By John L. Schlafer
Summary: As water treatment dealers know, there are almost as many filters as there are applications for these various filters. Selecting the right one for the job takes some careful consideration. Spotlighted here are granular bed filters and what these offer to dealers by way of contaminant reduction and efficiency.
Water filtration has existed for thousands of years. Egyptian Sanskrit writings dating to 2,000 B.C. describe early water purification as boiling water in copper vessels, exposing water to sunlight, and filtering through charcoal.1 Pictures of wick siphons are in the Egyptian tombs of pharaohs Amenhotep II (1,420 B.C.) and Rameses II (1,223 B.C.)—see Figure 1. on page 44, this issue. In A.D. 98, the water supply commissioner for the municipalities of Rome published two books on the water systems including a description of a settling basin at the entrance of one aqueduct and “pebble catchers” built into several aqueducts for treatment purposes. Sir Frances Bacon published, in 1627, results of experiments conducted on water purification by filtration, boiling, distillation and clarification. He claimed that clarifying water improves health and increases the “pleasure of the eye.”
Later, physician Luc Antonio Porzio published a book in 1685 describing the maintenance of the health of soldiers during the Austro-Turk war by use of sand filters to treat their drinking water. In 1856, Henry Darcy patented filter designs in England and France using hydraulic principals leading to today’s rapid-sand filter designs. The cholera outbreaks in London during this same time period and their link to water quality highlighted the importance of effective water treatment worldwide. Over the last 150 years in the United States, water filtration has become a common process used by water treatment municipalities as well as domestically at the point-of-use (POU) or point-of-entry (POE). The 1986 amendments to the 1974 Safe Drinking Water Act require filtration in many situations, and recognize for the first time the value of POU/POE in assuring additional consumer protection.
Drinking water filtration includes numerous mechanical technologies for the removal of suspended solids—sediment or particulate—from drinking water sources. Common filters include strainers or screens, membranes, bags, cartridges, precoat, centrifugal or cyclone, chemical feed with settling, and various granular, deep-bed filtrations. Each of these involves specific technologies, product designs and applications. Common, deep-bed, granular media filters have been used for years primarily for POE installations. They’re very effective in many applications when designed, applied and maintained properly. This article highlights many important design and application features of deep-bed, granular media filters.
Fundamentally, bed filtration involves three key phenomena—transport, retention and release.2 Transport addresses the movement of the particle from the raw influent water to the point of retention during the filtration process as well as the movement during backwash of the same particle back out of the filter to a waste stream. Transport mechanisms may include gravity (settling), diffusion, screening, interception, electrical (coagulation and colloidal charge) and inertia. The effectiveness depends on the size, chemical and electrical properties, and density of the unwanted particles in the water, the media particle’s chemical and electrical features, shape, media particle size, and size distribution as well as the water temperature and flow rate. Essentially, transport needs to provide sufficient penetration into the bed to give reasonable bed capacity between scheduled backwashes. Transport mechanisms are also important in the removal of solids during the cleaning backwash cycle.
Retention involves the fundamental physical and chemical mechanisms of electrical attraction, chemical bonding and mechanical straining. Their effectiveness depends on the chemical properties of the water and filter media, any pretreatment coagulants, and the shape and sizes of the openings and paths in the media. Retention requires that the solids be retained against varying flow rates and the increase in flow resistance—or pressure buildup—as the openings in the media near the top of the bed fill with solids. As the top layer of media accumulates solids, the openings in the top of the media reduce in size. This, in turn, causes water velocities to increase shearing some of the filtered solids and transporting them deeper into the bed where the media is cleaner and the water velocities slower. Continuation of this chain of events leads to increasing pressure loss. If backwash doesn’t occur at the proper time, the customer sees excessive pressure loss or a breakthrough of solids. Additionally, the media is more difficult to clean and re-bedding of the media may be necessary.
The third phenomenon—release or detachment—is critical for reasonable life of the filter. During filtration, the media needs to capture and retain the solids from the water so that it doesn’t pass through the filter, but allows the solids to remain mobile enough to pass down into the media for good capacity. Alternatively, the media must release and permit evacuation of the solids during the backwash/cleaning stage. The backwash water, generally flowing in the direction opposite to the normal service cycle, and the media properties must be such that the bed gradually lifts, expands and fluidizes. The solids trapped in the bed are released and the fluidized bed particles churn, rubbing and bumping against each other resulting in clean media particles. The backwash water additionally lifts the solids up and out of the tank to drain. Just as a clothes washer requires a certain period of time for agitation to loosen and suspend soil particles from clothes, the filter media needs a period of time during backwash to break up the solids, clean the media particles, and transport the filtered solids to waste. At the end of a backwash cycle, the backwash water must be relatively clean, indicating that the solids’ release from the filter medium has been completed.
The above phenomena are important considerations during the filter installation. It’s important to test the raw water for turbidity and sediment as well as for oxidized or red water iron if present. These data indicate the media solids loading rate and are needed for setting backwash frequency and length. The higher the levels, the more frequent and longer the backwash needs to be, particularly if oxidized iron is present.
The water supply pressure is very important for satisfactory filter backwash. The pressure at the filter must be sufficient during the entire backwash cycle to provide thorough cleaning of the media. One well-known water expert3 outlines the procedure to measure this capability. If the water delivery is insufficient, then two filters of smaller tank diameter operating in parallel, but scheduled for different backwash times, may be appropriate. Though more expensive, there’s another important advantage to two filters in that the treated water from one filter can be used as backwash water for the second. Thus, the filter medium after backwash is cleaner.
A second approach to low backwash water flow rate may be a lighter media. There are special lower-density filter media from several suppliers that backwash with lower flow rates. Following the supplier’s guidelines is very important because the lighter media may need a longer backwash cycle. Lower density media doesn’t create as strong of an abrasive and bumping action between particles as heavier media do and, thus, less cleaning of the particles results unless a longer cycle time is used. Also, if the media density is equal to or less than the solids, solids may remain in the bed after backwashing and will appear initially in the filtered water. This is because during the design of the filter, the backwash flow rate is established by the media properties so that the bed fluidizes but not all contaminants are transported out of the tank to waste. If the solids are denser than the media, the solids will tend to stratify under or near the bottom of the media and wash out early during the service cycle.
One symptom of inadequate backwash is the progressive appearance of spherical clusters of filtered solids and media near the top of the bed. These are commonly called “mud balls.”4 They start as clusters of insufficiently cleaned media, clumping and clustering together near the top of the bed. Each backwash fails to break them up and more and more solids and media roll onto the clumps. They start as pea size and may grow to 1 to 2 inches in diameter. Once they reach a certain size, their density will cause them to progressively move toward the bottom of the media bed.
In some locales, well water contains colloidal materials. Colloids are very small particles that carry an electrical charge on the surface.5 They’re so small that gravity is no longer a dominant force and the electrical surface charge controls their motion in water. They’re usually negatively charged, repel each other and tend to be uniformly suspended in the water. Filter media don’t effectively remove colloids because such particles bounce down through the filter bed and don’t become caught or attached to the media. Feeding of a coagulant, usually alum, with a follow-up detention time disrupts the effects of the electrical charges and causes the particles to clump together. Then, the filter media removes these larger clumps.
The careful balancing of the filtering process and the cleaning process establishes the relative design specifications. The tank diameter is established by the expected service flow rate and the recommended media filtration flow rate. For example, if the customer needs 7 gallons per minute (gpm) and the media supplier recommends 5 gpm per square foot service flow rate, then a 16-inch diameter tank is necessary. But, then the media backwash rate must be studied. If the media requires 15 gpm per square foot backwash flow rate, then the plumbing system must deliver 21 gpm for that tank. Many residential systems, particularly private wells, cannot deliver a sustained flow to meet this backwash requirement, so an alternative is necessary. The filter media supplier may have special lighter medias to consider for this application or systems with two filter tanks in parallel should be considered. Insufficient backwash flow rate and duration are the leading causes of filter failure.
Filter media choices are numerous including silica sand, crushed anthracite, greensand, garnet and several trademarked, specially formulated, proprietary media. Bed depths and flow rate requirements—during both filtration and backwash—are available from media suppliers.
One factor important to the life of the media, but often overlooked, is its uniformity coefficient. This is a measure of the distribution of the physical size of the media particles. When a filter backwashes, the bed stratifies and leaves the smaller particles on top and particles are progressively stratified down through the bed, leaving the largest particles at the bottom. This is opposite to that of a good depth filter where the larger particles preferably need to be on top. Thus, media uniformity coefficients need to be relatively small so the filter voids are more uniform throughout resulting in good depth capacity.6
For waters carrying high solids, a dual media filter in the same tank is often used. An example is sand with a layer of crushed anthracite on top. The larger voids and spacing of the anthracite particles allow accumulation and deeper penetration of the solids, resulting in a higher capacity filter. During backwash, the lighter anthracite and the sand release the solids when proper flow rate and duration time are used.
The filter design must include proper water distribution in the bottom of the tank, particularly during backwash. Under-bedding of relatively large-sized garnet or ilmenite, or a distributor designed to provide uniform collection of filtered water during service is necessary. Also, uniform distribution of water to the bottom of the bed during backwash is necessary to keep the bed clean. Poor distribution leads to channeling through the media resulting in non-uniform filtering and cleaning leading to regions of unused, unwashed media. Uneven distribution of water through the bed is another cause of “mud balls.” Gradually, water pressure drops and poorer water quality is the result.7
Granular activated carbon is sometimes used as a filtration media for solids. It provides a taste and odor removal benefit and its low density and irregular particle shape permits backwash fluidization with lower flow rates. But carbon has limitations. Many of carbon media can have relatively high uniformity coefficients; for example, 2.4 for carbon vs. sand at 1.7.8 There will be a greater density of fines on top of the bed requiring more frequent backwash. In addition, the backwash flow needs to be controlled closely to ensure proper stratification of the bed. Otherwise, media particles may redistribute, with some particles moving from the top to bottom and permitting desorption of organics, if present, into the effluent, particularly the more volatile, less-strongly held organics. Lastly, new carbon tends to resist wetting and retains air when first used. If the carbon hasn’t soaked long enough to release the air or if no top screen is used, the first backwash may waste a considerable amount of carbon to drain.
Water treatment professionals will recognize that there are more details to know regarding granular bed filters, the interrelationship of specific media being employed and to what end—i.e., the particular application. Again, selecting the right design for the job takes careful consideration. It’s hoped you come away from this article with a better idea of the issues involved in determining the proper contaminant reduction and efficiency required to achieve your goals.
- Viessman, W., and M.J. Hammer, “Water Supply and Pollution Control,” New York, N.Y., HarperCollins, pg. 1-4, 1993.
- McGhee, T.J., “Water Supply and Sewerage,” New York, N.Y., McGraw-Hill, pg. 208, 1991.
- McGowan, Wes, “Residential Water Processing,” Naperville, Ill., Water Quality Association, pgs. 6-7, 1997.
- Cleasby, J.L., and G.S. Logsdon, “Water Quality and Treatment: AWWA Handbook of Community Water Supplies,” New York, N.Y., McGraw-Hill, pg. 8.70, 1999.
- McGhee, T.J., “Water Supply and Sewerage,” New York, N.Y., McGraw-Hill, pg. 184, 1991.
- McGhee, T.J., “Water Supply and Sewerage,” New York, N.Y., McGraw-Hill, pg. 209, 1991.
- McGhee, T.J., “Water Supply and Sewerage,” New York, N.Y., McGraw-Hill, pg. 210, 1991.
- Cleasby, J.L., and G.S. Logsdon, “Water Quality and Treatment: AWWA Handbook of Community Water Supplies,” New York, N.Y., McGraw-Hill, pg. 8.22, 1999.
- Cleasby, J.L., and G.S. Logsdon, “Water Quality and Treatment: AWWA Handbook of Community Water Supplies,” New York, N.Y., McGraw-Hill, pg. 8.4, 1999.
About the author
John L. Schlafer has 40 years of engineering experience including 15 in the water treatment industry. He is a member of AWWA, WQA and the American Academy of Environmental Engineers. Schlafer is retired and living in Dow, Ill. He can be reached at email: [email protected]