LESSON 9- Water Treatment: Sedimentation
Sedimentation- the process in which the velocity of the water is lowered below the suspension velocity
and the particles settle out of the water due to gravity. This process is also known as settling or clarification.
This may not be necessary in low turbidity water of less than 10 NTU.
Types of Basins
1.) Rectangular- water flows horizontally through a long tank.
2.) Double Deck- two rectangular
3.) Clarifiers- square or circular basins with horizontal flow. More likely to have short circuiting
problems.
Short-circuiting- Water skips the usual path and flows straight to the outlet faster than normal.
FOUR ZONES of SEDIMENATION BASINS
1.) INLET ZONE-
• To distribute the water.
• Prevents short-circuiting.
• To control the water’s velocity
(water velocity > 0.5ft/s.)
floc in the water will break up due to
agitation.
• Prevent turbulence of water.
2.) SETTLING ZONE-
• Water velocity is greatly reduced.
• Bulk of floc settling occurs.
• Largest volume of the basin.
• Requires a slow, even flow of water.
3.) OUTLET ZONE-
• Controls the water flowing out
• Prevents short circuiting
• Ensures that well-settled water leaves the basin
• Controls water level
4.) SLUDGE ZONE-
• Where the sludge collects temporarily
• Velocity in this zone should be very slow.
• If sludge is not removed, volume of tank is decreased and reduced efficiency of the
sedimentation. May become septic
COMMON DEVICES USED TO THICKEN THE SLUDGE-
a.) Lagoons- small volume storage ponds.
b.) Drying Beds- faster than lagoons, similar design to a sand filter.
DISCRETE PARTICLE SETTLING- occurs when particles are discrete and do not interfere with one another
as they settle.
