Greensand Filters and Greensand Filtration
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Greensand is the name commonly ascribed to a sandy rock or sediment containing a high percentage of the mineral glauconite. Glauconite is a greenish-black to blue-green mineral that forms in shallow marine sedimentary deposits. Glauconite, coated with manganese dioxide, is an excellent greensand filtration media for catalyzing the oxidation of soluble iron and manganese, and hydrogen sulfide which is a gas. Recently, a second generation of greensand has been developed, which uses a silica sand core instead of glauconite. This allows greensand filters to better withstand challenging operating conditions such as higher operating temperatures and higher differential pressures, and work well in waters that are low in silica, total dissolved solids, and hardness.
Greensand Filter 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 a pressure differential switch when the differential pressure across the filter increases by 12 psi. Chemical Pretreatment: Filtration with manganese greensand often requires pretreatment with an oxidizing agent. In large flows, such as with industrial water filtration, pretreatment is typically accomplished with chlorine and/or potassium permanganate, a strong oxidizer. Since the oxidation reaction required to convert the soluble material into solid particles, followed by successful filtration, must be accomplished within 1-2 minutes of filter residence time, the water chemistry must be conducive for rapid reaction kinetics. 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 filters. 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, then the potassium feed is too high. Hydrogen sulfide pretreatment is similar to that for manganese. Start with a chlorine dose of about 2.0 times the hydrogen sulfide and adjust the permanganate quantity to achieve the same color.
Work with the Professionals The WaterProfessionals® have many years of experience designing, assembling, loading, starting up, and chemically balancing greensand filter installations. We can provide pilot testing skids where required by regulatory authorities, and we are accustomed to meeting the specific design requirements of scores of states and jurisdictions for industrial water filtration. Call WaterProfessionals® today at 1-800-999-4195
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