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The forms of filter media discussed in the previous parts in this section have all been constrained as pieces of material or structures. The remaining group of media materials, of particular interest to clarifying processes, are unconstrained (except by the walls of their containing vessel), being masses of coarse particulate substances, used as packed beds within which contaminants are removed by what can truly be called depth filtration.

Deep-bed filtration involves filtration vertically through a packed bed of granular or fibrous material, whose height is considerably greater than even the thickest of continuous filter media. It is typified by the conventional sand filter, which clarifies water by depth filtration mechanisms as it flows through a bed of graded sand that may be up to one metre in depth.

Deep-bed filters are of very simple construction: a vessel (usually cylindrical), a supporting grid at the base of the vessel, and the bed of granules – plus the necessary inlet and outlet piping. Effectively, the medium is the filter.