Filter (water)

Overview
A water filter is a device which removes impurities from water by means of a fine physical barrier, chemical process and/or biological process. Filters are used to cleanse water for irrigation, drinking water, aquariums, and swimming pools.

Uses
Water can be cleaned using a variety of different types of water filters, both for drinking and other purposes. Filtration forms a key stage in many waste water and drinking water treatement systems. In the case of drinking water, filtration tends to be carried out by the supplier prior to the water's entry into the distribution system. Additional filtration may be carried out by consumers at their premises using any of several types of domestic water filter.

Domestic water filters are primarily used to remove chemicals and metals such as chlorine and lead. Water filters can be used for preventative health reasons in the case of harmful substances, or for reasons of aesthetics, to improve the taste, colour or odour of drinking water. Domestic water filters can be attached directly to a tap, attached to the domestic water supply before the tap, or filters can be used at the point of use or in a portable unit.

Methods of filtration
The physics of water filters is based on sieving, adsorption, ion exchanges and other processes. Unlike a sieve or screen, a filter can remove particles much smaller than the holes through which the water passes.

Water treatment plant filters
Types of water filters include media filters, screen filters, disk filters, slow sand filter beds, rapid sand filters and cloth filters.

Point-of-use/domestic drinking water filters
A number of different filter types are available for use in the home, including granular activated carbon filters (GAC), metallic alloy filters, microporous ceramic filters, carbon block resin (CBR) and ultra-filtration membranes. Some filter units may be composite systems comprising several of these filter technologies. Jug filters can be used for small quantities of water for drinking. Some kettles have such filters built in, predominantly to reduce the effects of limescale.

Portable water filters
Water filters are also used by recreational hikers, by aid organizations during humanitarian emergencies, and by the military. These filters are usually small, portable and light (1-2 pounds or less) and usually filter water by working a mechanical hand pump, although some use a siphon drip system to force water through while others are built right into water bottles. Dirty water is pumped via a screen-filtered flexible silicon tube through a specialized filter, ending up in a container. These types of filters work to remove bacteria, protozoa and microbial cysts, all of which can cause disease. Filters include fine meshes, which must be replaced, and ceramic water filters which must have their outside abraded off when it becomes clogged with impurities.

These water filters should not be confused with devices or tablets that are water purifiers. Some of these devices have the ability to remove or kill viruses such as hepatitis A and rotavirus.