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As discussed in Iron/Rust - The Stainers iron and manganese require special filters designed to handle large amounts of iron or manganese removal without clogging causing flow rate reductions and without requiring excessive maintenance. The following discussion will list the most effective and most commonly used methods of removing iron and manganese from water.
Small to moderate amounts of iron (up to 3 mg/L) can be removed by a water softener which is properly designed for that purpose. In situations where both iron and hardness are present, this method has the advantage of eliminating the need for a separate iron filter. This can save homeowners money because it essentially gives them two filters for the price of one.
The ion exchange process (see water softeners) used by softeners to remove the hardness minerals calcium and magnesium can also be used to remove iron under certain conditions. Iron in solution forms positive charged particles known as cations, as do the hardness minerals calcium and magnesium. The iron cations can be exchanged for sodium in a softener just as easily as can calcium and magnesium.
Problems with this method can occur if precipitated iron is allowed to form a jelly-like substance on the inside of the water softener, coating the resin bed and greatly reducing the softening capacity of the softener. This can render the softener ineffective for both iron removal and softening.
avoid this occurrence certain precautions and design considerations should
be considered when considering water softeners for iron removal.
This class of filters removes large amounts of iron by first oxidizing it, causing it to come out of the dissolved state, and then filtering it out of solution, capturing it in the filter bed. The accumulated iron is periodically, automatically flushed out of the filter.
Unlike water softeners, these types of filters are designed to allow iron to oxidize in the filter bed. They are equipped with the proper type of medium to handle the precipitated iron and have strong enough backwash flow rates to expel the accumulated iron during the cleanout cycle.
There are several types of filters in this class. These include greensand filters, chlorination-filtration filters, ozone filters and air oxidation filters (known as Chemical Free Iron Filters). Although they each employ different substances as their oxidizers and medium they all work on the same "oxidation - filtration principle", which is, plainly put, "get the iron to rust, then filter it out."
These types of filters should be used when water softening is deemed not to be appropriate for the situation. High levels of iron, manganese, sulfur gas (H2S), high dissolved oxygen or high pH would suggest the use of an iron filter over a water softener for iron removal.