As relates to ion exchange, a measure of the effectiveness of the operational performance of an ion exchanger, usually based on the ratio of output per unit of input. This ratio is often expressed as the amount of regenerant required to produce a unit of contaminant reduction capacity. For example: pounds of salt per kilograins of hardness removed or pounds of acid per kilogram of salt removed. SEE A:LSO salt efficiency. In media filtration, efficiency is the percent of contaminant reduction which occurs with a specified medium volume and specified water contact time. In membrane filtration, the figure obtained (expressed as a percent) by dividing the volume (gallons or liters) of product water produced by the total volume (gallons or liters) of feedwater fed to the particular unit or system.