Filtracion

Filtration
Removal of suspended solids by filtration plays an important role in the natural treatment of groundwater as it percolates through the soil. It is also a major part of most water treatment. Groundwater that has been softened or treated through iron and manganese removal will require filtration to remove floc created by coagulation or oxidation processes. Since surface water sourcesare subject to run-off and do not undergo natural filtration, it must be filtered to remove particles and impurities. THE FILTRATION PROCESS The filter used in the filtration process can be compared to a sieve or microstrainer that traps suspended material between the grains of filter media. However, since most suspended particles can easily pass through the spaces between the grains of the filtermedia, straining is the least important process in filtration. Filtration primarily depends on a combination of complex physical and chemical mechanisms, the most important being adsorption. Adsorption is the process of particles sticking onto the surface of the individual filter grains or onto the previously deposited materials. The forces that attract and hold the particles to the grains are thesame as those that work in coagulation and flocculation. In fact, some coagulation and flocculation may occur in the filter bed, especially if coagulation and flocculation of the water before filtration was not properly controlled. Incomplete coagulation can cause serious problems in filter operation.

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TYPES OF FILTERS Several types of filters are used for water treatment. Theearliest ones developed were the slow sand filters. They typically have filter rates of around 0.05 gpm/ft2 of surface area. This type of filter requires large filter areas. The top several inches of the sand has to be removed regularly-usually by hand--due to the mass of growing material (“schmutzdecke") that collects in the filter. The sand removed is usually washed and returned to the filter.These filters are still in use in some small plants, especially in the western United States as well as in many developing countries. They may also be used as a final step in wastewater treatment.

Most filters are classified by filtration rate, type of filter media, or type of operation into: A. Gravity Filters 1. Rapid Sand Filters 2. High Rate Filters -Dual media -Multi-media B. PressureFilters -Sand or Multi-media

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RAPID SAND FILTERS Rapid sand filters can accommodate filter rates 40 times those of slow sand filters. The major parts of a rapid sand filter are: Filter tank or filter box Filter sand or mixed-media Gravel support bed Underdrain system Wash water troughs Filter bed agitators

The filter tank is generally constructed of concrete and is most oftenrectangular. Filters in large plants are usually constructed next to each other in a row, allowing the piping from the sedimentation basins to feed the filters from a central pipe gallery. Some smaller plants are designed with the filters forming a square of four filters with a central pipe gallery feeding the filters from a center well.

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Filter Sand The filter sand used inrapid sand filters is manufactured specifically for the purpose of water filtration. Most rapid sand filters contain 24-30 inches of sand, but some newer filters are deeper. The sand used is generally 0.4 to 0.6 mm in diameter. This is larger than the sand used in slow rate filtration. The coarser sand in the rapid filters has larger voids that do not fill as easily.

Graded Gravel The gravelinstalled under the sand layer(s) in the filter prevents the filter sand from being lost during the operation. The under-gravel also distributes the backwash water evenly across the total filter. This under-gravel supports the filter sand and is usually graded in three to five layers, each generally 6-18 inches in thickness, depending on the type of underdrain used.

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Underdrain...