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7.32 CHAPTER SEVEN

SLUDGE BLANKET CLARIFICATION

Sludge blanket clarification is a variation of solids contact clarification in which coagu-
lated water flows up through a blanket of previously formed solids. As the small coagu-
lated particles enter the blanket, they contact particles within the blanket and flocculation
takes place. Flocculated particles grow in size and become a part of the blanket. The sludge
blanket grows in thickness and is suspended by the flow velocity passing through the
blanket.
A blanket depth of several feet is required for efficient clarification. When the blan-
ket depth has reached the design depth, the top of the blanket is above the level of sludge
removal hoppers. Because there is no upward flow in the hopper area, the portion of the
blanket next to the hopper settles into the hopper. Cohesion in the sludge blanket helps
pull solids into the hopper as the blanket settles, creating a flow of solids along the top
surface of the blanket. Sludge is periodically removed from the hopper by gravity.

Design Criteria and Application

Sludge blanket clarification may be used for applications where flocculent suspensions
are formed, such as to remove turbidity, color, organic matter, tastes and odors, and iron
and manganese. The process may be applied to highly turbid waters if the turbidity is col-
loidal in nature. However, heavy suspended solids should be removed by presedimenta-
tion because they may not be supported by the upward flow in the blanket and may set-
tle out to create a maintenance problem. Waters with high levels of algae may be difficult
to treat because algae may float to the surface, carrying flocculated solids with them. Oth-
erwise, sludge blankets can be an efficient removal process operating at much higher load-
ing rates than conventional clarifiers.
For a sludge blanket to perform efficiently, it must be designed to provide uniform up-
ward flow, ideally with equal velocities across the entire cross section. This ideal is ap-
proached by introducing flow across the bottom of the basin from uniformly spaced dis-
tribution laterals, each with uniformly spaced orifices. Clarified water is then collected in
equally spaced launders of either the overflow weir or the submerged orifice type.
One of the first applications of sludge blanket clarification was the pyramid type, shown
in Figure 7.19, developed by Candy in the 1930s. In this unit, coagulated water is intro-
duced to the bottom of the pyramid and flows upward through the sludge blanket with a
reducing velocity resulting from the expanding area of the pyramid. Clarified water is col-
lected in uniformly spaced launders, and sludge overflows to a hopper.
Because the hydraulic design reduces short-circuiting, and because the sludge blanket
is heavier than the settling floc particles in a conventional basin, the surface loading rate
can be increased. Recent application of plates and tubes in sludge blanket clarifiers has
allowed even higher loading rates.

Pulsed Blanket Clarifier
The pulsed blanket clarifier (Hartman and Jacarrino, 1987), as shown in Figure 7.20, con-
sists of the following subsystems:
• Vacuum chamber
• Inlet distribution system
• Effluent collection system