Sedimentation
Sedimentation is also called
clarification. Sedimentation removes settleable solids by gravity. Water moves
slowly through the sedimentation tank/basin with a minimum of turbulence at
entry and exit points with minimum short-circuiting. Sludge accumulates at the
bottom of the tank/basin. Typical tanks or basins used in sedimentation include
conventional rectangular basin, conventional center-feed basins,
peripheral-feed basin, and spiral-flow basins.
In conventional treatment plants,
the amount of detention time required for settling can vary from 2 to 6 hours.
Detention time should be based on the total filter capacity when the filters
are passing 2 gpm per square foot of superficial sand area. For plants with
higher filter rates, the detention time is based on the filter rate of 3 to 4
gpm per square foot of sand area. The time requirement is dependent on the
weight of the floc, the temperature of the water, and how still the basin is.
Effective sedimentation is critical to optimize pathogen removal and to enhance
filter effluent quality.
A number of conditions affect
sedimentation: 1) uniformity of flow of water through the basin,
2)stratification of water due to difference in temperature between water
entering and water already in the basin, 3) release of gases that may collect
in small bubbles on suspended solids, causing them to rise and float as scum
rather than settle as sludge, 4) disintegration of previously formed floc, and
5) size and density of the floc.
Sedimentation basins are designed
to provide idle conditions, such as a uniform low velocity, proper detention
time, no short circuits, and no surface turbulence to the flocculated water for
an effective sedimentation. Velocity varies from 1 to 3 ft/minute. Detention
time (the time it takes to fill the tank) ranges from 1 to 6 hours, depending
on the design of the basin. A short circuit is the flowing of incoming water
through the tank, without proper stratification of the water in the basin.
Short circuits are generally caused by the stratification of the water in the
basin and are common during summer and winter. There should be a proper mixing
and baffling of the influent with the water in the basin. Surface turbulence is
due to wind action and movements of the equipment. An adequate wind-breaking
height of the basin wall above the water surface and the proper maintenance of
equipment are very helpful.
Sediments settle to the bottom of
the basin, and the effluent is strained from the top. These basins have inlet
and outlet valves for flow control, and the bottom floor slopes to a hopper
(pit) for the sludge collection. Sedimentation basins are classified as primary
and final sedimentation basins, according to their functions. A primary
sedimentation basin receives the flocculated water and discharges it to the
final sedimentation basin.
