Definitions of Terms

The following is a list of many of the commonly used definitions in water treatment:

Raw Water

Water in its natural state, prior to any treatment. Usually the water entering the first treatment process of a water treatment plant.


The clumping together of very fine particles into larger particles called (floc) caused by the use of chemicals (coagulants). The chemicals neutralize the electrical charges of the fine particles, allowing them to come closer and form larger clumps. This clumping together makes it easier to separate the solids from water by settling, skimming, draining, or filtering.


Clumps of bacteria and particulate impurities that have come together and formed a cluster. Found in flocculation tanks or sedimentation basins.


The gathering together of fine particles after coagulation to form larger particles by a process of gentle mixing.


A water treatment process in which solid particles settle out of the water being treated in a large clarifier or sedimentation basin.

Sedimentation Basin

A basin or tank in which water is retained to allow settleable matter, such as floc, to settle by gravity. A sedimentation basin is also called a settling basin, settling tank, settler, or sedimentation tank.


The removal of suspended materials by passage of the water through a filter media.


A selected group of materials used in filters and filter devices to form barriers to the passage of certain solids or molecules that are suspended or dissolved in water.  The term media is commonly used to refer to ion-exchange resin products, granular filtration material, or granular adsorption material.  Media is the plural form of medium and refers to more than one type of barrier material.


  • The Water Dictionary second edition by American Water Works Association
  • Water Treatment Plant Operation Volume 1, sixth edition