Freeboard

the vertical distance between a bed of filter media or ion exchange material and the overflow or collector for backwash water; the height above the bed of granular media available for bed expansion during backwashing; may be expressed either as a linear distance or a percentage of bed depth.

Definitioner

Media: the selected materials in a filter that form the barrier to the passage of certain suspended solids or dissolved molecules.
Ion Exchange: a reversible process in which ions are released from an insoluble permanent material in exchange for other ions in a surrounding solution; the direction of the exchange depends depends on the affinities of the ion exchanger for the ions present, and the concentrations of the ions in the solution.
Ion: an atom, or group of atoms which function as a unit, and has a positive or negative electrical charge, due to the gain or loss of of one or more electrons.
Filter: specifically, a device or system for the removal of solid particles(suspended solids); in general, includes mechanical, adsorptive, oxidizing, and neutralizing filters.
Bed Depth: the height of the ion exchange or filter media in the vessel after preparation for service
Bed: the ion exchange or filter media in a column or other tank or operational vessel.
Backwash: The process in which beds of filter or ion exchange media are subjected to flow opposite to the service flow direction to loosen the bed and to flush suspended matter, collected during the service run, to waste.