Contrast Between Slow Sand Filter And Slow Sand Drift

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Register to read the introduction… 22 2.2 Contrast Between BioSand Filter and Slow Sand Filter................................... 24 2.3 The Schmutzdecke ............................................................................................ 25 2.4 Biological Removal Mechanisms ..................................................................... 25 2.4.1 2.4.2 Metabolic breakdown........................................................................ 26 Bacterivory........................................................................................ 26

2.5 Physical Removal Mechanisms ........................................................................ 27 2.5.1 2.5.2 Surface straining ............................................................................... 27 Inter-particle Attraction ....................................................................
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In recent years, this parasite has been recognized as a significant threat to potable supplies. The resistant stage – an oocyst – is relatively untouched by chlorine disinfection. In experiments performed by Thames Water Utilities, United Kingdom, slow sand filters reduced concentrations of Cryptosporidium oocysts by 99.997% from 4000/L to 0/8L (Timms et al., 1994). Another study in British Columbia by Fogel contradicts the aforementioned study (Fogel et al., 1993). Fogel found removal efficiencies of 48%; this figure is significantly different from the 100% removals from previous literature. However, a point to note concerning the British Columbia filters is that they were operating well out of the range of the recommended design limits for the uniformity coefficient1 at 3.5 (Fogel et al., 1993). Furthermore, temperature can adversely affect the performance of a slow sand filter; the British Columbia filters were operating at extremely low temperatures of less than 1°C (Fogel et al., 1993). Overall, the literature supports data that strongly suggests slow sand filtration is a viable option for Cryptosporidia removals. Although slow sand filtration often has been replaced by faster and more advanced high-rate filtration methods, its low cost, ease of operation, minimal maintenance requirements, and success in …show more content…
Prior to attachment, the particles are transported along flow streamlines unless they are captured by interception or transported across the streamlines causing them to reach a grain surface. If the conditions at the grain surface provide favorable particle-to-grain interaction, attachment will occur. The efficiency of particle attachment is related to the net attractive force between the medium (consisting of sand and previously removed particles) and suspended particles. Viscous forces hinder attachment or cause detachment by shearing particles from the medium. Shearing forces are expected to be the highest in the filter cake, because shearing forces increase as

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