Clarifier Drive Torque: Selection and Clarity by William H. Boyle, P.E.

In order to provide an accurate determination of torque for specific applications, specifying engineers should be familiar with the application, selection, and terminology related to drive torque requirements. Also, required design specifications should be clearly communicated to suppliers.

The dictionary definition of torque is “a force or combination of forces that produces a twisting or rotating monitor.”

When engineers specify a particular torque value for a clarifier, they are limiting the suppliers to one particular design requirement. By selecting and specifying a torque, the designers are trying to assure the drive will function as intended. To truly receive what they are specifying, specifications should be made clear and simple.

In the circular clarifier field, the types of drives vary with each equipment supplier. The basic arrangements are: (1) the half bridge design where the drive rests on the center column; (2) the full bridge design where the drive rests on the bridge spanning the tank. While other arrangements do exist, the two described above are most commonly seen in the circular clarifier field. 

Torque Calculation

Calculation of torque for a circular drive unit is based on the simple cantilevered beam type of equation, with a uniform load L (#/ft. – pounds per foot) applied to the rotating sludge removal arm. Torque required to turn a rake arm would equal the resultant force of the uniform load (L*R) [R, Radius of the basin] multiplied by the moment arm (R/2). Since most circular clarifiers have two arms, the resulting equation will be T = L*R^2 (expressed in ft•lbs [foot pounds]).

Loading varies with the type of waste to be handled, material density, depth of sludge, and angle of repose of the solids, as well as other intangible parameters. Loadings for various industrial sludges are sometimes available based on pilot work or field data. The following table shows suggested uniform loadings (L) in lbs/ft. for various sewage treatment applications.

Application	Loading L - #/ft.
Grit	40
Primary w/grit treatment	8
Primary w/o grit treatment	10
Secondary (scraper)	6
Secondary (suction)	4
Gravity Thickener primary only	30
Gravity Thickener primary w/ secondary	20 - 30
Torque = L*R^2 = foot pounds    Radius R in ft.

There are times the skimmer could be hung up on the scum trough and possibly impart a torque to the center mechanism and subsequently to the drive unit. This is not a major factor since the skimmer and the supports would act as a torsion-bar (in most cases) and dissipate the torque energy, causing a mechanical failure before it got to the drive unit. There are some designs that use a fail-safe release to minimize this concern.

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.