strength and require weekly addition to be effective. Activated carbon units or reverse osmosis units should then be used to remove the remaining chlorine and possible halogenated hydrocarbons created from organics. It should be noted that acid prevents the complete oxidation of iron in water and acidity should be neutralized for effective removal of iron. Final choice of the method will depend on iron and manganese concentrations, pH of water, and the presence of the bacteria.

An alternative to iron removal is stabilization with polyphosphates. The application of the polyphosphate must take place before the iron is oxidized with aeration or chlorination. This process is also called sequestration. It does not work well where the concentration of iron is over 1 mg/l. Also, heat will convert polyphosphate to orthophosphate which causes it to lose its dispersing properties. The use of phosphates may stimulate the growth of bacteria so chlorination may still be required. As a result, chlorine might as well be used for iron and manganese removal in the first place.