There are multiple types of water filters, which through physical, chemical and biological processes remove impure substances from water. The most common type of filtration system is that which uses granular-activated carbon, usually in the form of charcoal sheets.
The charcoal is treated with an oxidization process that opens up millions of tiny pores between its carbon atoms, increasing – due to its large porous surface – its ability to absorb (chemically bond with) particulate matter from the water. Once the carbon sheet is saturated with impurities, it’s then cleaned by heating it in a furnace. Carbon-based systems are commonly found in household water filters.
Another method used mainly in laboratory or industrial settings (where macromolecular solutions need to be purified) is ultrafiltration.
This works by generating hydrostatic pressure to force liquid against a semi-permeable membrane. Solids and solutes of high molecular weight (ie, larger than a water molecule) are caught by the system’s membrane while water and low molecular weight solutes pass through into the collection area. This allows the geometry of the membrane to be varied according to local conditions and severity of impurities.