Suction Pipe Versus Suction Header Clarifier – Advantages and Disadvantages
Suction Pipe vs. Suction Header Clarifier Designs – Pros & Cons
Suction Pipe Design (Type C3S)
Removal of sludge is through differential head pressure between the liquid level of the clarifier and the liquid level in the sludge collection box. Sludge flows through vertical and horizontal lengths of PVC pipe into valved openings within a central sludge box, where it reaches a free surface condition, and then out of the clarifier through a return activated sludge (RAS) pipe that is inside and concentric with the center column. There are two flow control points – the sludge valves and the RAS pump.
PRO: Sludge collection box at liquid level allows operators to view and sample sludge from the walkway or platform. Sludge can conveniently be taken from individual pipes, representing the section of the floor covered by that pipe.
PRO: Plugged pipes can be visually detected and can usually be unplugged by closing all the valves but the one on the plugged pipe.
PRO: Sludge pickup can be varied among the pipes for selective operation.
CON: Balanced control of sludge pickup can be difficult to obtain with the multiple valves of the sludge pipes.
CON: One seal between the rotating sludge collection box and the stationary column must be maintained. The tank must be partially drained to service this seal.
CON: Pipes can plug at low flow velocities.
CON: Pipes are typically sized for a total withdrawal rate and not for the proportion of the floor covered by each pipe.
CON: For new mechanisms, special tank floor construction is required for the RAS pipe, which is installed inside the influent pipe.
Suction Header or Duct Design (Type C3D)
Removal of sludge is through a RAS pump. Sludge flows through orifices and into a tapered header, then into a center, submerged manifold and out of the clarifier through an embedded RAS pipe. The sludge never reaches a free surface condition as with the suction pipe design, and the RAS pump is the only flow control point.
PRO: RAS rate is controlled by the RAS pump and/or a single telescoping valve rather than the multiple valves required for the suction pipe design.
PRO: The header provides several collection orifices along its length, each sized for the proportional area covered. This provides a balanced flow at the design RAS rate.
PRO: For new mechanisms, the floor is usually flat, which is easier to construct than the 1:12 slope that is traditional for suction pipe clarifiers.
CON: The header is typically designed for the peak RAS flow, and the most balanced flow occurs at this rate. At lower flows, the distribution may not be proportional between the orifices.
CON: Orifices can plug with leaves or other material that enters the tank, especially at low RAS flows. The tank must be drained to unplug the orifices.
CON: Two seals must be maintained. These seals are located at the submerged, rotating sludge collection manifold around the column and at the tank floor. The tank must be fully drained to service these seals.