Granular filtration sand filters

Sand filters vary in sophistication. A simple filter will remove most particles down to 5 pm. Multi-media filters which use sand and anthracite, and possibly a third medium, in discrete layers, can yield very efficient filtration down to 2 pm. Granular activated carbon can be used instead of sand to add some measure of organic removal to the filtration process. The quality produced by any filter depends largely on the efficiency of the backwash. Sand filters in some form provide a satisfactory solution for the majority of water-filtration problems. 

Granular bed filters are used in porcelain enameling wastewater treatment to remove residual solids from clarifier effluent (sedimentation effluent or flotation effluent). Filtration polishes the effluent and reduces suspended solids and insoluble precipitated metals to very low levels. Fine sand and coal are media commonly utilized in granular bed filtration. The filter is backwashed after becoming loaded with solids and the backwash is returned to the treatment plant influent for removal of solids in the clarification step.10-12... 

Slow sand filters are operated at very low filtration rates without the use of coagulation. Slow sand filters are a simple, reliable and easy to operate system. The filtration rate for slow sand filters is typically 50-100 times slower than that of granular media filters. Therefore, a much larger area is needed for the filter bed to produce an equivalent amount of water. Contaminants are removed... 

Deep-bed filtration involves filtration vertically through a packed bed of granular or fibrous material, whose height is considerably greater than even the thickest of continuous filter media It is typified by the conventional sand filter, which clarifies water by depth filtration mechanisms as it flows through a bed of graded sand that may be up to one metre in depth. 

There are two major types of filtration "cake" and "filter-medium" filtration. In the former, solid particulates generate a cake on the surface of the filter medium. In filter-medium filtration (also referred to as clarification), solid particulates become entrapped within the complex pore structure of the filter medium. The filter medium for the latter case consists of cartridges or granular media. Among the most common examples of granular materials are sand or anthracite coal. 

Granular media filtration is used for treating aqueous waste streams. The filter media consists of a bed of granular particles (typically sand or sand with anthracite or coal). The anthracite has adsorptive characteristics and hence can be beneficial in removing some biological and chemical contaminants in the wastewater. This material may also be substituted for activated charcoal. 

A typical physical-chemical treatment system incorporates three "dual" medial (sand anthracite) filters connected in parallel in its treatment train. The major maintenance consideration with granular medial filtration is the handling of the backwash. The backwash will generally contain a high concentration of contaminants and require subsequent treatment. 

Both media filters and cartridge filters can be used in a pretreatment process. Granular media filters involve the filtration of large particles through different layers of fine particles, usually coal, pumice, sand or garnet (Bonnelye et al. 2004). Cartridge filters act as the final filtration step before the water passes through the membranes, and remove fine particles as small as 1 pm. 

The data from Fig. 4.36, that show the evolution of c s(H,t)/Cto versus time, have been used to identify the model parameters Uj2 and 023. Here, H is the height of the fine sand granular bed used as porous filter. We have also selected the following process factors the porous bed height (H), the mean diameter of the particles in the sand granular bed (dg), the filtrate flow rate (G, ), the content of Fe(OH)3 in the water (noted here as Cq and c o in the model) and the fluid temperature as an indirect consideration of the liquid viscosity (t). Table 4.7 shows the results of these computations. 

The Panel Bed Filter differs from other granular filters in that filtration takes place on the surface of the granular medium, through the building up of a filter cake with roots in the outside layer of sand. The size of the sand grains and the speed of the gas impinging on the filter surface are inqrortant parameters with regard to the formation of the filter cake and the efficiency of the filter. 

Sand and anthracite are the two most commonly used media in granular bed filters. Single, dual, or multiple media can be used. Finer granular provides higher filtration efficiency but has a... 

However, in upflow filtration on a noncompact granular medium, the aim is to associate the cyclic filtration of SS retained at the bottom of the filter bed with the limited and deliberate migration of oil. This corresponds in fact to the enlargement and breakaway of the thick oil film which has fed on the coalescence of HC droplets on the sand walls. 

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