Three kinds of filter media that may be used in connection with the operation of a rapid sand filter are ___

• A. Coal, Sand and Gamet
• B. Gravel, Sand and Garnet
• C. Coal, Sand and Garnet
• D. Sand, Alum and Coal

Answer: (A)

In rapid sand filtration, using several media layers improves filtration by providing different pore sizes and adsorption properties to remove a broader range of particles. The standard three-media combination is coal, sand, and garnet. Coal (anthracite) on top is light and highly porous, giving extra adsorption capacity and helping capture finer organic material and smaller particles at the surface. Below it, sand acts as the main filtration medium, physically removing particles through straining and inter-particle filtration. At the bottom, garnet is dense and finer than sand, catching the smallest remaining solids and minimizing breakthrough, which helps keep the effluent clearer and controls headloss as the filter loads. Gravel is typically used as the underdrain/support bed rather than counted as one of the filter media in this three-media scheme.