Slow sand filters

Neither rapid sand nor mixed-media filters remove appreciable quantities of coUoidal particles without adequate pretreatment. Although it is widely beheved that filters are an effective barrier against unsafe water, the effluent may be as colored, as turbid, or as bacteriologicaHy unsafe as the water appHed. In contrast, slow sand filters requite no pretreatment, as the slow passage through the bed allows the particles to contact and attach to the schmut ecke. 

The growth of community water supply systems in the United States started in the early 1800s. By 1860, over 400, and by the turn of the century over 3000 major water systems had been built to serve major cities and towns. Many older plants were equipped with slow sand filters. In the mid 1890s, the Louisville Water Company introduced the technologies of coagulation with rapid sand filtration. 

Porosity constitutes a important criterion in a description based on straining. Porosity is determined by the formula V /Vc, in which V c is the total or apparent volume limitated by the filter wall and is the free volume between the particles. The porosity of a filter layer changes as a function of the operation time of the filters. The grains become thicker because of the adherence of material removed from the water, whether by straining or by some other fixative mechanism of particles on the filtering sand. Simultaneously the interstices between the grains diminish in size. This effect assists the filtration process, in particular for slow sand filters, where a deposit is formed as a skin or layer of slime that has settled on the... 

For the purification of water supplies and for waste water treatment where the solid content is about 10 g/m3 or less, as noted by Cleasby(23) granular bed filters have largely replaced the former very slow sand filters. The beds are formed from granular material of grain size 0.6-1.2 mm in beds 0.6-1.8 m deep. The very fine particles of solids are removed by mechanical action although the particles finally adhere as a result of surface electric forces or adsorption, as Ives 24 points out. This operation has been analysed by Iwasaki 25) who proposes the following equation ... 

A review of the evolution of filter media design will provide a better insight into the differences between media systems. The slow sand filter was developed in... 

It may be noticed that some of the filters discussed are operated continuously and some are not. For example, the rapid sand filter, the slow sand filter, the pressure filter, and the rotary vacuum filter are all operated continuously. The plate-and-frame press is operated as a batch. Thus, filters may also be classified as continuous and discontinuous. Only the plate-and-frame press is discussed in this chapter as a representation of the discontinuous type, but others are used, such as the shell-and-leaf filters and the cartridge filters. The first operates in a mode that a leaf assembly is inserted into a shell while operating and retracted out from the shell when it is time to remove the cake. The second looks like a cartridge in outward appearance with the filter medium inside it. The medium could be thin circular plates or disks stacked on top of each other. The clearance between disks serves to filter out the solids. 

Example 7.2 A sharp filter sand has the sieve analysis shown below. The porosity of the nnstratified bed is 0.39, and that of the stratified bed is 0.42. The lowest temperatnre anticipated of the water to be filtered is 4°C. Find the head loss if the sand is to be nsed in (a) a slow-sand filter 76 cm dee ) operated at 9.33 mim d and (b) a rapid-sand filter 76 cm deep operated at 117 m /m d. 

Slow-sand filter—Gravity filter normally operated at a rate of 1.0 to 10 mVd m. ... 

Smutzdecke— Layer of dirt that collects on top of slow-sand filters. 

For the sharp filter sand of sieve analysis of Problem 7.10, determine the flow rate applied to the filter in mVm d if the sand is to be used in (a) a slow-sand filter of average porosity of 0.39 and (b) a rapid-sand filter of average porosity of 0.42. The temperature of the water to be filtered is 20°C. The head losses in the slow-sand and rapid-sand filters are 0.06 m and 0.71 m, respectively and the bed depths are 76 cm, respectively. 

The volume of water filtered through the plankton sampling net was carefully noted. For raw water, e.g. surface-water or reservoir water and influent slow sand filter, a sample of 15—20L was collected. For treated water, e.g. filtrate of rapid filters, 150 L samples were taken. The concentrate, collected into a small cylindrical container, was quantitatively carried over into a calibrated small polyethene bottle after that the small container was immediately rinsed with demi-water and the calibrated recipient was filled up to 50 mL. Netplankton samples were preserved with formalin. If necessary, further concentration was performed by a secondary nitration through a plankton net with size aperture of 30 nm. A suvolume of 50 juL of the concentrated sample, taken with a micropipet, was used for mounting and counting the preparation by means of a compound microscope. 

An overall removal/inactivation of 3-logs for Giardia and 4-logs for viruses is sufficient for this water treatment system. The Primacy Agency credits the slow sand filter, which produces water with turbidity ranging from 0.6 to 0.8 NTU, with a 2-log Giardia and virus removal. Disinfection must achieve an additional 1-log Giardia and 2-log virus removal/inactivation to meet overall treatment objectives. 

The slow sand filter has a dimension of approx 34 ft L x 20 ft W, and is packed with... 

Slow sand filters are large beds of sand through which the water passes slowly. These beds are cleaned occasionally by draining and removing the upper surface of slime and dirt. 

Bourne, D.G. et al.. Biodegradation of the cyanobacterial toxin microcystin LR in natural water and biologically active slow sand filters. Water Res., 40, 1294, 2006. 

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