Sedimentation is the process of sediments settling out of a liquid or air. This occurs when particles that are suspended in a fluid stop moving and come to rest against a surface. This happens because forces like gravity, spinning motion, or electrical forces push the particles downward. Settling is the movement of suspended particles through a liquid, while sedimentation is the final outcome of this settling process.

In geology, sedimentation refers to the process of sediments being deposited, which leads to the creation of sedimentary rock. The term is used to describe all steps involved in forming sedimentary rock, including erosion, transport, settling, and the hardening of sediments into rock. However, the strict geological meaning of sedimentation is the mechanical settling of sediment particles from a suspension in air or water.

Sedimentation can involve particles of many different sizes, such as large rocks in flowing water, tiny dust or pollen particles, cells, or even single molecules like proteins and peptides. Even very small molecules can create enough force to cause noticeable sedimentation.

Principles

Settling is the process where tiny particles in a liquid move downward and form a layer of solid material called sediment. When particles are pushed by forces such as gravity or spinning motion, they move in the same direction as the force. In gravity settling, particles fall to the bottom of a container, creating a thick layer of sludge or slurry. Settling is an important step in many processes, including mining, treating wastewater and drinking water, biological research, space fuel ignition, and scooping.

Types of Sedimentation:
– Type 1 sedimentation happens when particles settle individually at a steady speed without sticking together. This type is also seen when Iron-Rich minerals deposit along streamlines toward a source. Example: sand and grit.
– Type 2 sedimentation occurs when particles stick together during settling, causing their size and speed to change over time. Example: materials like alum or iron that cause clumping.
– Type 3 sedimentation, also called zone sedimentation, happens when particles are very dense (more than 1000 mg/L) and settle as a group. This creates two clear layers: one with clean liquid and one with thick sludge. This type is seen in lime-softening, active sludge processes, and sludge thickeners.

When particles settle and reach a solid surface, the concentration of particles at the surface is balanced by their random movement. This creates a stable distribution of sediment near the surface, called sedimentation equilibrium. Studying this distribution helps scientists learn about the properties of the particles.

In geology

In geology, sedimentation refers to the processes that create sedimentary rock. These processes include erosion of rock particles, movement and settling of sediments, and the hardening of sediments into rock. The term is most commonly used to describe the deposition of sediments, which happens when particles settle out of air or water. Sedimentation forms land features and the layers of rock that record Earth's history. When sediment builds up on land surfaces, especially in river valleys, this process is called aggradation.

The rate of sedimentation measures how thick a layer of sediment forms over time. For sediments carried in water, this can be calculated using a formula called the Exner equation. Sedimentation rates vary widely. For example, deep ocean sediments may accumulate less than 3 millimeters (0.12 inches) every thousand years, while river deltas can accumulate several meters in the same time. However, over long periods, the amount of sediment that accumulates depends more on how much the land sinks, which creates space for sediments to settle.

Most sediment layers in Earth's history formed during short periods of deposition, with long times of no deposition or erosion in between.

In estuaries, the presence of plants like mangroves can affect how quickly sediments settle. These trees reduce wave and current strength, helping particles settle.

Excess sediment movement and deposition, called siltation, is a major pollution problem in some areas. It often results from poor land use and frequent flooding. If not controlled, siltation can harm ecosystems like coral reefs. Climate change also influences siltation rates.

Sedimentation-enhancing strategies are projects designed to help restore natural land-building processes in deltas. Deltas naturally sink over time, so adding sediment is essential to keep them at the same level, especially as sea levels rise. These strategies aim to increase sediment buildup on delta plains by restoring the flow of water and sediments between rivers and low-lying areas. These methods help raise land levels to counteract rising seas. Interest in these strategies has grown because they support the long-term survival of deltas.

In chemistry

In chemistry, scientists use sedimentation to measure the size of large molecules, called macromolecules. They use a special machine called an ultracentrifuge to combine the force of gravity with centrifugal force.

In water treatment

Sedimentation is a physical process where sediment settles out of water. This process is used in water treatment to remove suspended solids by using gravity. Solid particles that are carried along by the movement of water can settle out naturally in still water, like in lakes and oceans. Settling basins are ponds built to remove solid particles from water through sedimentation. Clarifiers are tanks with machines that continuously remove solids from water using sedimentation. However, they do not remove dissolved solids.