16593 08 tủ tài liệu bách khoa
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16593 08 tủ tài liệu bách khoa
_____CHAPTERS_____GRANULAR BED AND PRECOAT FILTRATIONJohn L. Cleasby, Ph.D., P.E.Professor Emeritus Department of Civil anti Construction Engineering 16593 08 tủ tài liệu bách khoa Iowa State UniversityAmes, IowaGary s. Logsdon, D.Sc., P.E.Director. Water Process Research Black anti Veaich. Engineers-Architects Cincinnati, OhioAN OVERVIEW OF POTABLE WATER FILTRATIONThe filtration processes discussed in this chapter are used primarily to remove particulate material from water. 16593 08 tủ tài liệu bách khoa Filtration is one of the unit processes used in the product ion of potable water. Particulates removed may be those already present in the source wat16593 08 tủ tài liệu bách khoa
er or those generated during treatment processes. Examples of particulates include clay and silt panicles; microorganisms (bacteria, viruses, and prot_____CHAPTERS_____GRANULAR BED AND PRECOAT FILTRATIONJohn L. Cleasby, Ph.D., P.E.Professor Emeritus Department of Civil anti Construction Engineering 16593 08 tủ tài liệu bách khoa or iron used in coagulation: calcium carbonate and magnesium hydroxide precipitates from lime softening: and iron and manganese precipitates.Types of FiltersA number of different types of filters are used in potable water filtration, and theymay be described by various classification schemes. The g 16593 08 tủ tài liệu bách khoaranular bed and precoat filters discussed herein arc comprised of porous granular material. In recent years. intĩarii>?f 11 •11' /It-riiiin tn thd Iif16593 08 tủ tài liệu bách khoa
ia Ilf ntùmhrfiltr-itiim tn 1*1 i'll Ilf nr in 'izlilitmn tn8.10CHAIT ER EIGHTFixed-Bed Porosity. Fixed-bed porosity is the ratio of void volume to to_____CHAPTERS_____GRANULAR BED AND PRECOAT FILTRATIONJohn L. Cleasby, Ph.D., P.E.Professor Emeritus Department of Civil anti Construction Engineering 16593 08 tủ tài liệu bách khoaad loss, and the solids-holding capacity of the medium. Fixed bed porosity is affected by the grain spheric ity; angular grains (i.e., those with lower sphericity) have higher fixed-bed porosity (Cleasby and Fan. 1981), as is evident in Table 8.2. For the low ƯC commonly specified for filter media, 16593 08 tủ tài liệu bách khoauc has no effect on porosity. However, for natural materials with very high ƯC, the nesting of small grains within the voids of the large grains can r16593 08 tủ tài liệu bách khoa
educe the average bed porosity.Fixed-bed porosity IS determined by placing a sample of known mass and density in a transparent tube of known internal _____CHAPTERS_____GRANULAR BED AND PRECOAT FILTRATIONJohn L. Cleasby, Ph.D., P.E.Professor Emeritus Department of Civil anti Construction Engineering 16593 08 tủ tài liệu bách khoavided by the density. The void volume is thus the bed volume minus the grain volume. The fixed-bed porosity is substantially affected by the extent of compaction of the medium placed in the column and by the column diameter. The loose-bed porosity can be measured in a column of water. If the bed is 16593 08 tủ tài liệu bách khoaagitated by inversion and then allowed to settle freely in the water with no compaction, the highest porosity (i.e.. the loose-bed porosity) will be o16593 08 tủ tài liệu bách khoa
btained. It may be as much as 5 percent greater than the porosity measured alter gentle compaction of the bed. Materials of lower sphericity show grea_____CHAPTERS_____GRANULAR BED AND PRECOAT FILTRATIONJohn L. Cleasby, Ph.D., P.E.Professor Emeritus Department of Civil anti Construction Engineering 16593 08 tủ tài liệu bách khoalter. This can be important in small pilot scale filter columns because it causes the average bed porosity to be higher than in full-scale filters, and affects the particulate removal and head loss behavior during filtration studies. It is common to make the diameter of such filter columns at least 16593 08 tủ tài liệu bách khoa50 times the grain size of the coarser filter grains to be studied (e.g.. di>) to minimize such wall effects.Sieve Analysis ConsiderationsThe standard16593 08 tủ tài liệu bách khoa
procedure for conducting a sieve analysis of a filter medium is detailed in A STM Standard Test Cl 36-92 (ASTM. 1993). This standard docs not specify_____CHAPTERS_____GRANULAR BED AND PRECOAT FILTRATIONJohn L. Cleasby, Ph.D., P.E.Professor Emeritus Department of Civil anti Construction Engineering 16593 08 tủ tài liệu bách khoaient period and in such manner that, after completion, not more than 1 weight percent of the residue on any individual sieve will pass that sieve during one minute of hand sieving” conducted in a manner described in the AST M standard. With softer materials such as anthracite coal or G AC, abrasion 16593 08 tủ tài liệu bách khoaof the material may occur when attempting to meet the 1 percent passing test.TABLE 8.2 Typical Properties of Common Filter Media for Granular-Bed Fill16593 08 tủ tài liệu bách khoa
ers (Clcasby and Fan, 19X1: Dharma rajah and Clcashy, 1986: Clcasby and Woods, 1975)Silica sandAnthracite coalGranular activated carbonGarnetIlmeniteG_____CHAPTERS_____GRANULAR BED AND PRECOAT FILTRATIONJohn L. Cleasby, Ph.D., P.E.Professor Emeritus Department of Civil anti Construction Engineering 16593 08 tủ tài liệu bách khoa44GRANULAR BED AND PRECOAT FILTRATION8.11When sieving a 100-g sample of hard materials such as sand, on 8-in sieves, and using a Ro-Tap type of sieving machine, it is common to require three sieving periods of 5 min each to satisfy the 1 percent passing test. The Ro-Tap machine imparts both a rotary 16593 08 tủ tài liệu bách khoa shaking and a vertical hammering motion to the nest of sieves. With some other sieving machines, the ASTM requirement will not be achieved even after16593 08 tủ tài liệu bách khoa
three 5-min periods of sievingWhen sieving anthracite coal, the sample should be reduced to 50 g because of its lower density.The Ro-Tap machine shou_____CHAPTERS_____GRANULAR BED AND PRECOAT FILTRATIONJohn L. Cleasby, Ph.D., P.E.Professor Emeritus Department of Civil anti Construction Engineering 16593 08 tủ tài liệu bách khoathracite, yielding a more erroneous result.Because of sieving and sampling difficulties, and because of the tolerance allowed in manufacture of the sieves themselves (ASTM Standard Test El 1-87, Wire-cloth Sieves for Testing Purposes, 1993). a reasonable tolerance should be allowed in the specified 16593 08 tủ tài liệu bách khoasize when specifying filter media. Otherwise, producers of filter material may not be able to meet the specification, or a premium price will be charg16593 08 tủ tài liệu bách khoa
ed. A tolerance of 1Ơ percent plus or minus IS suggested. For example, if a sand of 0.5 mm ES is desired, the specification should read 0.45 to 0.55 m_____CHAPTERS_____GRANULAR BED AND PRECOAT FILTRATIONJohn L. Cleasby, Ph.D., P.E.Professor Emeritus Department of Civil anti Construction Engineering 16593 08 tủ tài liệu bách khoahe prior understanding of the importance of various properties of granular filtering materials.Table 8.2 is presented to illustrate typical measured values for some properties, rhe large difference in grain densities evident 111 Table 8.2 allows the construction of dual- and triple-media filters, wi 16593 08 tủ tài liệu bách khoath coarse grains of low-density material on top and finer grains of higher density beneath. Alluvial sands have the highest sphericity, and crushed ma16593 08 tủ tài liệu bách khoa
terials such as anthracite, ilmenite, and some garnet have lower sphericity. Some anthracites contain an excessive amount of fiat, elongated jagged gr_____CHAPTERS_____GRANULAR BED AND PRECOAT FILTRATIONJohn L. Cleasby, Ph.D., P.E.Professor Emeritus Department of Civil anti Construction Engineering 16593 08 tủ tài liệu bách khoa-bed porosity). An approximate empirical relationship between sphericity and loose-bed porosity was used in developing a predictive model for fluidization that will be presented later (Cleasby and Fan. 1981).HYDRAULICS OF FLOW THROUGH POROUS MEDIAHead Loss for Fixed-Bed FlowThe head loss (Ĩ.C., pres 16593 08 tủ tài liệu bách khoasure drop) that occurs when clean water flows through a clean filter medium can be calculated from well-known equations.The flow through a clean filte16593 08 tủ tài liệu bách khoa
r of ordinary grain size (i.e.,0.5 min to 1.0 mm) at ordinary filtration velocities (2 to 5 gpm/fth 4.9 to 12.2 m.''h) would be in the laminar range o_____CHAPTERS_____GRANULAR BED AND PRECOAT FILTRATIONJohn L. Cleasby, Ph.D., P.E.Professor Emeritus Department of Civil anti Construction Engineering 16593 08 tủ tài liệu bách khoare dimensionally homogeneous*):8.12CHAPTER EIGHT_____CHAPTERS_____GRANULAR BED AND PRECOAT FILTRATIONJohn L. Cleasby, Ph.D., P.E.Professor Emeritus Department of Civil anti Construction EngineeringGọi ngay
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