Conventional gravity and pressure rapid filters operate downflow. The filter medium is usually a 15-30 in. deep bed of sand or anthracite. Single or multiple grades of sand or anthracite may be used.
A large particle bed supports the filter media to prevent fine sand or anthracite from escaping into the underdrain system. The support bed also serves to distribute backwash water. Typical support beds consist of 1 8-1 in. gravel or anthracite in graded layers to a depth of 12-16 in.
TYPES OF MEDIA
Quartz sand, silica sand, anthracite coal, garnet, magnetite, and other materials may be used as filtration media. Silica sand and anthracite are the most commonly used types. When silica is not suitable (e.g., in filters following a hot process softener where the treated water is intended for boiler feed), anthracite is usually used.
The size and shape of the filter media affect the efficiency of the solids removal. Sharp, angular media form large voids and remove less fine material than rounded media of equivalent size. The media must be coarse enough to allow solids to penetrate the bed for 2-4 in. Although most suspended solids are trapped at the surface or in the first 1-2 in. of bed depth, some penetration is essential to prevent a rapid increase in pressure drop.
Sand and anthracite for filters are rated by effective particle size and uniformity. The effective size is such that approximately 10% of the total grains by weight are smaller and 90% are larger. Therefore, the effective size is the minimum size of most of the particles. Uniformity is measured by comparison of effective size to the size at which 60% of the grains by weight are smaller and 40% are larger. This latter size, divided by the effective size, is called the uniformity coefficient-the smaller the uniformity coefficient, the more uniform the media particle sizes.
Finer sands result in shallower zones for the retention of suspended matter. The most desirable media size depends on the suspended solids characteristics as well as the effluent quality requirements and the specific filter design. In general, rapid sand filters use sand with an effective size of 0.35-0.60 mm (0.014-0.024 in.) and a maximum uniformity coefficient of 1.7. Coarse media, often 0.6-1.0 mm (0.024-0.04 in.), are used for closely controlled coagulation and sedimentation.