With a carbon filter process, water passes through activated carbon which is porous, trapping certain particles that are attracted to the porous material. The size of particles attracted depends on the size of the pores or micron rating. However, not all particles are attracted to the active carbon filter. And at some point it loses its ability to attract particulates and must be changed in order to be effective.
Coal is the most common element used to activate carbon filters, but manufacturers will often enhance carbon filters by using other elements and blends of materials in their designs, including spun fibers. Such layered filters are considered more effective since they have the ability to filter smaller particles from the water. It really depends on what the filter is designed to achieve in filtering out waterborne particles. And carbon filters vary in size, design, intensity, function, efficiency and lifecycle.
Each activated carbon filter is designed to allow a certain flow of water through the filtering process. Costs of carbon filters will also vary. Filter effectiveness is measured in terms of size of particles they can attract. This is referred to as microns, with the higher number being the worst or least effective and the smallest number being the best. A higher micron number means it can only remove large particles, but a filter with a rating of 0.05 microns will filter out minute particles - being most effective.
Carbon filters should not be confused with sediment or sand filters usually installed at the home's main water intake. These are effective in removing sand, soil, silt and other sediment. I should also add that filters in general are not helpful in reducing hard water. As effective as carbon filters are, they are limited by design and cannot reduce the amount of bacteria and microbes in water. For bacterial removal, a UV or ultraviolet light system is more effective.
For a better understanding of filters, read the North Dakota State University's resource on Filtration
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