Rapid sand filters

Rapid-sand filters force water through a 0.45-lm layer of sand (d=0.4-1.2mm) and work faster, needing a smaller area. They need frequent back-washing. 

Precipitation is often applied to the removal of most metals from wastewater including zinc, cadmium, chromium, copper, fluoride, lead, manganese, and mercury. Also, certain anionic species can be removed by precipitation, such as phosphate, sulfate, and fluoride. Note that in some cases, organic compounds may form organometallic complexes with metals, which could inhibit precipitation. Cyanide and other ions in the wastewater may also complex with metals, making treatment by precipitation less efficient. A cutaway view of a rapid sand filter that is most often used in a municipal treatment plant is illustrated in Figure 4. The design features of this filter have been relied upon for more than 60 years in municipal applications. 

It should be noted that the total loss of head of a filter bed is in inverse ratio to the depth of penetration of the matter in suspension. In a normal wastewater treatment plant, the water is brought onto a series of rapid sand filters and the impurities are removed by coagulation-flocculation-filtration. Backwashing is typically performed in the counterfiow mode, using air and water. One type of common filter is illustrated in Figure 6, consisting of closed horizontal pressurized filters. 

Belfort, Georges "Evaluation of a Rapid Sand Filter", Filtration Experiment, Rensselaer Polytechnic Institute, 1990. 

No plant-specific identification number was available for this facility. The wastewater from Plant B contains pollutants from both metals processing and finishing operations. It is treated by precipitation-settling followed by filtration with a rapid sand filter. A clarifier is used to remove much of the solids load. Table 5.14 summarizes the data on pollutant removal efficiency at Plant B. 

In the early 1900 s, the rapid sand filter was introduced, together with chemical coagulation and periodic backwashing of the filter. Effective size of the rapid sand filter media might vary from 0.3S to 1.0 mm typical size is 0.5 mm. This type of filter has demonstrated the ability to remove effectively applied suspended solids of 5 to 10 ppm at flow rates of 2 to 3 gal/min per square foot of bed area. 

Fig. 1 describes a typical rapid sand filter. Note that hydraulic grading of the sand occurs in the backwash cycle, with the very finest sand forming the top of the bed. Filtration occurs in the top few inches. Once a suspended solid passes the top layer, larger and larger voids occur, and the chance of passing through the filter is greatly increased. 

Fig. 1—Cross-section through single-media bed. such as conventional rapid sand filter.

The third material — garnet or ilmenite — restores the breakthrough efficiency and surge resistance of the filter, providing the same media-particle surface area as the rapid sand filter. The small media particles also increase filter efficiency. The initial hcadloss through a 30-in.-deep mixed-media filter b 1.5 ft at a 5-gal/min per square foot of filter flow rate, comparable to a rapid sand filter of 0.5 mm and 30-in. depth run at the same rate. 

Mixed media has also been used to convert existing filters. A considerable number of rapid sand filters, either in pressure or concrete containers, have been expanded in capacity through conversion to mixed media. Generally, certain hydraulic revisions are also required, particularly to the influent and effluent piping system. However, it is not unusual to more than double the capacity of an existing plant by the installation of mixed-media materials with the appropriate hydraulic changes. Each system must be looked at individually, however, to assure that the conversion is economic. 

Washing rates for rapid sand filter beds. Wafer Works Sewer., 77 123-125. 

The design and care of rapid-sand filters. J. Am. Water Works Assoc. 26 445-460. 

Slow-sandfilters normally operate at a rate of 1.0 to 10 mVd-m, while rapid-sand filters normally operate at a rate of 100 to 200 mlA-m. A section of a typical gravity filter is shown in Figure 7.2. 

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. 

In the early development of filters, units that had been clogged were renewed by scraping the topmost layers of sand. The scraped sands were then cleaned by sand washers. In the nineteenth century, studies to clean the sand in place, rather than taking out of the unit, led to the development of the rapid-sand filter. The method of cleaning is called backwashing. 

Rapid-sand filter—Gravity filter normally operated at a rate of 100 to 200 m /d m. ... 

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. 

Clasen, J. (1997). Efficiency control of particle removal by rapid sand filters in treatment plants fed with reservoir water A snrvey of different methods. Water Science Technol., Proc. 1997 1st lAWQ-lWSA Joint Specialist Conf. on Reservoir Manage, and Water Supply—An Integrated Syst., May 19-23, Prague, Czech Republic, 37, 2, 19-26. Elsevier Science Ltd., Exeter, England. 

Effluent rapid sand filters, dominant alga Pediastrum ... 

Manganese passed through the rapid sand filters therefore, ozonation could not be applied to filtered water without loading the distribution system with mineralized manganese. Point of ozonation was chosen as the presettled water before application of the chemicals. 

Korth A., Bendinger B., Czekalla C., Wichmann K. Biodegradation of NOM in rapid sand filters for removing iron and manganese. Acta Hydrochimica et Hydrobiologica 2002 29(5) 289-295. 

Ellms, J.W. (1916). A study on the behavior of rapid sand filters subjected to the high-velocity method of washing. Trans. ASCE 80 1342-1428. 

Anonymous (1961). Giles, R.V. Lexerdyearbook. 134. Drexel University Philadelphia PA. P Giles, R.V. (1936). A model of a rapid sand filter plant. Civil Engineering 6(11) 779. 

Figure 1. CutawtQf view of a rapid sand filter.

Figure 1. Design details of a Rapid Sand Filter. 274...

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