Garnet

Garnet is a high hardness, high density granular filter media. It is normally used as the lower (final) filtration layer of a multi-bed down flow filtration system. Called Multi-Media or Mixed Media filtration, the high density small grain size of Garnet solves a major filtration problem. In a single media granular filter such as a sand filter, the material will hydraulically classify during back wash according to granual size, the smallest rinsing to the top. When water flows downward through the sand, the fine particles at the top of the bed do most of the straining of the sediment. The solids form a cake on the surface with filtration typically taking place in the top few inches. As the cake forms, the filtration becomes finer and head loss increases exponentially in time. The ideal situation would be to have the large grains of media at the top to trap the large solids and a uniformly decreasing grain size in the direction of fluid flow with the small grains at the bottom to do the final polishing. The penetration of the solids through the entire bed allows increased solids storage, longer filter runs and higher filtration rates. A properly designed multi-media system will maintain its unique inverse void gradation and provide superior performance even after long periods of use in backwash. This stable condition of large grains above finer ones is achieved by the use of materials of different sizes and specific gravities. Garnet with its high specific gravity of 4.0 forms the lower fine grain layer, its 0.3 mm effective size can filter down to the 10-20 micron range. Filter sand (e.s. =0.5 mm) and Anthracite (e.s. = 0.9 mm) or Filter-Ag can form the larger less dense layers.