Sedimentation
Sedimentation is the removal of settleable solids by gravity. The velocity of the water is slowed and turbulence is lessoned, allowing floc, sand, pollutants, and other impurities to be filtered out.
In the same way that laminar and turbulent flow affect flocculation and coagulation, the flow and velocity of the treatment water affect sedimentation. In addition, the forces acting on the suspended particles and the characteristics of the particles and the forces acting on the suspended particles (gravity and drag) affect the rate of settlement (Benefield, 1982).
The fluid dynamics equation, Stoke’s Law, predicts the settling velocity of a sphere in a fluid. The equation demonstrates the
settling velocity is dependent on characteristics of both the particle and the fluid including density, radius of the particle, and viscosity (the property of resistance to flow, "thickness") of the fluid.
settling velocity is dependent on characteristics of both the particle and the fluid including density, radius of the particle, and viscosity (the property of resistance to flow, "thickness") of the fluid.
Basin Types
There are two main types of tanks used during sedimentation. The first is a rectangular tank in which the flow of water is parallel to the bottom of the basin. The second is a center-feed tank in which the water flows radially from the center to the outside of the tank. Both types of tanks incorporate the use of multiple zones (Pizzi, 2010).
Basin Zones
Inlet Zone
o In the inlet zone, the water’s velocity is decreased in order to prevent the turbulence from breaking up the floc, and the water is evenly distributed across the basin. An even distribution throughout the basin is critical because improper distribution can cause the water to short-circuit or go through the sedimentation process faster than the normal detention time.
Settling Zone
o The settling zone is a calm area, allowing the particles to settle. This zone is often the largest portion of the sedimentation tank.
Outlet Zone
o The outlet zone controls the discharge of water from the sedimentation tank and is meant to remove water that only contains a minimal amount of floc.
·
Sludge Zone
o The sludge zone stretches across the bottom of the sedimentation bank and is where the precipitated impurities settle. The water velocity here is low, preventing the settled floc from reentering the water. A drain in the tank allows for the removal of sludge, after which it may be disposed.
Basin Zones
Inlet Zone
o In the inlet zone, the water’s velocity is decreased in order to prevent the turbulence from breaking up the floc, and the water is evenly distributed across the basin. An even distribution throughout the basin is critical because improper distribution can cause the water to short-circuit or go through the sedimentation process faster than the normal detention time.
Settling Zone
o The settling zone is a calm area, allowing the particles to settle. This zone is often the largest portion of the sedimentation tank.
Outlet Zone
o The outlet zone controls the discharge of water from the sedimentation tank and is meant to remove water that only contains a minimal amount of floc.
·
Sludge Zone
o The sludge zone stretches across the bottom of the sedimentation bank and is where the precipitated impurities settle. The water velocity here is low, preventing the settled floc from reentering the water. A drain in the tank allows for the removal of sludge, after which it may be disposed.