Suction Duct Header vs. Spiral Blade Clarifier

What are the pros and cons of the spiral blade clarifier versus the suction duct header clarifier?

Suction Header or Duct Design (Type C3D)

Sludge flows through orifices located on a tapered header attached to one or more rake arm, and is channeled into a submerged center manifold where it flows out of the clarifier through a return activated sludge (RAS) pipe embedded in the center of the tank. Plow blades on the opposite or stub arms move heavy solids to a separate hopper at tank center.

PRO:   The RAS pump controls RAS rate.

PRO:   The header provides several collection orifices along its length, each sized for the proportional area covered, providing a balanced flow at the calculated RAS rate.

PRO:   New units are designed with nearly flat tank floors, which are easier to build than traditional tanks with a 1:12 slope.

CON:  Header design is based on the calculated peak RAS flow rates where the most balanced flow occurs. At lower flow rates, the distribution may not be proportional between the orifices.

CON:  At low RAS flows, orifices can plug with leaves or other material entering the tank, requiring the tank to be drained to clear the orifices.

CON:  To ensure functionality, two submerged manifold seals, located around the column and at the tank floor must be maintained, requiring the tank to be fully drained to service these seals.

Spiral Blade Design (Type C4)

Continuous, tapered spiral blades move sludge to the center of the sloped tank for withdrawal through the Ovivo® rotating sludge manifold, which collects sludge continually from the leading edge of the spiral blades. Sludge then flows within the manifold to a withdrawal point on the tank floor and exits the clarifier through an embedded RAS pipe.

PRO:   Tapered blades direct movement of deeper and deeper masses of sludge along the sloped floor to the tank center, helping to consolidate and thicken the sludge prior to withdrawal.

PRO:   Blades oriented at a continuous 30º angle of attack promote rapid return of the sludge to the tank center and withdrawal point.

PRO:   Spiral blades are low maintenance and require minimal operator attention. Manifold seals are not as critical to performance as with the suction header design and there are no orifices, so no problems with plugging.

PRO:   The rotating sludge manifold collects the thickest sludge that the clarifier can produce.

PRO:   No hydraulic limitations other than RAS pipe size and RAS pumping capacity.

CON:  Sludge return time is not as rapid as with suction header designs.

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