Filtration is the process in which solid particles in a liquid or gaseous fluid are removed by the use of a filter medium that allows the fluid to pass through while retaining the solid particles. It may mean the use of a physical barrier, chemical, and/or a biological process. The removal of particles takes place with processes including: straining, flocculation, sedimentation and surface capture. 

Water filtration can remove or reduce the concentration of suspended particles, parasites, bacteria, algae, viruses, fungi, and more chemical and biological contaminants.

In order for water to be filtered, it can only pass through the filter medium if some driving force is applied, which may be caused by gravity, centrifugation, application of pressure on the fluid above the filter, or other processes that use pumps, valves and pipes to produce enough pressure to push the water through the filter.