Used to reduce the level of suspended solids in feed water applications, EN-FAB high rate, multi media water filter design is effective in removing rust, dirt, silt, and other suspended solids from water down to 5 to 10 microns. The effectiveness increases industry’s ability to conserve, treat and recycle large amounts of water.
Older sand filter systems depend on bacterial action for water purification, while rapid sand filters, which operate on a surface effect principle, make use of disinfectants and coagulants to accomplish the same objective. Extensive investigative research of filter media and improved coagulants have made high-rate filtration practical and efficient.
A wide variety of high-rate filter types using down-flow, up-flow, center injection and radial outward flow have been developed. However the down-flow type has become the most popular and seems to be the most versatile and reliable. The backwash cycle in newer filters makes them perfect for those applications in which heavy loads cause short cycles. The filter’s ability to be backwashed rapidly, using less clean water makes it very versatile for a wide range applications.
Perhaps the most attractive feature of this filter design is the low operating and maintenance costs. Routine maintenance of process equipment has always been a costly part of plant maintenance. Now, many plant engineers are choosing to remove suspended solids with filters before they can foul water passages. The accrued savings in labor resulting from infrequent manual cleaning of major plant equipment often pays for the filters in a very short time. Improved operating efficiency of the process equipment is usually considered a bonus.
Almost all high-rate filter media are granular solid materials of reasonably uniform size. In this type of filter there has been some discussion on the issue of retention of solids in the media, but only three types of retention have been deemed adequate and most efficient: straining at the surface, attachment of small particles to the media grains in the filter bed, and settling.
Straining at the surface will take place for all particles too large to enter the pores in the media. When the surface has been saturated or covered with collected solids, no smaller particles will enter the bed. The filter will eventually lug, resulting in a high differential pressure across the filter.
Attachment of smaller solids to media grains is the basic principle of high-rate filtration. Many natural solids will adhere to media surfaces to some extent. This bond’s resistance to the shear forces of the water flow is the measure of success of the filtration.
Settling occurs within the media when solids are sheltered from the main flow, and, in effect, miniature-settling tanks are formed where small particles can drop out of suspension.
Recently, some researchers have postulated the “sheath flow” principle, which holds that the earliest solids retained by the filter are held near the media surface and tend to remain there for the whole cycle. As solids adhere to the upper grains, they tend to choke the pores in the media. With constant applied flow rate, water velocity will rise in the restricted passages, shear forces will be increased, and attachment of solids will be reduced. After passing the loading zone, solids will adhere in the expected way. This theory seems to be borne out in practice, and it accounts for the depth penetration of collected solids in the media.
With a high-rate filter in service, some demonstration of theory can be seen. Sample points located at regular increments of depth show increasing solids present as the filter run proceeds. The rate of penetration appears to be proportional to the total weight of solids in the influent, but it is not necessarily dependent upon the rate of flow. The filter media must be selected depending upon the clarity of effluent required. The addition of coagulants will change the filter performance by providing strengthened bonds to capture even sub-micron particles in the media.
Backwash usage is usually stated as a percentage of water produced. Production runs at 15 gpm per sq ft filter rate can vary from 48 hr on stream with low suspended solids (5 ppm) to very short runs of 2 hr with extremely heavy organic particle loading. In the latter case, backwash water usage is approximately 2 percent of produced water, but in most installations, the wash water usage is found to be less than 0.1 percent. Ten percent wash water consumption is regarded as excessive.
The selection of filter media does not change the basic filter design. Therefore, it is possible change the media bed in a filter to suit a particular service without modifying any of the internal or external filter piping requirements. Media grains can be of any hard, compatible material. Grain shapes can vary considerably and good results are obtained with either rounded or sharp-edged granules. Uniformity of size is of paramount importance. Media effective size is usually defined as the theoretical square screen opening which will retain 90 percent by weight of media. Uniformity coefficient is the ration of the size of opening which will pass 60 percent media to the effective size. The more uniform the grains, the closer the uniform the grains, the closer the uniformity coefficient approaches unity.
As a typical field example, a fine media with effective size of 0.3 mm and uniformity coefficient of 1.4 maximum is usually selected. A course media might have 0.45 mm effective size and 1.4 maximum uniformity coefficients. The support sand may have an effective size of 1.2 mm and a uniformity coefficient of 1.6 maximum. In many cases, the fine media will remove 95 percent of the particles down to 6 microns, and the coarse media will remove particles down to 15 microns. Actual efficiency is so dependent upon site conditions that these numbers should only be used as a general guide.
The practice of using two media in a filter is quite popular. Large particle material through which the water first passes is capable of removing larger solids, while small solids will pass through the course material to be filtered in the second layer located above the collector system. If similar materials were used for both layers, the first backwash would cause the complete integration of filter media and probably even a large degree of reverse classification. Such a condition would allow the finer media to lie on top of the bed and the courser media to settle to the bottom, thus defeating the purpose of the single media system.
The obvious solution to this particular problem is the use of large particles of low specific gravity and low settling rate with smaller particles of high specific gravity and high settling rate. Under backwash conditions, the media bed fluidizes in two patterns with small, heavy material cycling below the coarse materials.
Some suggestions have been made to use additional media layers. However, with the commercial materials available, it has been found undesirable to introduce the third media layer. This is because it generally integrates with both the coarse and fine layers.
Integration of various sizes in unsatisfactory because the dirt-holding capacity of the filter is largely reduced and the ability to achieve uniform depth penetration is generally weakened. Whatever media system is chosen, the scouring action in back-flush is essential. Unless full scouring occurs, the detachment of particles adhering to media grains is not effective, and the filter does not off-load solids efficiently.
In special cases where highly “polished” effluent is required, secondary or polishing filters should be used downstream of high rate filters. In this case, it is customary to use the high rate filters as roughing filters to remove the bulk of the contaminants allowing the more expensive polish filters to produce the final filtration desired. The net result of this arrangement is a lower operating cost.
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