By James A. Hunt
Summary: With filtration products, determining effectiveness of a particular media can get fairly involved. Each may involve various processes and determining their capabilities for the intended application often is very scientific. Thus, selecting the proper media for your solution can be a true art as well.
This discussion of filter media is intended to be a comparative summary of the products commonly used in residential and commercial pressure filtration. The topics covered here include descriptions, applications and operation guidelines.
For basic principles of filter media, see Figure 1. For a comparison of filter media, see Table 1. There are other media that could be included in this list, but it offers a basic understanding of the principles involved.
Activated alumina (AA)
Activated alumina is a mixture of amorphous and gamma aluminum oxide prepared by low temperature (300-600oC) dehydration of Al(OH)3.1 The filtration process is termed surface adsorption but relies on the exchange of a surface hydroxide for the contaminant. AA is regenerated with sodium hydroxide and in some applications requires an acid rinse. AA is used to remove fluoride, arsenic, selenium, silica and humic acids. Arsenic and fluoride treatment are extremely pH dependent (5.5-6).
* Pounds per cubic foot.
** Gallons per minute per square foot.
Anthracite coal when crushed and graded makes an ideal medium-weight filter media. Because of its irregular shape, sediment penetrates deeper into the bed resulting in longer service runs.2 In recent times, anthracite has been primarily used in dual or multi-media filters.
Birm is a plastic filter media (see Filter AG) coated with magnesium oxide. The result is a lightweight product that has catalytic oxidizing capabilities. The catalytic activity is between dissolved oxygen in water and iron, and manganese in the water supply. The chemical reaction causes the iron and/or manganese to precipitate (change from dissolved to a particulate) and the particulate then adsorbs to the surface of the media.3
Corosex is a highly reactive magnesium oxide used to neutralize free carbon dioxide in water. Corosex is used in high flow and/or high pH correction situations. Limitations include the propensity to overcorrect in low flow or intermittent use applications. In waters containing medium to high hardness, calcium may precipitate resulting in cementing of the media.4
Crushed marble (calcite)
Calcite is a naturally occurring calcium carbonate media.5 It’s crushed and screened marble, which dissolves in the presence of acid. This is a self-limiting neutralizer because it dissolves in the presence of acidic water but is practically insoluble in non-acidic water. When the acid is neutralized, the media no longer dissolves. The higher the alkalinity of the water, the greater the amount of calcite that must be dissolved (longer contact time).
Filter-Ag is a non-hydrous silicon dioxide material designed to be a lightweight “sand.” Ag weighs 25 lb/ft3 vs. sand at 100 lb/ft3. It has an irregular surface, which increases surface area over many natural silica products. Ag is intended for removal of suspended solids (turbidity).6
Garnet is a high density and small mesh size filter media. Due to its density, it classifies with the media fines on the top, resulting in very fine filtration in the 10-20 micron range.7 Because of this fine top layer, filtration garnet is seldom used alone as filter runs would be very short. Garnet is used in dual or multi-media filters where larger particles are filtered by larger mesh media.
Granular activated carbon (GAC)
To manufacture activated carbon, charcoal is crushed to a powder, reconstituted with a binder to the desired mesh size, then treated with high temperature steam. The steam treatment is the activation process, which creates cavities in the charcoal, resulting in a high surface area to volume ratio. The high surface area gives GAC much greater adsorptive capacity. Charcoal from coal (anthracite), petroleum, animal bones, wood and coconut shells are used to make GAC.
Bituminous coal accounts for most of the GAC in this country with coconut GAC being used in point-of-use drinking water applications. Acid washing is an optional preparation, which reduces carbon fines and limits pH spikes. Acid or water washing also reduces any contaminants naturally present in the carbon source.
There are several measures of GAC quality including Iodine Number, which is a measure of the amount of iodine adsorbed by weight. Iodine is used as an indicator of the adsorptive capacity but there’s no direct correlation between Iodine and other constituents.
GAC is used to reduce organics, volatile organic compounds (VOCs), and taste and odor causing constituents. The best uses of GAC don’t include sediment removal. Most suspended solids will fill the cavities in GAC, thus reducing its surface area and adsorptive capacity.
KDF process media are high purity copper-zinc granules (50 percent Cu; 50 percent Zn) that use the redox (the exchange of electrons) to remove chlorine, heavy metals and is bacteriastatic. KDF has an unusually high service flow rate compared to other filter media.8 KDF cannot be used in aggressive water and is often preceded by some form of neutralization.
KDF process media are high purity copper-zinc (85 percent Cu; 15 percent Zn) granules that use the redox (the exchange of electrons) in products to remove iron, manganese, hydrogen sulfide and is bacteriastatic. KDF has an unusually high service flow rate compared to other filter media. KDF cannot be used in aggressive water and is often preceded by some form of neutralization.
Manganese greensand is naturally occurring glauconitic greensand coated with manganese resulting in a purple-black media. The intended use is as a catalyst to precipitation of iron, manganese and hydrogen sulfide. Greensand is continuously regenerated (CR) with KMnO4 (potassium permanganate) and/or chlorine. Greensand may also be intermittently regenerated (IR) with KMnO4. CR capacity is 10,000 parts per million (ppm) per ft3 when regenerated with 2-4 ounces of potassium permanganate and is calculated by KMnO4 demand; where iron = 1:1, manganese = 1:2, and hydrogen sulfide = 1:4 ppm. That translates as removal of 10,000 ppm iron alone, 5,000 ppm manganese alone, or 2,500 ppm hydrogen sulfide alone. IR has half the capacity of CR or 5,000 ppm. It’s common practice to place a 15-inch layer of anthracite on top of the greensand in CR applications.9
MTM is a granular manganese dioxide filtering media used for reducing iron, manganese and hydrogen sulfide from water.10 MTM works in the same fashion as greensand including regeneration with KMnO4 and/or chlorine. The primary difference between MTM and greensand is that MTM is manufactured with a lightweight (plastic) core. This lightweight property reduces backwash rates and pressure drop of service flow.
The practice of layering several different media (usually 3 or 5) results in higher service flow rates and finer filtration down to 10 micron. The media are loaded by density and reverse grading. The most dense media with the smallest mesh size is loaded first and the least dense with the largest mesh size is loaded last with intervening media layered in the same manner. As a result, larger particles are filtered in the first or top layer and successively smaller particles are adsorbed in succeeding layers. The most common media mix from top to bottom is: anthracite, filter sand, garnet 30×40, garnet 8×12, gravel 1/8×1/16, and gravel 1/4×1/8. While any combination of media can be labeled multi-media, the above section has come to be called multi-media.
Pyrolox is a mined ore (manganese dioxide) used for iron, manganese and hydrogen sulfide reduction. Pyrolox—like Birm, greensand and MTM—acts as a catalyst to oxidation. Waters low in dissolved oxygen cannot use the catalytic properties of Pyrolox. It doesn’t require a regenerant but must be backwashed aggressively. Backwashing causes the media to abrade itself resulting in a new exposed surface. Inadequate backwash rates and low dissolved oxygen are the two primary causes of filter failure when using Pyrolox.
Filter sand is naturally occurring, graded and washed sand that’s high in silica and low in calcium.11 It’s used for sediment filtration and is often part of a multi-media mix. Sand filters may be the oldest man-made filters and they mimic nature’s filtration.
Using the formulas or values in the Tank Formula Chart referred to in Table 2, one can size applications appropriate for each. In the strictest sense, all filter applications and sizing are pure science. In practice, you often don’t have the space, flow rates, pressure or budget required. The art of filter applications is balancing what you want with what you have. Thus, the proper application of filter media is both an art and a science.
- Clifford, Dennis, Ion Exchange and Inorganic Adsorption, Water Quality and Treatment Fourth Edition, p.568, McGraw Hill, 1990
- Anthracite, Form No. 2354, Clack Corp., Windsor, Wis., February 1999.
- Birm, Product Specification sheet, Clack Corp., Windsor, Wis.
- Corosex and Corosex II, Form No.2387, Clack Corporation, Windsor, Wis., February 1999.
- Calcite, Form No. 2386, Clack Corp., Windsor, Wis., February 1999.
- Filter-Ag, Product Specification sheet, Clack Corp., Windsor, Wis.
- Garnet, Form No. 2355, Clack Corp., Windsor, Wis., February 1999.
- KDF 55 Process Medium in Point-of-Entry Water Treatment Systems—Chlorine Reduction, Form No. 1003-596 AMS, KDF Fluid Treatment, Three Rivers, Mich.
- Point-of-Use Iron, Manganese, And Hydrogen Sulfide Removal From Well Waters Using Manganese Greensand, RV-2-1-87, Inversand Company, Clayton, N.J.
- MTM, Form No. 2353, Clack Corp., Windsor, Wis., February 1999.
- Filter Sand and Gravel, Form No. 2352, Clack Corp., Windsor, Wis., February 1999.
About the author
Jim Hunt is president of DI Water Inc., of Amesbury, Mass., offering RO, EDI, UF and portable exchange tank services. He also is a representative for DWC, a division of the Duff Co., a provider of products for quality water. And he’s a member of the WC&P Technical Review Committee. Hunt can be contacted at (800) 840-0901 or (978) 834-0764, (978) 834-3169 (fax), email: email@example.com and website: www.diwaterinc.com