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CHAPTER GRANULAR BED AND PRECOAT FILTRATION John L Cleasby, Ph.D., P.E Professor Emeritus Department of Civil and Construction Engineering Iowa State University Ames, Iowa Gary S Logsdon, D.Sc., P.E Director, Water Process Research Black and Veatch, Engineers-Architects Cincinnati, Ohio AN OVERVIEW OF POTABLE WATER FILTRATION The filtration processes discussed in this chapter are used primarily to remove particulate material from water Filtration is one of the unit processes used in the production of potable water Particulates removed may be those already present in the source water or those generated during treatment processes Examples of particulates include clay and silt particles; microorganisms (bacteria, viruses, and protozoan cysts); colloidal and precipitated humic substances and other natural organic particulates from the decay of vegetation; precipitates of aluminum or iron used in coagulation; calcium carbonate and magnesium hydroxide precipitates from lime softening; and iron and manganese precipitates Types of Filters A number of different types of filters are used in potable water filtration, and they may be described by various classification schemes The granular bed and precoat filters discussed herein are comprised of porous granular material In recent years, interest has grown in the use of membrane filtration in place of, or in addition to, granular bed filtration Membrane processes are discussed in Chapter 11 8.1 8.2 CHAPTER EIGHT FIGURE 8.1 Company.) A rapid sand filtration system (Source: Courtesy of F B Leopold One classification scheme for granular bed filters is based on the type of media used These filters commonly use a substantial depth of sand or anthracite coal or granular activated carbon or combinations thereof A typical granular bed filter is shown in Figure 8.1 In contrast, precoat filters use a thin layer of very fine medium such as diatomaceous earth (DE) that is disposed of after each filter cycle—typically about a day in duration Recovery, cleaning, and reuse of the medium is possible but not common A typical precoat filter is shown in Figure 8.2, with the circular flatplate septa that support the precoat Filters also may be described by the hydraulic arrangement employed to pass water through the medium Gravity filters are open to the atmosphere, and flow through the medium is achieved by gravity, such as shown in Figure 8.1 Pressure filters utilize a pressure vessel to contain the filter medium Water is delivered to the vessel under pressure and leaves the vessel at slightly reduced pressure The two systems are merely two ways to provide a hydraulic gradient across the filter Filters may also be described by the rate of filtration, that is, the flow rate per unit area Granular bed filters can be operated at various rates, for example, rapid granular bed filters provide higher filtration rates than slow sand filters, which favor surface removal of particulates at the top of the sand bed Finally, filtration can be classified as depth filtration if the solids are removed within the granular material, or cake filtration if the solids are removed on the entering face of the granular material Rapid granular bed filters are of the former type, while precoat and membrane filters are of the latter type Slow sand filters utilize both cake and depth mechanisms, as will be explained later After a period of operation referred to as a filter cycle, the filter becomes clogged with removed particulates and must be cleaned Rapid filters are cleaned by backwashing, using an upward, high-rate flow of water Slow sand filters are cleaned by scraping off the dirty layer from the surface Thus, a filter can be fully described by an appropriate choice of adjectives For example, a rapid, gravity, dual-media filter would describe a deep bed comprised of two media (usually anthracite coal on top of sand) operated at high enough rates to GRANULAR BED AND PRECOAT FILTRATION 8.3 FIGURE 8.2 Precoat filter of rotating leaf type (sluice type during backwash) (Source: Courtesy of Manville Products Corporation.) encourage depth removal of particulates within the bed, and operated by gravity in an open tank Dominant Mechanisms, Performance, and Applications Cake filtration of particulates involves physical removal by straining at the surface In addition, for the slow sand filter the surface cake of accumulated particulates includes a variety of living and dead micro- and macroorganisms The biological metabolism of the organisms causes some alteration in the chemical composition of the water, and the development of this dirty layer (or schmutzdecke) enhances removal of particulates as well As the filter cake develops, the cake itself assumes a dominant role in particulate removal Because of this, filtrate turbidity improves as the filter run progresses, and deterioration of the filtrate turbidity is normally not observed at the end of the filter cycle Because the mechanism of cake filtration is largely physical straining, chemical pretreatments such as coagulation and sedimentation are not generally provided To obtain reasonable filter cycles, however, the source water must be of quite good quality (which will be defined later) In contrast, depth filtration involves a variety of complex mechanisms to achieve particulate removal Particles to be removed are generally much smaller than the size of the interstices formed between filter grains Transport mechanisms are needed to carry the small particles into contact with the surface of the individual filter grains, and then attachment mechanisms hold the particles to the surfaces These mechanisms will be discussed in more detail later Chemical pretreatment is essential to particulate removal in depth filtration It serves to flocculate the colloidal-sized particulates into larger particles, which enhances their partial removal in pretreatment processes (such as sedimentation, flotation, or coarse-bed filtration, all located ahead of the filter) and/or enhances the transport mechanisms in filtration In addition, chemical pretreatment enhances the 8.4 CHAPTER EIGHT attachment forces retaining the particles in the filter The focal point of particulate removal in depth filtration moves progressively deeper into the bed as the cycle progresses, and if the cycle continues long enough, deterioration of the filtrate may be observed The provision of pretreatment makes the depth filtration process more versatile in meeting a variety of source water conditions With appropriate coagulation, flocculation, and solids separation ahead of depth filtration, source waters of high turbidity or color can be treated successfully Better quality source waters may be treated by coagulation, flocculation, and depth filtration, a process referred to as direct filtration, or by in-line filtration, which utilizes only coagulation and very limited flocculation before depth filtration In some cases, biological metabolism will result in partial removal of biodegradable organic matter in depth filters, if biological growth is allowed to develop in the filter medium This is especially important if ozonation precedes filtration Chapter 13 includes information on biological filtration Regulatory Requirements for Filtration The U.S Environmental Protection Agency’s (USEPA) Surface Water Treatment Rule (SWTR), promulgated on June 29, 1989 (Federal Register 40 CFR Parts 140 and 141, p 27486–27568), requires community water systems to disinfect all surface waters and requires filtration for most surface water sources The Surface Water Treatment Rule imposed stricter turbidity limits on filtration processes and made them specific to the type of process used (see Table 8.1) The total extent of inactivation and physical removal must be at least 3-log (99.9 percent) for Giardia cysts and 4-log (99.99 percent) for viruses The supplementary information published with the rule presented recommended minimum levels of disinfection and assumed log removals to be credited to the following defined filtration processes: conventional filtration, direct filtration, diatomaceous earth filtration, and slow sand filtration Conventional filtration and direct filtration were defined in the SWTR to include chemical coagulation; flocculation; and in the case of conventional filtration, sedimentation; ahead of the filtration process Slow sand filtration was defined as filtration of water, without chemical coagulation, through a bed of sand at rates of up to 0.16 gpm/ft2 (0.40 m/h) Filtration processes that not function on the principles of TABLE 8.1 SWTR Assumed Log Removals and Turbidity Requirements Log removals* Giardia Virus Turbidity requirement Conventional Filtration process 2.5 2.0 = or < 0.5 ntu in 95% of samples each month and never > ntu Direct 2.0 1.0 = or < 0.5 ntu in 95% of samples each month and never > ntu Slow sand 2.0 2.0 = or < ntu in 95% of samples each month** and never > ntu Diatomaceous earth 2.0 1.0 = or < ntu in 95% of samples each month and never > ntu * From Table IV-2 in Supplementary Information, p 27511 ** Special provision was made for slow sand filters to exceed ntu in some cases, providing effective disinfection was maintained GRANULAR BED AND PRECOAT FILTRATION 8.5 the processes defined in the SWTR are called alternative filtration processes, and the log removal for Giardia cysts or viruses that can be allowed for alternative processes must be determined for each alternative process Removal of Microorganisms by Granular Bed and Precoat Filtration In North America, waterborne disease outbreaks caused by Giardia lamblia and Cryptosporidium parvum, pathogenic protozoa with high resistance to disinfectants, have resulted in numerous studies of pilot plants and full-scale treatment plants to evaluate removal of microorganisms Many of these studies focused on filtration process trains involving coagulation, but some investigations of the efficacy of slow sand filtration and diatomaceous earth filtration have also been carried out Pilot plant studies (Logsdon et al., 1985; Al-Ani et al., 1986) established the need for attaining effective coagulation and filtered water turbidity in the range of 0.1 to 0.2 ntu for effective removal of Giardia cysts Additional research by Nieminski and Ongerth (1995) on Giardia and Cryptosporidium confirmed the necessity of attaining low filtered water turbidity and the importance of maintaining proper coagulation chemistry They evaluated a small (4900 m3/d) plant for removal of protozoan cysts and concluded that a properly operated treatment plant producing finished water turbidity of 0.1 to 0.2 ntu, using either the direct filtration mode or the conventional treatment mode, could achieve 3-log removal of Giardia cysts and about 2.6-log removal of Cryptosporidium However, when they corrected the test results for recovery efficiency in both the influent and the effluent samples, they reported 3.7- to 4.0-log removals for Giardia and Cryptosporidium in both conventional treatment and direct filtration The similarity of results for direct filtration and conventional treatment is different from the outcome reported by Patania et al (1995), who observed 1.4- to 1.8-log additional removal for Cryptosporidium and 0.2- to 1.8-log additional removal for Giardia when sedimentation was included in the treatment train, in comparison with in-line filtration treatment employing only coagulation and filtration Nieminski and Ongerth (1995) reported that oocyst removals calculated on the basis of the oocyst concentration in the seed suspension being fed to the raw water were 1-log higher than oocyst removals calculated on the basis of the concentration of oocysts actually measured in the seeded influent water, which is the more rational method of evaluation LeChevallier et al (1991) examined raw and filtered water samples from 66 surface water treatment plants in 14 states and Canadian province Log removals of Giardia and Cryptosporidium ranged from less than 0.5 to greater than 3.0 Log removals averaged slightly above 2.0 for each organism Some of these plants were practicing disinfection (usually chlorination) during clarification, so the Giardia results may have been influenced somewhat by disinfection They reported that production of low turbidity water (