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.

Riser Pipe Sludge Removal Design Considerations by William H. Boyle, PE

The riser pipe as developed in the early 1960s went through a number of design iterations for sludge removal. Due to the lack of concern for uniform floor pick up by the original designers, the original riser pipes were such that each pipe was of the same diameter and was adjustable to vary the return sludge flow. The design allowed the operator to adjust each pipe for the perceived conditions that might be found at various points within the clarifier. This flexibility to remove sludge from the various areas in the basin was thought to be paramount. Although this flexibility was and is a good design feature, other problems associated with the pick up of the sludge manifested themselves in the field. 

One of the major drawbacks was that low velocities caused solids deposition and blockage within the pipes and elbows at low flow conditions. This became a major problem for some of the installations that utilized the riser pipe design. Also, there were problems due to the increased headloss that was required for some flow control devices (particularly the telescopic sludge valve type). Since it was most economical to go with a single size for the withdrawal pipes and space them equally on the radius of the basin, uniform pick up of the sludge was compromised.   

  

Currently, the challenge for engineers is to design the riser pipe systems to ensure uniform pick up across the tank floor and to minimize the clogging problems. The main sludge withdrawal control systems utilize a submerged orifice device. To overcome the velocity and headloss limitations, various sizes of pipes with long radius elbows are used. These are designed to withdrawal sludge uniformly over the total area of the clarifier and minimize any clogging potential.

Riser Pipe by William H. Boyle, PE

The riser pipe sludge removal device was developed in the early 1960s. At the time, Rex Chain Belt (now Envirex/Siemens) patented the current Tow-Bro™ / unitube header product. As a result, hydraulic removal for activated sludge final clarifiers became the standard. To overcome this patent and still hold to hydraulic removal of activated sludge, the riser pipe design was developed and promoted. It was marketed and furnished heavily in the Northeast part of the USA by Dorr-Oliver, Walker, and Eimco who were the main players at that time. There have been various names for the product over the years such as: riser pipe, pipe organ, site well clarifier (so that it can be seen when there is a clogging problem), and suction removal pipe device.  

The general thinking was that a competitive offering was needed that could overcome the dominance of the Tow-Bro™ with unique features that it did not include. The riser pipe offered the plant operators the ability to see the sludge in the sludge box and the ability to control the sludge withdrawal rate from each area/region in the tank. There are several different flow control device designs for the pipes as they discharge into the sludge box. The two main control layouts are: 1) individual compartments with a submerged orifice gate and 2) a pipe over pipe design with a submerged orifice to control flow by varying the size of the orifice.

Despite these unique design features, there was not a lot of concern given to uniform pick up of sludge, the size of the pipes, low flow velocities, or pipe clogging problems. There were also thoughts at the time that the heavier sludge settles out near the center of the basin, making uniform pick up across the basin diameter less critical. There were some designers that also specified and promoted a heavy solids line at the center of the tank with scrapers directing the sludge into a heavy solids hopper. The outer area towards the wall then would be considered for the riser pipe removal of sludge.

After the Tow-Bro patent ran out in the mid-to-late seventies most manufacturers furnished the same header design configuration as the Tow-Bro™. The original equipment will now be 20 to 40 years old and in need of replacement at many plants.