Filtering is the separation of different components that are mixed in a substance. For example, the water in a river is not pure water. This water has sediment (soil, small rocks and sand), plant material and sometimes pollutants in it. When this water flows through a wetland, a lot of the sediment becomes trapped by plants and land; plant material is eaten by wetland animals or absorbed as nutrients by other plants and some pollutants are even used by certain plants as sources of nutrients. When this water leaves the wetland, it has been filtered and is more pure than when it entered.