Rapid Sand Filter In Hindi Video Civil Engineering
Rapid Sand Filters
Rapid sand filters are divided into two main types: (1) gravity filters and (2) pressure filters. The principles of the two types of filters are identical. The pressure filter is operated at elevated pressures, thus prolonging the filter cycle and/or increasing the rate of flow of water through the filter. Gravity filters are commonly operated at 2 GPM/sq ft*, whereas pressure filters are operated at 3 GPM/sq ft and higher.
The rapid sand filter is operated with clarification ahead of the filter. This step reduces the load on the filter, allowing longer filter runs and high-quality effluent at higher flow rates. Rapid sand filters have a layer of sand on layers of graded gravel and do not utilize a “Schmutzdecke” layer for the filtration action. Instead, the particulate matter is adsorbed on the sand in the layers below the surface. A considerable amount of support for the adsorption of solids (causing turbidity) as the predominant removal mechanism of rapids and filters was gained from the report of O'Melia and Crapps (1964) in their study on the chemical aspects of filtration.
Rapid sand filters are customarily operated with sand on top of a graded gravel bed. A considerable amount of interest, however, has been shown in some areas in the use of sized coal in place of sand. Coal has the advantage of lower density, occupying greater volume per unit weight and, more important, requiring lower velocity of the backwash water to suspend the coal bed during the washing or scrubbing cycle. Coal, however, is soft and abrades rapidly with reduction in particle size. This results in losses during the backwash cycle and, consequently, coal replacement is much more frequent than that of sand.
A skid-mounted bank of three high-rate rapid sand filters ready for shipment to the field is presented in Figure 7.7. Figure 7.8 is a cutaway drawing of a high-rate rapid sand filter showing the internals and the media. Figure 7.9 shows the inlet distributor, whereas Figure 7.10 shows the bottom drain collector for a high-rate rapid sand filter. The openings are spaced to obtain an equal flow through each.
Filter Media
The sand filter designs use either graded sand (fine to coarse or heterogeneous) or coarse monograde sand (uniform size or homogeneous). There is no single media specification (size and depth) that can be applied universally for all waters. The choice depends on the water quality and upstream processes, filtered water quality objectives, cleaning method, filtration rate and length of filter runs. In graded sand filters the bed depth typically comprises 0.7 m of 0.6–1.18 mm fine sand (effective size 0.75 mm), 0.1 m of 1.18–2.8 mm coarse sand, 0.1 m of 2.36–4.75 mm fine gravel and 0.15 m of 6.7–13.2 mm coarse gravel. For applications requiring a finer sand the two upper layers are changed to 0.7 m of 0.5–1.0 mm sand (effective size 0.55 mm) and 0.1 m of 1.0–2.0 mm coarse sand, the gravel layers remaining the same. Effective size = size of aperture through which 10% by weight of sand passes (D10). Depending on the slot size of the nozzles the bottom gravel layer can be omitted and replaced by more of the adjoining media. The homogeneous sand filter has a 0.9–1 m deep bed and typically of 0.85–1.7 mm of sand (effective size 0.9 mm) placed on a 50 mm layer of 4–8 mm or 75 mm of 6.7–13.2 mm gravel. Homogeneous sand of effective size up to 1.3 mm has also been used. The stated size ranges for sand and gravel are generally 5 and 95 percentiles. For estimating the sand depth some employ the rule that the depth of sand should be ≥1000 times its effective size (Kawamura, 2000). Some filter plant designers use the term ‘hydraulic size’ in place of effective size (Stevenson, 1994). It is defined as the size particles would have to be, if all were the same size, in order to match the surface area of a sample covering a range of sizes. For media with size range 1:2 hydraulic size is approximately 1.36 × the lower size in the range, for example for 0.85–1.7 mm sand it is 1.16 mm.
Other filter media such as anthracite (Section 8.6), granular activated carbon (Section 8.8), garnet, pumice (Farizoglu, 2003), expanded clay or glass are used in filtration application. Garnet is a dense (s.g. 3.8–4.2) medium which is used as the bottom layer of multimedia filters containing anthracite and sand. It occupies about 15% of the bed depth and the effective size could be as low as 0.35 mm. Being dense, it requires about 3 times the wash rate as anthracite to give the same bed expansion. Pumice and expanded clay are porous media and could be used in biological filtration (Sections 10.12 and 10.29). Glass is a suitable filter medium of similar specific gravity to sand.
The sand should be of the quartz grade with a specific gravity in the range 2.6–2.7. The uniformity coefficient (UC) should be less than 1.6 and usually lies between 1.3 and 1.5. Bulk density is about 1.56 g/cc.
where D60 is the size of aperture through which 60% of sand passes and D10 is the size of aperture through which 10% of sand passes. Lower UC values would make the medium costly as a high proportion of fine and coarse medium is discarded and higher values would reduce the voidage. Typically sand has a voidage of 37–40%. Voidage = 100 × (particle density—bulk density)/particle density. Loss in weight on ignition at 450°C should be <2% and the loss in weight on acid washing ( hydrochloric acid for 24 hours at 20°C) should be <2%. The sand should not be too friable to ensure that washing operations do not produce fines. It should therefore be tested for friability (BW, 1996).
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