Water filtration is an expansive topic that can refer to dozens of different processes all designed to remove impurities from water using a chemical process, a biological process, or a physical barrier.
Filtration can be used in irrigation, aquariums, swimming pools, and drinking water, as well as for other purposes.
Chemical filtration changes the chemical composition of the water through the use of a filtering substance.
One example of chemical filtration is the removal of unpleasant tastes and odors from drinking water using a Granular Activated Carbon (GAC) filter. The activated carbon within a GAC filter is tremendously effective in its adsorption of impurities due to its enormous surface area (one gram has more than 500 square meters of SA). The positively charged carbon is designed to attract negatively charged water contaminants such as chlorine and sediment. Carbon filters, commonly used in refrigerator drinking systems and portable water bottles, are unable to remove minerals, sodium, bacteria, and various chemicals.
Any water filtration that uses an ion-exchange process to remove unwanted particles (including traditional water softening) is considered chemical filtration.
Biological filtration is a filtration method that causes a biological reaction in water contaminants during filtration. The best example of biological filtration is the conversion of soluble to insoluble iron.
Concentrations as low as 3 parts per million of ferrous (dissolved) iron in your water supply can leave red or brown stains in plumbing fixtures and on clothes. It also gives the water an unsavory, metallic taste. Iron filtration systems inject pressurized air into the water system. The dissolved iron in the water reacts with the oxygen in a biological process known as oxidation and becomes rust. The rust, in the form of insoluble iron particles, binds to an internal media resin before being rinsed away in an ion-exchange process similar to water softening.
Physical filtration uses a barrier to remove unwanted particles and dissolved solids from water. This can be as rudimentary as a coffee filter or as complex as reverse osmosis.
European municipalities began using slow sand filters in the 19th century to purify their drinking water. Slow sand filters work through the formation of a biofilm at the very top of the fine sand layer. This biofilm serves as the primary means of purification by trapping particles of foreign matter and adsorbing dissolved solids from the water as it filters through. Similar formations, known as rapid sand filters, are in widespread use across the United States today.
In residential homes, reverse osmosis drinking water purification systems frequently filter water through membranes that are so fine they allow no particle larger than 0.001 microns to pass through. Reverse osmosis filtration can remove over 99.99% of dissolved solids and impurities.