Sedimentaation

Sedimentation, sometimes spelled sedimentation, is the process by which particles suspended in a fluid settle to the bottom under gravity, density differences, or centrifugal forces. In natural environments, it shapes the accumulation of sediments in rivers, lakes, and oceans. In engineered systems, sedimentation is used to remove solids from liquids in water treatment, wastewater treatment, and mining operations.

The rate of sedimentation depends on particle size and density, fluid density and viscosity, and the level of turbulence. For small, spherical particles in laminar flow, Stokes’ law gives the settling velocity v = (2/9) r^2 (Δρ) g / μ. In real suspensions, non-sphericity, particle interactions, flocculation, and high solids concentration cause deviations from the ideal behavior.

In engineering practice, sedimentation occurs in gravity settling basins and clarifiers. Primary clarification targets settleable solids from raw water or sludge, while secondary clarifiers remove solids formed during biological treatment. Pre-treatment often uses coagulation and flocculation to create larger aggregates; sedimentation may be followed by filtration. Centrifugation and sludge handling are used when gravity is insufficient.

Natural sedimentation contributes to sedimentary rock formation; layers of deposited material lithify into sandstone, siltstone, shale, and related rocks. Sedimentation rates and grain sizes reflect energy conditions of the depositional environment.

Limitations include non-uniform particle sizes, uneven density, resuspension by flow or turbulence, and the presence of colloids stabilized by electrostatic or steric forces. Very fine particles may require chemical treatment to promote settling. Proper design and operation rely on flow management, particle characterization, and routine maintenance of basins and sludge removal.