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STEM Drive Mixing system eliminates accumulation in wet well and allows removal of rags and

grit at new headworks facilities – Carlisle WwTP – Penn, USA

The Carlisle Regional Water Pollution Control Facility, a 7 MGD wastewater treatment facility, serves the Borough of Carlisle and portions of North Middleton, South Middleton and Middlesex Townships. Constructed in 1981, the raw wastewater enters the facility through a gravity sewer over 20 feet below grade. The raw wastewater discharges into a pump station wet well, where it is lifted to the primary clarifier splitter box, where it begins treatment before being discharged to the Conodoguinet Creek. As the wastewater is discharged to the pump station’s wet well, it passes through one of two large grinders where rags and other large solids are macerated to prevent jamming of the lift pumps.

Unfortunately, when the raw wastewater enters the wet well the grit settles to the bottom of the wet well. Lighter solids, such as the macerated rags, form a floating mat on the water surface. These accumulations require an annual cleaning that is highly unpopular and expensive. Annual cleaning costs are as much as $25,000.


The Borough of Carlisle decided that it would be best if screening and grit removal equipment were installed to alleviate the wet well accumulations and to remove this inorganic material from the sludge stream where it was ending up after passing through the lift pumps. The most ideal place for this equipment would be upstream of the raw influent pumping station. But to do this, the equipment would have to be installed in a structure over 20 feet below grade, making this option financially prohibitive. It was determined that the screening and degritting facilities would be located at grade, adjacent to the primary clarifiers, and that the raw wastewater lift pumps would discharge into the screen channel. This meant that a mixing system would need to be installed in the pump station wet well that would keep the grit suspended and the rags entrained without interfering with the pump operation.

When selecting a mixing system, the nature of the basin to be mixed must be considered, as well as the size and contents. A raw wastewater wet well is a confined space, likely to contain explosive and toxic gasses and is considered a dirty spot in a dirty industry. This application therefore required a mixing system that was explosion proof, low maintenance, and easy to install. Several systems were examined, including recirculation pumps, mechanical mixers, and compressed air driven mixers.



With a recirculating pump, a relatively small solids handling, dry-pit submersible pump is installed in the existing pump chamber and discharges into the wet well through a directed pressure line to resuspend material settling to the bottom of the wet well. The pump could be operated on a time basis or tied to the existing pump controls so that it operated for a designated period of time prior to startup of the lift pumps. The advantage of this option is that it places no mechanical or electrical equipment into the wet well, eliminating the need to enter the wet well to maintain such equipment. It also offers flexibility of operation, as the frequency of operation and/or pump rate (if paired with a VFD) can be varied to meet the existing needs. The disadvantage is the availability of space in the existing pump chamber for another pump, typical maintenance required by a raw wastewater pump, and higher installation costs as opposed to the other two options.



A mechanical mixer option would involve installing a propeller type mixer within the wet well, directed so that its operation would resuspend material that settles to the bottom of the wet well. Like the recirculation pump, the mixer could be operated on a time basis or tied to the existing pump controls so that it operated for a designated period of time prior to startup of the lift pumps. The chief advantage of this option is its relative simple installation. The disadvantages are the installation of electrical and mechanical equipment in the wet well, requiring entry into the wet well for periodic maintenance and a requirement for explosion-proof components.




The air driven mixing unit operates using compressed air. It was developed in Scotland, to mix sludge and wet well contents, including rags and grit. It works somewhat like a submerged air-lift pump. The compressed air is released in a tube and nozzle, which induces a water jet from the nozzle. Several units are used, arranged based on the tank geometry, and are pulsed so that air entrainment by the pumps is not a problem. There are no moving parts or electrical components in the wet well, so no routine maintenance is necessary.





solution3The air driven mixing units considered were manufactured by STEM Drive Limited, based in Scotland. STEM Drive refers to their mixers as Fluidic Mixers. They use a pressurized fluid, usually compressed air, to generate a motive force on other flowable materials. The manufacturer also claims that their units can be used for most mixing tasks within a wastewater treatment facility. The units are constructed of stainless steel and come in four different models, for various uses. Since the lifting and entrainment of settled grit was a goal of this project, the Hi-Lift model was chosen.

The United States East Coast representative in Richmond, VA was contacted about other installations that could be contacted to discuss their experiences with the product. David Boucher, of STEM Drive USA, informed us that at that time there were no installation on the East Coast, but offered to provide a demonstration unit for use in the wet well. Mr. Boucher shipped the demonstration unit to our offices and we forwarded it and installation instructions to the operations staff at the Carlisle Regional Water Pollution Control Facility. Jeff Shearer, Treatment Plant Supervisor, and his staff were initially skeptical of the technology. But once the demonstration unit was installed, they were surprised at the effectiveness of the mixer.

Following the demonstration, Carlisle decided to order and install a STEM Drive mixing system in the raw wastewater lift station wet well. While Carlisle’s staff was evaluating the mixer, the construction of the screening and degritting facilities were completed and put into operation.


The wet well at Carlisle is composed of two cells and four lift pumps. The selected StemDrive mixing system included four 100 mm Mk3 fluidic mixing units, a 4-way compressed air distribution manifold and sequencing unit, a 20 hp rotary vane compressor, and stainless steel and PVC air lines. The system placed two StemDrive mixing units into each wet well cell. The four mixing units are operated sequentially, alternating wet well cells. Each operates for 5 minutes, more or less, and then remains idle for 2 to 4 hours. The system can also be operated manually. The sequencing control panel automatically controls the operation of the mixing units and compressor according to the operator adjusted program. Installation was completed in March, 2010, a few months after start-up of the new headworks facilities, for a total cost of $44,500. Installation services were provided by Tuckey Mechanical Services of Carlisle and Universal Air Products of Norfolk, VA. Start-up of the mixing system resulted in a visible and measurable increase in the material removed by the new mechanical screen and degrit unit.

Prior to the installation of the StemDrive mixing system, the Carlisle Regional Water Pollution Control Facility required a cleaning of their raw lift station wet well annually to remove settled grit and the floating mat of rags. The cost of this annual cleaning ranged between $20,000 and $25,000. Since the start-up of the mixers, it appears that an annual cleaning will not be required. If no cleaning are required going forward, as anticipated, the payback on the mixing system will be only two years. In addition, the biosolids product produced by Carlisle, which is disposed of by land application, no longer contains grit and rags and the sludge volume should be reduced due to the removal of these inorganic materials which previously were removed with the primary sludge. It has also reduced or eliminated the accumulation of material in the clarifier and thickener center wells, and clogging of solids pumps, which previously required shutdowns for repairs.

The STEM Drive Mixing system met the demanding requirements for installation in the harsh environment of Carlisle’s raw lift station wet well at a reasonable price. Installation was relatively simple and less intrusive than other options considered. And if annual wet well cleanings can be avoided completely, the mixing system will pay for itself in only two years.


By P. Fred Heerbrandt, P.E., Wm. F. Hill & Assoc., Inc.