Greensand Filters and Greensand Filtration
Greensand is the name commonly applied to a sandy rock or sediment containing a high percentage of the mineral glauconite. Glauconite ((K,Na)(Fe3+, Al,Mg)2(Si, Al)4O10 (OH)2) is a greenish-black to blue-green mineral which forms in shallow marine sedimentary deposits.
Early in the 1900’s, glauconite was recognized as having chemical exchange properties in water solutions, which led to its use as a water softener media. With the development of higher efficiency polymer-based ion exchange resins, its use for this essentially ceased in the 1970’s. Glauconite, coated with manganese dioxide, was found to be an excellent greensand filtration media for catalyzing the oxidation of soluble iron and manganese and the gas hydrogen sulfide.
Once oxidized to the form of tiny particles, manganese greensand serves as an effective filtration media to filter the particles. Recently a second generation of greensand has been developed which uses a silica sand core instead of glauconite, which allows it to better withstand operating conditions in waters that are low in silica, total dissolved solids and hardness as well as higher operating temperatures and higher differential pressures (see http://www.inversand.com).
Iron, Manganese and Hydrogen Sulfide Water Contaminants
Iron and manganese are naturally occurring minerals comprising about 5% and 0.1% of the earth’s crust, respectively. If present as part of the geological strata in contact with underground aquifers, both will dissolve in water, particularly when the water has a depressed pH. Both have USEPA secondary maximum contaminant level’s for potable water based on aesthetics (i.e., they are regulated because of staining, taste and discoloration of beverages such as tea rather than health concerns).
Iron will cause staining at concentrations ≥ 0.3 parts per million (ppm) while manganese begins staining at ≥ 0.05 ppm. These thresholds are also the current MCL’s for each. Both can be present in the soluble or oxidized form as the water is pumped from the well. If the water appears clear, but turns red upon exposure to air (oxygen), the majority of the iron is dissolved (called soluble or "clear water" iron which is present as Fe+2) while oxidized or "red water iron" is present as tiny particles of Fe+3.
Manganese behaves similarly except it has a purplish-black appearance in the oxidized form. If 100% of either metal is oxidized in the aquifer, filtration alone, without any chemical addition, will remove the contaminant. There is another form of iron called "heme iron" where it is bound with organic molecules (e.g., tannins). Removal of heme iron typically requires more severe chemical pretreatment to break the iron-organic bond before filtration. Hydrogen sulfide, an odorous gas that smells like rotten eggs, is dissolved in the water present in some aquifers.
Hydrogen sulfide dissipates from water quickly once it is pumped to the surface and exposed to air. In fact, particularly in climates where freezing is not a problem, aeration is a relatively inexpensive way to treat high flows.
Chemical Treatment and Filtration
Filter Design: Greensand filters are typically designed to flow at 3-5 gpm per square foot of filter surface area and need a minimum of 12 gpm per square foot for backwash. Backwash should be initiated manually or by pressure differential switch when the differential pressure across the filter increases by 12 psi. Excessive delta P can damage the media.
Chemical Pretreatment: Filtration with manganese greensand often requires pretreatment with an oxidizing agent. On small commercial or residential systems, this may be accomplished by oxygen in the air using a small air pump and a contact tank upstream of the greensand filter. In large flows such as with industrial water filtration, it is typically accomplished with chlorine and / or potassium permanganate, a strong oxidizer. Because the oxidation reaction required to convert the soluble material into solid particles, followed by successful filtration must be accomplished in 1-2 minutes of filter residence time, the water chemistry must be conducive to rapid reaction kinetics. Technical literature typically recommends pH in the 6.2 to 6.5 range. Our experience with waters in the SE US demonstrates pH 6.8 to be optimal. Alkalinity must be ≥ 120 ppm. These parameters can be adjusted by metering a dilute caustic and / or a soda ash solution into the water upstream of the filter to adjust pH and alkalinity, respectively. For iron alone, chlorine fed at a concentration of 1.0 to 1.2 times the iron should be adequate to oxidize the iron and “regenerate” the manganese greensand. Since excess chlorine is not deleterious to greensand filtration, it is possible to feed enough to provide adequate free chlorine residual in the filtered water sufficient for introducing into the clearwell and pumping to service.
For water containing manganese and iron, feed chlorine followed by potassium permanganate upstream of the greensand filter. Start with a chlorine dose calculated as 1.0 times the iron and 2.0 times the manganese concentration and introduce a small feed of permanganate. Collect minimum one liter samples in a glass container (a one liter Erlenmeyer flask is ideal) just upstream of the greensand filter and observe the color in good light. Chlorine alone should cause a yellowish tinge. When a pink tinge is first observed, the permanganate dose should be about right. If the color is distinctly pink, the potassium feed is too high.
Hydrogen sulfide pretreatment is similar to that for manganese. Start with a chlorine dose about 2.0 times the hydrogen sulfide and adjust permanganate to achieve the same color.
Deal with Experts
The WaterProfessionals® have many years of designing, assembling, loading, starting up and chemically balancing greensand filter installations. We can provide pilot testing skids where required by regulatory authorities and are accustomed to meeting the specific design requirements of scores of states and jurisdictions for industrial water filtration.
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