Sub-Zero refrigerator filters use an extruded carbon block to remove sediment and bacteria from drinking water. The filter has a series of very small pores through which the water passes. Chlorine, bacteria and sediment adhere to the carbon as the water passes through the filter.
Manufacturers produce two types of carbon filters. Granular carbon filters are common in gravity-flow water purification pitchers. They allow enough flow that gravity pulls the water through the filter and the carbon absorbs most of the chlorine to improve the flavor. Extruded block filters provide better filtration, but they also require more pressure to force the water through the unit. The block filter reduces the fine bits of carbon and potential for water bypass that are common with granular filters.
Extruded carbon filters provide openings as small as 0.5 microns. The Sub-Zero filtration system removes greater than 99.99 percent of bacteria, 99.99 percent of viruses and 99.95 percent of cysts. It is also effective in removing sediment, lead, volatile organic compounds, and the flavor and odor of chlorine.
History records using charcoal to filter water as early as 2000 B.C. The first recorded use of carbon filtration for potable water was in 19th-century Europe. Carbon filters find use in homes, health care facilities and industry for purifying water and air, as of 2015.