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What Is Greensand Filtration?

Greensand is a common term for sandy rock or sediment that contains a high percentage of glauconite. Filters made with glauconite, otherwise known as greensand filters, are good ways to remove iron, manganese and sulfides from drinking water sources.

  1. Glauconite

    • Glauconite is the greenish mineral that gives greensand its name. It is found in shallow marine sedimentary deposits. Glauconite was identified in the early 1900s as having chemical exchange properties in water. These properties make glauconite an ideal media for water softeners. In the 1970s, new technology created a new use for glauconite as a filtration media when coated with manganese dioxide.

    Purpose of Greensand Filters

    • Greensand filters are ideal for removing iron, manganese and hydrogen sulfide contaminants from water. Iron and manganese are naturally occurring minerals that dissolve in the water of some underground aquifers when they are present in the geological strata within the aquifer. The U.S. Environmental Protection Agency created maximum contaminant levels for iron and manganese but not because these minerals are dangerous. Rather, the levels are regulated because of aesthetic concerns such as staining, taste and discoloration. Greensand filters remove hydrogen sulfide, too, but it is of less concern for most applications since it dissipates quickly once the water is exposed to surface air.

    Filter Design

    • Modern greensand filters flow at a rate of 3 to 5 gallons per minute (gpm) per square foot of filter surface area. The filters are designed to require a minimum backwash of 12 gpm. Backwash is the process of reversing the water's flow to flush particles out of the filter. To prevent damage to greensand filters, backwash should be initiated when the differential pressure across the filter increases by 12 psi.

    Pretreatment

    • Manganese greensand filters often require the water to be pretreated with an oxidizing agent to prompt the oxidation reaction required to filter the water. For small commercial or residential filtration systems, this pretreatment may mean installing a contact tank upstream from the filter. Since oxygen is an oxidizing agent, a small air pump in the tank can incorporate enough oxygen in the water to allow the necessary oxidation reaction in the filter. Larger systems typically use chlorine or potassium permanganate as oxidizers.

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