Filtration consists of passing the water through some porous material to remove the suspended impurities. Filtration is one of the oldest and simplest procedures known to man for removing suspended matter from water and other fluids.
It is a common misconception that filtration removes suspended solids by a simple straining process whereby particles too large to pass through openings in the filter media are retained on the media. The mechanism involved in removing suspended solids by filtration is very complex. While straining is important at the filter media surface, most solid removal in deep granular filters occurs within the filter bed.
Flocculation and sedimentation in the pore spaces between filter media particles are an important removal mechanism as well as absorption of particles onto the filter media surfaces. Additional straining between media particles within the filter also contributes to overall solids removal.
The simplest form of water filter is the sand filter. This filter resembles a small reservoir, whose bottom is a bed of filter sand that rests on abed of well-graded aggregate with the largest size aggregate being at the bottom. An underdrain system of tile or brick is provided beneath the gravel to collect the water from the filter area. The underdrain system consists of a header or main conduit extending across the filter bed. Means are provided for regulating the flow of water out of the filter through this header and also for controlling the rate of flow onto the filter. This allows the filter to be operated at controlled rates that should not exceed 3.0 gph per square foot of filter area. An average filter bed consists of about 12 to 20 inches of gravel and 20 to 40 inches of sand, The depth of water over the sand bed varies from 3 to 5 feet.
The cartridge filter basically comes in two types of cartridge filtration: (1) depth filtration, where solid particles become trapped within the filter medium, and (2) surface filtration, where solid particles form a cake on the surface of the filter medium. Wound fiber cartridges function primarily as depth filters and are the standard cartridge used in the 600 gph ROWPU. (See cartridge filter in figure 9-13(C).) The most effective filtration system ever devised and one of the most effective portable systems in existence is the diatomite filter unit.
Figure 9-13(C). - Cartridge filter.Continue Reading