Riser Pipe Sludge Box and Inlet Design Considerations by William H. Boyle, P.E.

The riser pipe sludge removal design raises some concerns with regards to the inlet distribution into the final clarifier. Based on the inherent need for a sludge collection box, the inlet for the mixed liquor flow is introduced into the clarifier well below the water surface. The influent well depth is then lowered accordingly, and the resulting effect on the side water depth is a key element to be considered. 

It is imperative to have the sludge collection box be as shallow as possible. This will minimize the headloss required yet still allow for proper uniform sludge pick up and adequate flow velocities within the riser pipes.

In addition, the arrangement of the riser pipes at the inlet of the sludge collection box creates a baffle wall that produces a strong, defined flow jetting pattern. As a result, the sludge pulsates as it enters the clarifier because the baffle walls block the inlet ports. This resulting flow pattern accentuates the normal sludge flow pattern (sometimes called the wall effect) in the final clarifier.

Robert Crosby recognized the wall effect and did something about it.  He and Jeanette Siemens (Brown) developed the energy dissipating wall baffle at the Stamford Connecticut WWTP.

To help minimize the jetting effect of the riser pipe baffle wall, simple energy dissipating baffles can be utilized. However, the traditional energy dissipating inlet (EDI) well design of a tub with a floor must take into account the maintenance and replacement of the sludge seal between the sludge collection box and center influent column.


Taking these design considerations into account will ultimately produce a more efficient, well-balanced riser-pipe clarifier.