Introduction to Sedimentation
Sedimentation is a physical water treatment process that uses gravity to remove suspended materials from water. Solid particles entrained by the turbulence of moving water can be naturally removed by sedimentation in the still water of lakes and seas.
Settling basins are ponds used to remove entrained sediments by sedimentation. Clarifiers are tanks equipped with mechanical systems for continuously eliminating sediments accumulated by sedimentation. Clarification does not remove dissolved organisms. Sedimentation is the process of accumulating sediment.
A particle's theoretical dropping speed in clear and motionless water is its limit sedimentation velocity. A particle will settle only if: -
1: The ascending water velocity in a vertical ascending flow is less than the limit sedimentation velocity.
2: In a longitudinal flow, the ratio of tank length to tank height is greater than the ratio of water velocity to limit sedimentation velocity.
The removal of suspended particles by sedimentation is dependent on particle size, zeta potential, and specific gravity. Suspended solids kept on a filter may remain suspended if their specific gravity is comparable to that of water, however excessively dense particles passing through the filter may settle. Settleable solids are defined as the observable volume accumulated at the bottom of an Imhoff cone after one hour of wastewater settlement.
What is the difference between unhindered and hindered sedimentation?
1: Sedimentation that is unhindered
Unhindered settling is a method of removing single particles in very low concentrations without interference from neighbouring particles. In general, sedimentation will be deemed discrete if the solution concentration is less than 500 mg/Lt of total suspended solids.
During discrete settling, the particles maintain their size and form while moving at their own velocity. With such low suspended particle concentrations, the chance of particle collisions is very low, and as a result, the rate of flocculation is tiny enough to be ignored in most calculations. As a result, the surface area of the settling basin becomes the most important determinant in sedimentation rate.
Every continuous flow settling basin is separated into four sections: the inflow zone, the settling zone, the sludge zone, and the exit zone.
Flow is established in the intake zone in the same forward direction. As the water flows towards the outlet zone, sedimentation develops in the settling zone. The cleared liquid is subsequently discharged from the output zone.
In the Sludge zone, the settled particles will be gathered here, and we normally expect that they will be removed from the water flow once they reach the sludge zone.
In an ideal rectangular sedimentation tank, the critical particle enters at the top of the settling zone, and the settle velocity is the smallest value to reach the sludge zone, and at the end of the outlet zone, the velocity component of this critical particle is the settling velocity in vertical direction and horizontal direction.
2: Hindered/Zone Sedimentation
Flow is established in the intake zone in the same forward direction.
As the water flows towards the outlet zone, sedimentation develops in the settling zone. The cleared liquid is subsequently discharged from the output zone.
In the sludge zone, the settled particles will be gathered here, and we normally expect that they will be removed from the water/wastewater flow once they reach the sludge zone.
In an ideal rectangular sedimentation tank, the critical particle enters at the top of the settling zone, and the settle velocity is the smallest value to reach the sludge zone, and the velocity component of this critical particle is the settling velocity in vertical and horizontal directions at the end of the outlet zone.
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