Sedimentation (Clarification and Settling)

Sedimentation (used interchangeably with settling and clarification) is the separation of suspended solids from water or wastewater using gravity. Suspended solids include settleable and colloidal solids. Colloidal solids must be chemically treated (coagulation and flocculation) to enhance removal by settling.

The process usually incorporates a rectangular or circular tank (clarifier) that holds the water or wastewater during a set period of time to allow the solids to settle to the bottom. Most clarifiers have a cone or sloped bottom section that concentrates the settled solids, The solids can then be pumped to storage or further treatment.

Some systems include a surface skimmer to remove materials that float and a bottom skimmer to push the sludge to a sludge hopper for pumping to the dewatering facility. These type systems are more common in municipal water and wastewater applications.

Clarifiers used in many industrial applications are rectangular and include inclined plate packs that enhance settling. These plates are usually set at an angle of 45 degrees to 60 degrees. Inclined plate clarifiers work very well for metal finishing wastewater. Wastewater with heavy oils and grease or other viscous materials are not well suited for inclined plate clarifiers as they tend to clog the units. Most inclined plate clarifiers include integral mixing chambers for chemically-enhanced settling.

Basis:

The settling of discrete particles can be analyzed by the classic sedimentation laws of Newton and Stokes. Newton’s law can be used to show that the terminal velocity of a particle equates to the gravitational force on the particle to the frictional resistance, referred to as drag. Drag is dependent on velocity, fluid density, fluid viscosity, particle diameter, and the sedimentation (drag) coefficient Cd. Newton’s Law:

v_p(t) = Terminal velocity of particle (m/s)
d_p = Diameter of particle (m)
sg_p = Specific gravity of particle
C_d = Drag coefficient (dimensionless)

For particles that are small (low Reynolds Number), the drag force varies linearly to the terminal velocity. This is Stokes Flow:

Where: f depends only on the properties of the
particle and the surrounding fluid

“Sedimentation/settling occurring by gravity can be described by the Mason-Weaver equation, which has a simple solution with regard to sedimentation. The sedimentation coefficient Cd in this case equals mb / f, where mb is the buoyant mass.”