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.
(fig. 9-14). In the diatomite filter, water is passed through a layer of diatomaceous silica (also called diatomaceous earth). It consists of skeletal remains of minute algae (diatoms) found in marine deposits that have lifted above sea level.
The diatomite filter accomplishes highly efficient filtration. Properly operated diatomite filters are capable of removing from coagulated and settled water, amoebic cysts, the cercariae of schistosomes, and approximately 90 percent of the bacteria. They also produce water with less than one unit of turbidity.
Before filtering, water is normally pretreated by passing it through sedimentation basins or holding tanks. This process removes heavier suspended solids that may cause rapid clogging of the filter. This water is brought onto the filters as the next step in the purification process. This water contains very finely divided suspended matter such as minute particles of floc, clay, and mud that have not settled, and bacteria and microscopic organisms that have not been removed by sedimentation. The purpose of the filter is to remove this suspended matter and give the water a clear, sparkling, and attractive appearance.
There are basically three types of filters. These are slow sand filters, rapid sand filters, and pressure filters.
Slow sand filters contain fine-grain sand and have low filtration rates. They are usually used when coagulation is not included in the treatment process. Their capacity is about 2 to 10 million gallons per day (mgd) per acre of filter surface. Use of slow sand filters has been practically discontinued because of their high cost per unit of capacity and the labor required to clean them. Rapid sand filters are now universally used in modern water treatment plants. There are two types, gravity and pressure. Gravity filters (fig. 9-1 5) are essentially open-top rectangular concrete boxes about 10 feet deep. An underdrain system at the bottom is covered by gravel, which, in turn, supports a 24- to 30-inch layer of fine filter sand (fig. 9-16). Gravity filters are usually designed to filter about 2 gpm per square foot of filter-bed area. However, in an emergency, up to 4 gpm per square foot can be obtained if prior treatment by flocculation and sedimentation produces very low turbidity and precholorination and postchlorina tion or both are effectively disinfecting the water. Approval must be obtained from the major command to operate filters at rates in excess of 2 gpm
Figure 9-14.-Diatomite filter, showing one filter element.
Figure 9-15.-Battery of three gravity-type rapid sand filters.
Figure 9-16.-Typical sand filter showing relationship of filter media.
per square foot. Pressure filters (fig. 9-17) have the filter bed enclosed in a pressure vessel. Water is either pumped into the vessel and forced through the filter or is drawn into the vessel and through the filter by a pump. The diatomite filter is classified as a pressure filter.