Slow sand filtration

Slow sand filtration invdves removing material in suspension and/or dissolved in water by percolation at slow speed. In principle, a slow filter comprises a certain volume of areal surfiice, with or without construction of artificial containment, in which filtration sand is placed at a sufficient depth to allow free flow of water through the bed. When the available head loss reaches a limit of approximately 1 m, the filter must be pulled out of service, drained, and cleaned. The thickness of the usual sand layer is approximately of 1 - 1.50 m, but the formation of biochemically active deposits and clogging of the filter beds takes place in the few topmost centimeters of the bed. 

The filter mass is pored onto gravels of increasing permeability with each layer having a thickness of approximately 10 - 25 cm. The lower-permeability layer can reach a total thickness of 50 - 60 cm. So-called gravels 18 - 36 cm in size are used and their dimensions are gradually diminished to sizes of 10 - 12 cm or less for the upper support layer. 

Sometimes slow filtration is used without previous coagulation. This is generally practiced with water that does not ctmtain much suspended matter. If the water is loaded (periodically or permanently) with clay particles in suspension, pretreatment by coagulation-flocculation is necessary. Previous adequate oxidation of the water, in this case preozonization producing biodegradable and metabolizable orgairic derivatives issuing from disstdved substances, can be favorable because of the biochemical activity in slow filters. 

There are several disadvantages to the use of slow filters. They may require a significant surface area and volume, and may therefore involve high investment costs. 

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Explain the difference between, and compare the advantages and disadvantages between rapid- sand and slow-sand filtrations. 

In degradation experiments of radiolabelled LAS in soil columns, Branner et al. [29] observed that microbial transformation products, believed to be SPC, were virtually not retained on the column at all. Conversely, the results from the waterworks Rhine study [23] show that the subsequent slow sand filtration leads to a nearly total elimination of the SPC homologues. The residues detected in the water after this step... 

RookUdge S.J., E.R. Bums, and J.P. Bolte (2005). Modeling antimicrobial contaminant removal in slow sand filtration. Water Research 39 331-339. 

Bank filtration (slow sand filtration) Adsorption, biodegradation Decolorization, elimination of organic carbon... 

Eighmy, T. T., M. R. Collins, and J. R, Malley Jr. (1993). Biologically Enhanced Slow Sand Filtration for Removal of Natural Organic Matter. Am. Water Works Assoc., Denver. 

In water treatment besides particle size and shape, an important characteristic of particles in suspension, be it colloidal clay particles, floes or algae, is their clogging properties in relation to rapid and slow sand filtration and membrane filtration. 

MFI measurements are based on the formation of a cake or gel when water containing colloids are filtered through a membrane filter. Recently the use of the MFI parameter has been studied in relation to slow sand filtration performance 5 and in relation to clogging of injection wells [6 ]. 

Additional work however is needed to assess the significance of these treshold values in their relation to clogging of slow sand filtration. 

Slow sand filtration Established Especially small, but all sizes Operationally simple low cost, but requires large land areas... 

Conventinal filtration Direct filtration Slow sand filtration Package plant filtration Diatomaceous earth filtration Membrane filtration (reverse osmosis)... 

Conventional treatment, which includes coagulation, flocculation, clarification (sedimentation or flotation), and filtration, along with disinfection, can achieve 99.9% inactivation of Giardia cysts and 99.99% inactivation of enteric viruses when properly designed and operated. Direct filtration, slow sand filtration, and diatomaceous earth filtration systems, each combined with disinfection, have also achieved these reductions. 

A 50,000 GPD slow sand filtration water treatment plant supplies a community of 500 people with drinking water from a reservoir in a protected watershed. The raw water supply has the following characteristics (a) turbidity = 5-10 NTU (b) total estimated Giardia cyst level = less than 1 per 100 mL (c) pH = 6.5-7.5 (d) temperature = 5-15°C. 

Donner, C., Remmier, F., ZuUei-Seibert, N., et al. (2002). Enhanced removal of herbicides by different in-site barrier systems (GAC, FAC, anthracite, lignite coke) in slow sand filtration.FFijier Sci. TechnoL-. Water Supply, 2(1), 123-8. 

Carlo PL, Owens LP, Hanna Jr. GP, et al. 1992. The removal of selenium from water by slow sand filtration. Proceedings of the Sixteenth Biennial Conference of the International Association of Water Pollution Research and Control. Water Sci Technol 26(1-11) 2137-2140. 

Slow sand filtration has been the classical form of biological treatment and it has been used as a means to reduce water-borne outbreaks of typhoid and cholera [30]. A biofilm is formed on the surface of the filter, which removes bacteria, protozoa, viruses and also organic nitrogen. The mechanical process of filtration of the water through the sand bed increases removal of microorganisms as well as various chemical constituents. 

Protozoa may become another problem of water treatment. Usually, removal or inactivation of Entamoeba has been the central problem, but recent investigations also concerns Giardia [43]. Proper coagulation, flocculation, sedimentation, and filtration can remove up to 99% of protozoan cysts slow sand filtration can remove Giardia cysts [16]. Improper operation of treatment facilities can cause outbreaks of water-borne illnesses caused by protozoa [44]. Chlorination and disinfectants such as chloramines and ozone can further reduce levels of protozoa [45-47]. 

R. Collins (eds.), Advances in Slow Sand Filtration and Alternative Biological Filaation, John Wiley Sons, New York, 1996. 

They can also form in deep bed filtration, in which a dispersion is passed through a bed of granular material such as fine sand (this is also known as depth filtration, granular media filtration or slow sand filtration). 

High 0.2 300 Surface derived waters with a total organic content >3 mg/l, including high alkalinity waters affected by algae or containing treated sewage effluent. Particularly the case where water treatment is based on slow-sand filtration or where physico-chemical treatment is limited or poorly controlled. 

The graded qnartz sand with a size between 0.3 and 2.0 mm is used in direct-flow sand filters. The height of sand bed in these filters is between 0.5 and 2 m. The process of water clarification is realized as qnick sand filtration or slow sand filtration depending on the fraction of used sand. In the first case, the filtration rate is about 5.5-14 m/h, and in the second case, it is about 0.1-0.2 m/h. The parameters of these processes were taken from [22-24]. They are represented in Table 8.1. The depicted data allow to determine the rate constants of water clarification at quick and slow sand filtration nsing Equation 8.32. For slow sand filtration with the height of packed bed equal to 0.5 m, the calculation gives the value of rate constant of water clarification equal to T.T-IO"" s", if the initial turbidity level is 1500 mg/L, finish turbidity level is 1.5 mg/L, and filtration rate is 0.2 m/h. And for fast sand filtration, the value of rate constant of water clarification is equal to 5.26-10 s , if water turbidity is reduced by 50%, the height of filter bed is 2 m, and filtration rate is 5.5 m/h. 

Charles River. Water from the river was more polluted than that from the wells, and therefore had to be filtered. The company installed a slow-sand filtration system, which not only eliminated bacterial contaminants, it also reduced the lead solvency of the river water." The introduction of these measures in 1902 and 1903 reduced the average water-lead levels in area homes from 1.39 ppm (627 times the modern EPA standard) to 0.27 ppm (175 times the modern EPA standard). Although 0.27 ppm is a high lead level by modern standards, it was well below the 0.5-ppm threshold then considered safe by the Massachusetts Board of Health." ... 

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