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Oxidizing Filter Filtration

These units are usually installed as Point-or-Entry (POE) and are placed at the incoming water supply.

These filters often referred to as iron filters or red water filters and use a "greensand" resin bed (coated with manganese) to oxidize iron and manganese that are in solution. Oxidizing filters are most effective in water with a pH of 7 or above. If water is acidic (pH below 7), a chemical feed pump may also be needed.

As the water flows through the resin bed, the iron and manganese are oxidized and changed from their soluble to an insoluble form. The media also acts as a filter and traps iron and manganese precipitates that have been oxidized prior to reaching the filter.

When iron and manganese are above 1 mg/L,  or if the water is pH below 7 (acidic), a strong oxidizing substance should be applied prior to greensand filtration. Super chlorination is commonly used to oxidize iron and manganese in order to adjust the pH.  At a pH of 7 or above, iron changes more readily from its soluble to its insoluble form, which can then be removed by filtration. This process also kills any iron bacteria present, however, the excess chlorine will need to be removed prior to use.

Oxidizing Filters (greensand) is effective at filtering or reducing:

  • sulfur (hydrogen sulphide) concentrations up to ~5 mg/L.
  • combined iron/manganese concentrations up to 10 mg/L.

  Considerations:

Greensand filter do not tolerate iron bacteria because the slimy material  produced fouls the greensand by coating it.  Consider testing before investing. 

Oxidation occurs as water flows through the filter bed, because much of the precipitate is filtered out near the discharge side of the greensand bed. it requires thorough backwashing. Insufficient backwashing can cause the precipitated iron and manganese to be expelled from the filter in a disgusting discharge from a faucet or it can ruin a washer load of clothes. Consider maintenance.

The backwash requirement rate for a greensand filter is considerably more than for a Ion Exchange water softener due to the media weight and density. Consider whether the water supply is sufficient to backwash the filter.

The condition of the water, the size of the unit, and the amount of water consumed all affect how quickly the oxygen is depleted.  The oxygen oxidizing filters must be regenerated with a solution of potassium permanganate.  As intimated above, backwashing these filters takes a considerable amount of water. Consider ongoing costs and disposal of the discharge water.

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Last modified: November 13, 2018