1、外文资料 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
2、filtration to remove floc created by coagulation or oxidation processes. Since surface water sources are 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
3、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 filter media, straining is the least important process in filtration. Filtration primarily depends on a c
4、ombination 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 t
5、he same 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 opera
6、tion. TYPES OF FILTERS Several types of filters are used for water treatment. The earliest 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
7、 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 developin
8、g countries. They may also be used as a final step in wastewater treatment. Most 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. Pressure Filters -Sand or Multi-
9、media 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
10、is generally constructed of concrete and is most often rectangular. 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 formin
11、g a square of four filters with a central pipe gallery feeding the filters from a center well. Filter Sand The filter sand used in rapid 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 dee
12、per. 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 gravel installed under the sand layer(s) in the filter prevents the filter sand f
13、rom 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. Filtr
14、 action Underdrain The filter underdrain can be one of many types, such as: Pipe laterals False floor Leopold system Porous plates or strainer nozzles Pipe laterals A pipe lateral system uses a control manifold with several perforated laterals on each side. Piping materials include cast iron, asbest
15、os cement, and PVC. The perforations are usually placed on the underside of the laterals to prevent them from plugging with sand. This also allows the backwash to be directed against the floor, which helps keep the gravel and sand beds from being directly disturbed by the high velocity water jets. F
16、alse floor The false floor design of a filter underdrain is used together with a porous plate design or with screens that retain the sand when there is no undergravel layer. This type of underdrain allows the plenum or open space under the floor to act as the collection area for the filtered water and for the distribution of the filter backwash water. Leopold system The Leopold system consist
