Sedimentation

Principles

Settling

Classification

Classification of sedimentation:

• Type 1 sedimentation is characterized by particles that settle discretely at a constant settling velocity, or by the deposition of Iron-Rich minerals to streamlines down to the point source. They settle as individual particles and do not flocculate (stick to each other) during settling. Example: sand and grit material
• Type 2 sedimentation is characterized by particles that flocculate during sedimentation and because of this their size is constantly changing and therefore their settling velocity is changing. Example: alum or iron coagulation
• Type 3 sedimentation is also known as zone sedimentation. In this process the particles are at a high concentration (greater than 1000 mg/L) such that the particles tend to settle as a mass and a distinct clear zone and sludge zone are present. Zone settling occurs in lime-softening, sedimentation, active sludge sedimentation and sludge thickeners.

Sedimentation equilibrium

Main article: Sedimentation equilibrium

When particles settling from a suspension reach a hard boundary, the concentration of particles at the boundary is opposed by the diffusion of the particles. The distribution of sediment near the boundary comes into sedimentation equilibrium. Measurements of the distribution yields information on the nature of the particles.

In geology

In geology, the term sedimentation is broadly applied to the entire range of processes that result in the formation of sedimentary rock, from initial formation of sediments by erosion of particles from rock outcrops, through sediment transport and settling, to the lithification of the sediments. However, the term is more particularly applied to the deposition of sediments, and in the strictest sense, it applies only to the mechanical deposition of sediment particles from an initial suspension in air or water. Sedimentation results in the formation of depositional landforms and the rocks that constitute the sedimentary record. The building up of land surfaces by sedimentation, particularly in river valleys, is called aggradation.

Rate of sedimentation

The rate of sedimentation is the thickness of sediment accumulated per unit time. For suspended load, this can be expressed mathematically by the Exner equation. Rates of sedimentation vary from less than 3 mm per thousand years for pelagic sediment to several meters per thousand years in portions of major river deltas. However, long-term accumulation of sediments is determined less by rate of sedimentation than by rate of subsidence, which creates accommodation space for sediments to accumulate over geological time scales.

Depositional episode

Most sedimentation in the geologic record occurred in relative brief depositional episodes separated by long intervals of nondeposition or even erosion.

In estuarine environments, settling can be influenced by the presence or absence of vegetation. Trees such as mangroves are crucial to the attenuation of waves or currents, promoting the settlement of suspended particles.

Siltation

Main article: Siltation

An undesired increased transport and sedimentation of suspended material is called siltation, and it is a major source of pollution in waterways in some parts of the world. |access-date = 2009-11-16 |archive-url = https://web.archive.org/web/20091221081934/http://blackwarriorriver.org/siltation-sedimentation.html |archive-date = 2009-12-21 |access-date=2009-11-16 High sedimentation rates can be a result of poor land management and a high frequency of flooding events. If not managed properly, it can be detrimental to fragile ecosystems on the receiving end, such as coral reefs. Climate change also affects siltation rates.{{cite web |access-date=2009-11-16 |display-authors=etal |archive-date=2017-03-18 |archive-url=https://web.archive.org/web/20170318092805/http://ijc.cgpublisher.com/product/pub.185/prod.38

Human-enhanced sedimentation

In chemistry

In chemistry, sedimentation has been used to measure the size of large molecules (macromolecule), where the force of gravity is augmented with centrifugal force in an ultracentrifuge.

In water treatment

Notes

References

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