Sedimentation is a process that allows particles in suspension in water to precipitate out of suspension under the influence of gravity. The particles that settle from the suspension become sediment, and in water treatment are known as sludge. When a thick layer of sediment continues to settle, this is known as consolidation. When sediment consolidation, or sludge, is assisted by mechanical means, this is known as thickening.

In water treatment, sedimentation can be used to reduce the concentration of particles in a suspension prior to application of coagulation, to reduce the amount of coagulation chemical required, or after coagulation and, possibly, flocculation. When sedimentation is applied after coagulation, the aim is usually to reduce the concentration of solids in the suspension so that subsequent filtration can function most effectively.

Sedimentation is one of several methods for pre-filtration applications: other options include dissolved air flotation and some filtration methods. In general, the solid-liquid separation process is sometimes referred to as a clarification process.

There are various methods for applying sedimentation and include: horizontal flow, radial flow, plate plate, ballast floc, and herd blanket sedimentation.

The simplest form of sedimentation is to fill a jar or tank with water, leave it on long enough for the particles to settle and then pour out the resulting water without sediment. In practice, it is rarely successful in treating water for the city, and therefore the sedimentation tank is operated continuously.

The simplest sedimentation method is to use a rectangular tank with horizontal flow through it. Water with particles in suspension is introduced to one end of the tank, then when the water flows to the other end of the tank, the particles settle in the water. The goal is for most of the settling particles to reach the tank floor before water is drawn out of the tank at the outlet end. Such horizontal flow tanks are usually constructed with the floor sloping slowly to the end of the inlet to the hopper. The tank is equipped with a mechanism to scrape the sediment from the end of the outlet back to the end of the inlet and into the hopper from which it can be hydraulically drained. In the design of such a tank, detailed attention should be paid to the ends of the inlet and outlet so that the water flows from one end to the other as evenly as possible.

Partly because the rectangular tank has a large footprint, multilayered (i.e. two or three deck) tanks have been constructed. These tanks are usually multi-pass because water flows along one layer before returning to the next layer.

The radial flow tank is circular in shape with an inlet for water in the center and a peripheral outlet. Attention should be paid to the inlet design to support uniform flow distribution throughout the tank. Sediment is scraped off to a central hopper for disposal.
Some round tanks include additional features in the middle for flocculation (i.e. premix design) and even settled particle recirculation (i.e. premix recirculation design).