Scum. It’s the plastics, grease and other floatables that hover on the surface during wastewater clarification. No one likes scum, and no one really wants to be the one to remove it.

But when it comes to the “traditional method” of skimming scum off the top of the clarifier surface and placing it into a scum collection box, from which the material flows into a wet well, someone has to do it.

OPERATION AND MAINTENANCE ISSUES

A wet well is equipped with a level-control system that starts and stops the associated pumps as the water rises and falls. The process typically incorporates a submersible pump in the wet well or a dry-pit pump piped to the well.

As the level rises and falls, the layer of scum will not get removed, because the pit never gets completely pumped down to the bottom. This causes the layer to continue to build on itself until the pump operator removes it. That is an inherent downside of a wet well.

To combat this continuous buildup, the operator must use a high-pressure hose to break up the scum layer and mix the material so it can be pumped. However, no matter how much breaking up and mixing is done to the scum, the entire content of the well never gets fully removed.

The level-control system becomes coated with floatables and stringy material that creates an unpleasant odor. If the pump fails, it must be lifted from the wet well. This can be challenging and expensive if it is necessary to enter the well, because a confined-space permit is required.

AN ALTERNATE SOLUTION

To eliminate these challenges, a double disc pump is a solution. It is a completely different system; in fact, it eliminates the wet well altogether, along with the maintenance, odors and expense associated with it.

With the wet well eliminated, the scum collection box can be piped directly to the suction side of the pump. The enclosed piping becomes the well, with a much lower volume. The piping for a double disc pump is typically 6 inches or larger to hold the volume during a few rotations from the clarifier mechanism.

The pumps are then operated using a limit switch trigger located by the clarifier arm. A timer system clears out all the contents from the entire line, preventing a scum layer from building up. Double disc technology is specifically used in this design because of the pump’s unique ability to run dry and pass large solids and stringy materials.

DESIGN ADVANTAGES

Double disc pump technology is based on a noncaptive free disc design where two discs are staged in the pump housing. These discs work in unison to perform the duties of both the pumping element and valving element.

The hydraulic interaction between the discs creates vacuum and pressure simultaneously to move fluid from suction to discharge. The discs have an integral elastomer design with a rigid core and do not have any external metal components.

The double disc pump has a short stroke length created by eccentric cams pinned to the drive shaft. This design provides for positive valving on every stroke since the discs are mechanically driven on the seat.

The pump incorporates an inlet-side foot valve, also known as a clack valve. This device is a priming and repriming aid and only sees action under a suction lift condition or when the suction line is under vacuum. This design allows the pump to pass larger solids, rags and debris that would routinely cause problems like clogging and failure on other types of pumps.

LESS HANDS-ON ATTENTION

No level or float controls are required when using a double disc pump. These pumps also eliminate the washdown and cleanup maintenance required with traditional scum pits. That saves money because there is no need for operators to maintain clogged pumps and the level-control system.

No odor is emitted from the double disc pump, and all regular maintenance associated with a traditional scum system is eliminated as well. The pump can be run dry without causing damage. Both large solids and line-size semisolids pass easily through the pump.

There are only five wet end components. The pump is self-priming and self-valving. A trunnion seal design eliminates stuffing box and mechanical seals, so there is no seal water. Double disc pumps feature a leak-free operation for clean installation. Its maintain-in-place design provides easy access.

SIGNIFICANT SAVINGS

With new clarifier installations, thousands of dollars can be saved by not having to construct the wet-well structure. The pump is mounted on grade outside the clarifier, and the suction piping runs from the pump to the scum collection trough. Existing installations can be retrofitted by bypassing the wet well and directly piping to the pump inlet.

By removing the wet well and installing a double disc pump, industries that need to treat wastewater can reduce capital costs, eliminate routine maintenance, eradicate odors and save money over the long haul.

ABOUT THE AUTHOR

Leslie Burrage (info@pennvalleypump.com) is an engineering director with Penn Valley Pump, a manufacturer of double disc pumps for municipal, industrial, chemical and food applications, based in Warrington, Pennsylvania.