A water filter is a device or device for filtering substances from water. The substances in the water can be particles, microorganisms or dissolved substances. A distinction is made between mechanical filters and chemical-physical water filters.

Physical filters basically remove particles by sieving them out. The smaller the diameter of the filter medium, the smaller particles are filtered out. Chemical-physical processes are used to filter substances that have already been dissolved (such as salts). An exception to this distinction is the reverse osmosis process , in which dissolved substances are also removed by mechanical filtration.