Flocculation refers to the separation of a solution. Most commonly, flocculation is used to describe the removal of a sediment from a fluid. In addition to occurring naturally, flocculation can also be forced through agitation or the addition of flocculating agents. Numerous manufacturing industries use flocculation as part of their processing techniques, and it is also extensively employed in water treatment. The technique is also widely used in the medical world to analyze various fluids.

The term is derived from floc, another word for flakes of material. When a solution is flocculated, the sediment in the material is formed into clumps of aggregate which are easier to see. When flocculation is initiated intentionally, the floc is usually removed with filters or screens. When a solution was flocculated by accident, attempts may be made to turn it back into a solution so that it will be usable again.

The technique of flocculation can be used to separate out visible sediments and materials, and to treat colloids. A colloid is a solution which looks uniform, but actually consists of one or more components blended together. Some common examples of solutions with visible sediment are swimming pools and wastewater, both of which are flocculated for purity. Frequently encountered colloids include seafoam, milk, glues, and inks. The separate elements within the colloid are thoroughly blended to create a new compound, and they can be separated again through flocculation.

Depending on the circumstances, flocculation may be desired or undesired. Flocculation is often deliberately achieved in the treatment of wastewater, removing harmful substances so that they can be treated separately. Other examples of deliberate flocculation include grease traps in commercial kitchens, centrifuges for processing blood, and facilities for extracting salt from seawater. A classic case of undesired flocculation is soured milk, in which the milk colloid separates, forming a layer of flakes of material on the top. If flocculation is not desired in a manufacturing environment, steps are taken to minimize it.

There are a number of ways to flocculate a solution. Agitation is a common technique, since it can encourage flakes of sediment to clump together and gather on the bottom or the top, depending on their weight. Flocculant chemicals are also used to encourage clumping in a solution. Typically, a flocculant has a different electrical charge than the substances being precipitated out of the solution, attracting the material to itself in clumps of particles.