RAPID GRANULAR BED FILTRATION
Rapid Gravity Sand Filter
Rapid granular bed filtration, formerly known as “rapid sand filtration,” usually consists of passage of pretreated water through a granular bed at rates of between 2 and use of upflow filters is reported in Latin America, Russia, and the Netherlands. Both gravity and pressure filters are used, although some restrictions are imposed against the use of pressure filters on surface waters or other polluted source waters or following lime soda softening.
During operation, solids are removed from the water and accumulate within the voids and on the top surface of the filter medium. This clogging results in a gradual
increase in head loss (i.e., clogging head loss) if the flow rate is to be sustained. The total head loss may approach the maximum head loss provided in the plant, sometimes called the available head loss. After a period of operation, the rapid filter is cleaned by backwashing with an upward flow of water, usually assisted by some auxiliary scouring system.The operating time between backwashes is referred to as a filter cycle or a filter run. The head loss at the end of the filter run is called the terminal head loss.
The need for backwash is indicated by one of the following three criteria,
whichever occurs first:
1. The head loss across the filter increases to the available limit or to a lower established limit (usually 8 to 10 ft, i.e., 2.4 to 3.0 m) of water.
2. The filtrate begins to deteriorate in quality or reaches some set upper limit.
3. Some maximum time limit (usually 3 or 4 days) has been reached.
Typical filter cycles range from about 12 hours to 96 hours, although some plants operate with longer cycles. Setting an upper time limit for the cycle is desirable because of concern with bacterial growth in the filter, and because of concern that compaction of the solids accumulated in the filter will make backwashing difficult. Pretreatment of surface waters by chemical coagulation (see Chapter 6) is essential to achieve efficient removal of particulates in rapid filters. In addition, filteraiding polymers may be added to the water just ahead of filtration to strengthen the attachment of the particles to the filter media. Groundwaters treated for iron and manganese removal by oxidation, precipitation, and filtration generally do not need other chemical pretreatment.
Filter Media for Rapid Filters
Common filter materials used in rapid filters are sand, crushed anthracite coal, GAC, and garnet or ilmenite.Typical configurations of filter media are shown in Below Figure The most commonly used of these configurations are the conventional sand and dual-media filters, but a substantial number of triple-media filters have been installed in the United States. Granular activated carbon replaces sand or anthracite in filter-adsorbers. It can be used alone or in dual- or triple-media configurations. The first three configurations in Figure 8.5 are backwashed with full fluidization and expansion of the bed. Fluidization results in stratification of the finer grains of each medium near the top of that layer of medium.
The single-medium deep-bed filter using coarse sand or anthracite coal [Figure)] differs from the conventional sand filter in two ways. First, because the medium is coarser, a deeper bed is required to achieve comparable removal of particulates.
Second, because excessive wash rates would be required to fluidize the coarse medium, it is washed without fluidization by the concurrent upflow of air and water.The air-water wash causes mixing of the medium, and little or no stratification by size occurs.
An up flow filter is used in some wastewater filtration plants and in a few potable water treatment plants in other countries. It may include a restraining grid to resist
uplift, as shown in Figure 8.5(5), or it may be operated with a deeper sand layer and to a limited terminal head loss so that the mass of the sand itself acts to resist uplift. The up flow filter is backwashed using air and water together during part of the backwash cycle. Adoption of the up flow filter as the only filtration step in potable water treatment is doubtful because of the potential for contamination of the filtered water caused by both the dirty backwash water and the filtered water exiting above the filter medium.
However, up flow filtration exists in pre engineered, package, potable water treatment plants in the United States, where it is used in the pretreatment process in place of sedimentation or other solids separation processes, ahead of down flow filtration. Typical grain sizes used in rapid filters are presented in Table 8.3 for various potable water applications. The UC of the filter medium is usually specified to be less than 1.65 or 1.7. Use of a lower UC is beneficial for coarser filter media sizes that are to be backwashed with fluidization, however, because this will minimize the d90 size and thereby reduce the required backwash flow rate. But the lower the specified UC, the more costly the filter medium, because a greater portion of the raw material falls outside the specified size range. Therefore, the lowest practical UC is about 1.4. Anthracite coal that will meet this UC is commercially available.
There is growing interest in the use of deeper beds of filtering materials, either mono or dual media, especially for direct filtration applications. For example, deep beds of dual media were used in pilot studies of cyst removal by Patania et al. (1995) in Seattle. These studies included in-line filtration using dual media with 80 in (2.0 m) of 1.25-mm ES anthracite over 10 in (0.25 m) of 0.6-mm ES sand. The media were selected to meet stringent turbidity and particle removal goals and operated with optimized chemical pretreatment. Pilot studies for proposed plants for Sydney, Australia, included deep-bed mono- and dual-media configurations (Murray and Roddy, 1993). The dual media studied included anthracite 39 to 118 in (1.0 to 3.0 m) in depth, with 1.7- to 2.5-mm ES over 6 to 12 in (0.15 to 0.30 m) of crushed sand with 0.65- to 1.0-mm ES. The coarser dual media performed best in production per cycle and in filtrate quality. One reason expressed for the interest in deep beds was the possible future conversion of the filters to GAC with empty-bed contact times of at least 7.5 minutes.
In addition to the configurations of filtering materials shown in Figure 8.5, other proprietary media are being used in some applications. The following examples are discussed in more detail in the section titled “Other Filters” in this chapter.
A buoyant crushed plastic medium is being used in an up flow mode as a contact flocculator and pretreatment filter ahead of a down flow triple-media bed. Several manufacturers are marketing traveling backwash filters in which the filter is divided into cells and utilizes a shallow layer of fine sand, usually about 12 in. in depth.
The Concept of Equivalent Depth of Filter Media
When considering the use of deeper beds of coarser media, the provision of a deeper bed is sometimes used so that the ratio of bed depth to grain diameter L/d is held constant. Hopefully, this will result in equal filtrate quality when filtering the same influent suspension at the same filtration rate. The concept is supported by the experiments of Ives and Sholji (1965). However, these authors also analyzed the work of other investigators and suggested the relationship should be L/dβ, with β values from 1.5 to 1.67.
Nevertheless, the use of L/d has grown in recent years. For graded beds, the effective size de has been suggested for the diameter term (Montgomery 1985, p. 538). For dual and multimedia filters, the sum of the L/de for each layer is calculated (i.e., the weighted average L/de for the bed). Some typical values for L/de for common bed configurations, quoted directly from Kawamura (1991, p 211), are as follows:
_ L/de ≥ 1000 for ordinary fine sand and dual-media beds
_ L/de ≥ 1250 for triple-media (anthracite, sand, garnet) beds
_ L/de ≥ 1250 for a deep, mono medium beds (1.5 mm> de >1.0 mm)
_ L/de = 1250–1500 for very coarse, deep, mono medium beds (2.0 mm> de > 1.5mm)
For a common dual-media filter with 2.0 ft (0.6 m) of anthracite of 1.0-mm ES over 1.0 ft (0.3 m) of sand of 0.5-mm ES, the total L/de would be 600/1.0 plus 300/0.5 = 1200, meeting Kawamura’s criterion. Of course, this is a simplistic concept for a complex process, but it does assist in selecting alternative filter media for pilot scale evaluations.
Mono medium Versus Multimedia Filters
The media utilized in rapid filters evolved from fine sand, mono medium filters
about 2 to 3 ft (0.5 to 0.9 m) deep to dual media and later to triple (mixed) media of about the same depth. Still later, success at Los Angeles had led to strong interest in deep-bed (4 to 6 ft, 1.2 to 1.8 m), coarse, monomedium filters.The rationale for this evolution is discussed in this section.
Early research on rapid sand filters with sand about 0.4- to 0.5-mm ES demonstrated that most of the solids were removed in the top few inches of the sand, and that the full bed depth was not being well-utilized. The dual-media filter bed, consisting of a layer of coarser anthracite coal on top of a layer of finer silica sand, was therefore developed to encourage penetration of solids into the bed.
The use of dual-media filters is now widespread in the United States. It is not a new idea. Camp (1961) reported using dual media for swimming pool filters beginning about 1940, and later in municipal treatment plant filters. Baylis (1960) described early
work in the mid-1930s at the Chicago Experimental Filtration Plant, where a 3-in layer (7.5 cm) of 1.5-mm ES anthracite over a layer of 0.5-mm ES sand greatly reduced the rate of head loss development in treatment of Lake Michigan water.
The benefit of dual media in reducing the rate of head loss development—thus
lengthening the filter run—is well-proven by a number of later studies (Conley and
Pitman, 1960a; Conley, 1961;Tuepker and Buescher, 1968). However, the presumed benefit to the quality of the filtrate is not well-demonstrated.
Based on their experiences, Conley and Pitman (1960b) concluded that the alum dosage should be adjusted to achieve low levels of uncoagulated matter in the filtrate (low turbidity) early in the filter run (after 1 h), and that the filter-aiding polymer should be adjusted to the minimum level required to prevent terminal breakthrough of alum floc near the end of the run. At the same time, to prevent excessive head loss development, the dosage of polymer should not be higher than
necessary. Further research comparing dual media with a fine sand medium was reported by Robeck, Dostal, and Woodward (1964).They compared three filter media during filtration of alum-coagulated surface water. These comparisons were made by running filters in parallel, so the benefits of dual media were more conclusively demonstrated. The rate of head loss development for the dual-media filter was about one half of that for the sand medium, but the effluent turbidity was essentially the same prior to breakthrough, which was observed under some weak flocculation conditions.
The evidence clearly demonstrated lower head loss for a dual-media filter, as compared to a traditional fine sand filter. For a typical dual media with anthracite ES that is about double the sand ES, the head loss development rate should be about one-half the rate of the fine sand filter when both are operated at the same filtration rate on the same influent water.
The benefits gained by the use of dual-media filters led to the development of the triple-media filter, in which an even finer layer of high-density media (garnet or ilmenite) is added as a bottom layer .The bottom layer of finer material should improve the filtrate quality in some cases, especially at higher filtration rates. There is growing evidence to support that expectation.
The triple-media filter is sometimes referred to as a mixed-media filter because in the original development, the sizes and uniformity coefficients of the three layers were selected to encourage substantial intermixing between the adjacent layers. This was done to come closer to the presumed ideal configuration of “coarse to fine” filtration. The original patents have now expired, and other specifications for triple media are being used with various degrees of intermixing.
The initial clean-bed head loss will be higher for the triple-media filter due to the added layer of fine garnet or ilmenite. Thus, for a plant with a particular total available filter system head loss, the clogging head loss available to sustain the run is reduced, which may shorten the run compared to the run obtained with a dual media filter .
Some comparisons of triple media versus dual media have demonstrated triple media to be superior in filtered water quality. On Lake Superior water at Duluth, a mixed-media (triple-media) filter was superior to dual media in amphibole fiber removal.Twenty-nine out of 32 samples of filtrate were below or near detection level for the dual-media filter, and 18 out of 18 for the mixed-media filter. Mixed media was recommended for the plant. Mixed media was also reported to be superior in resisting the detrimental effects of flow disturbances on filtrate quality . In contrast, Kirmeyer (1979) reported pilot studies for the Seattle water supply in
which two mixed-media and two dual-media filters were compared. No difference in filtrate quality was observed in either turbidity or asbestos fiber content with filtration rates of 5.5 to 10 gpm/ft2 (13.4 to 24.4 m/h). Some differences in production per unit head loss were observed, favoring dual media at lower rates and mixed media at higher rates.
A number of laboratory studies have compared triple media versus single medium (fine sand) filters. All of these studies have clearly shown the head loss benefit gained by filtering in the direction of coarse grains to fine grains. Three of the studies also clearly showed benefits to the filtrate quality for the triple media.
Rates of Filtration
Fuller (1898) is commonly credited with establishing a standard rate of filtration of
2 gpm/ft2 (5 m/h) for chemically pretreated surface waters. This filtration rate was considered practically inviolable for the first half of the twentieth century in the United States. Fuller observed, however, that with properly pretreated water, higher rates gave practically the same water quality. Of equal importance, Fuller acknowledged that without adequate chemical pretreatment, no assurance of acceptable filtered water existed even at filtration rates of 2 gpm/ft2 (5 m/h).
In the 1950s and 1960s, many plant-scale studies were conducted comparing filter performance at different filtration rates. These studies were generally conducted as utilities were considering uprating existing filters, or building new plants with filtration rates higher than the traditional 2 gpm/ft2 (5 m/h).
The results of such studies as illustrated in Tables 8.4 and 8.5, and other similar studies demonstrated that higher filtration rates do result in somewhat poorer filtrate quality, as both theory and intuition would predict. However, at the time, the turbidity standard was 1 Jackson Turbidity Unit, and it was presumed that chlorine disinfection would handle any pathogens that happened to pass through the filters. Nevertheless, there was a gradual acceptance of filtration rates of up to 4 gpm/ft2 (9.8 m/h), with conventional pretreat ment and without the use of filter-aiding polymers to increase filtration efficiency. Pioneering work at even higher filtration rates, assisted by filter-aiding polymers, included the following studies. Robeck, Dostal, and Woodward (1964) compared performance of pilot filters at 2 to 6 gpm/ft2 (5 to 15 m/h), filtering alum-coagulated surface waters through single and dual-media filters.
They concluded that with proper coagulation ahead of the filters, the effluent turbidity, coliform bacteria, polio virus, and powdered carbon removal was as good at 6 gpm/ft2 (15 m/h) as at 4 or 2 gpm/ft2 (10 or 5 m/h). Pretreatment included activated silica when necessary to aid flocculation and a polyelectrolyte as a filter aid (referred to as a coagulant aid in the original article). The benefit of using filter-aiding polymers in retarding terminal breakthrough is shown in Figure 8.6.
Conley and Pitman (1960a) showed the detrimental effect of high filtration rates of up to 15 gpm/ft2 (37 m/h) in the treatment of Columbia River water, using alum coagulation followed by short detention flocculation and sedimentation before filtration. A proper dose of nonionic polymer added to the water as it entered the filters, however, resulted in the same filtrate quality from 2 to 35 gpm/ft2 (5 to 85 m/h). [Note that the turbidity unit being reported in Conley’s studies was later acknowledged (Conley, 1961) to be equivalent to about 50 Jackson Turbidity Units.]
However, the more recent concerns about Giardia and Cryptosporidium have emphasized the need to achieve filtered water turbidities at or below 0.10 ntu, and
have emphasized log reduction of cysts or cyst-sized particles to ensure the absence of protozoan pathogens. Some results of such studies were presented in the Introduction section of this chapter, and more follow.
When chemical pretreatment was optimized for turbidity and particle removal, resulting in filtered water turbidity below 0.10 ntu, Patania et al. (1995) found no difference in Giardia cyst and Cryptosporidium oocyst removal in pilot studies at Contra Costa, California, at filtration rates of 3 and 6 gpm/ft2 (7 to 15 m/h), and at Seattle at 5 and 8 gpm/ft2 (12 to 20 m/h). Conventional pretreatment preceded filtration at Contra Costa, whereas in-line filtration was used at Seattle. Both used deep-bed dual-media filters and dual coagulants (alum plus cationic polymer). Filter-aiding polymer was also used in the Seattle pilot plant. Design and operation guidelines for optimization of high-rate filtration plants were reported by Cleasby et al. (1989, 1992).The reports were based on a survey of 21 surface water treatment plants with consistent operational success in producing filtered water turbidity below 0.2 ntu at filtration rates at or above 4 gpm/ft2 (10 m/h).These plants were characterized by management support of a low-turbidity goal, optimal chemical pretreatment, the use of polymeric flocculation and/or filter aiding chemicals, the use of dual- or tri-media filters, continuous monitoring of each
filter effluent turbidity, and good operator training.
The turbidity and particle count results of this study were summarized by Bellamy et al. (1993) as presented in Tables 8.6 and 8.7. Table 8.6 shows that when source water turbidities were above 5 ntu, the log reductions from source water to finished water turbidity agreed well with log reductions in total particle count and cyst-sized particle count. Log reductions in turbidity and particle counts were 2-log or higher in all of the plants in Table 8.6. However, with lower source water turbidity (Table 8.7), the agreement between log reductions in turbidity and particle count was not good because of the inability to measure turbidities below about 0.05 ntu. In spite of this, the log reduction in cyst-sized particles was near 2-log except in 4 of the
11 plants treating such waters.
The use of unusually high filtration rates was reported at the Contra Costa
County Water District plant. By precoating the dual-media filters with a small dose
of polymer during the backwash operation, Harris (1970) reported successful operation at 10 gpm/ft2 (24 m/h). Harris also reported that the initial period of poorer water quality was eliminated by this precoating operation.The Contra Costa County Water District plant is now authorized by the State of California to operate at 10 gpm/ft2 (24 m/h).
Acceptable Run Length and Production per Run
Higher filtration rates have two conflicting impacts on plant production. First, the clean-bed head loss is increased. Thus, for a given amount of available head loss in the plant hydraulic profile, less clogging head loss is available to sustain the filter cycle. This will shorten the filter cycle length if it is terminated based on reaching maximum available head loss. Second, the head loss increases at a faster rate per hour at higher filtration rates because solids are captured at a faster rate. However,
the production per unit time also increases, so that production per day is increased substantially unless the filter cycles get too short. Therefore, the common importance attached to the length of filter run can be misleading. Rather than emphasizing the effect of filtration rate on length of run, one should emphasize the effect of filtration rate on filtrate quality, net plant production, and production efficiency.
Figure illustrates the effect of unit filter run volume on net water production at three filtration rates. Unit filter run volume is the actual throughput of a filter during one filter run; it also can be called gross production per filter run. The figure is based on the assumption that 100 gal/ft2 (4 m3/m2) is used for each backwash operation, and 30 min downtime is required per backwash. Similar figures can be constructed with other assumptions.The following example illustrates the preparation of the figure.
_ Assume four cycles per day per filter.
_ Use a run time per cycle = 360 min/cycle − 30 min/wash = 330 min/cycle.
_ Assume desired maximum filtration rate = 5 gpm/ft2.