Advanced deterioration and the increasing failure of aging pumps led the Department of Operational Services for the city of Shreveport, La., to recently replace the Stoner Lift Station. Rated at 4-mgd, the facility is among the largest lift stations along the sanitary sewage system. It is also a part of $86 million worth of improvements that are being made to the system's infrastructure. The design specifications and equipment highlight major advancements that have been made since the 1970s when the Stoner Lift Station first entered service. The city's engineering group considers the replacement facility the most advanced in the city's system.
The sewage system serves approximately 200,000 residents along the Red River. Major elements include two returned activated sludge (RAS) wastewater treatment plants with a combined capacity of 51.4 mgd, a Class A sludge treatment plant, 1,024 miles of mains up to 60 in. diameter, the network of 115 lift stations and a modern effluent monitoring laboratory. Both treatment plants have been largely rebuilt in recent years involving projects that addressed nearly every aspect of the RAS process, including the screening, aeration, clarification and sludge handling, replacement of chemical with UV disinfection, high-flow equalization facilities and various pump retrofits or replacements. A wireless SCADA system also adds a new dimension in remote monitoring and control over the treatment plant and strategic lift station operations.
Stoner Lift Station, located approximately eight miles upline from the 40 mg (80 mgd/hr) Lucas WWTP, is among the more significant pump station improvements. Built in the 1970s, the dry pit-type of station originally had four, 200 hp vertical line- shaft pumps to serve a large drainage area that encompasses the commercial and casino district, low-income housing and older residential areas. The station became a candidate for major upgrades as recurring problems emerged toward the end of the station's expected life cycle, according to Ali Mustapha, PE, assistant engineer with the city of Shreveport.
"The station had recurring odor emissions, pump failures, and the general physical deterioration," said Mustapha. "Pump failures, in particular, required a lot of maintenance and a basket screen at the intake hadn't worked for nearly ten years. The wetwell, in fact, had enough concrete corrosion to expose the rebar."
As city engineers and outside consultants met to establish the needed scope of improvements at the Stoner facility, the most logical investment turned out to be a total replacement. A deciding factor was the high cost that would result from bypass pumping the influent around the existing facility and through the 36 in. force main to the treatment plant.
"Our study projected six to ten months of bypass pumping during the rehabilitation would add several hundred thousands of dollars to the project," said Ben Rauschenbach, PE, with Balar Associates, Inc., the city's engineering consultants. "Building a new station next to it would have cost more, but the city would be better off in the long term to just replace it. In the process, they improved capacity, energy efficiency and now have the built-in redundancy to better handle contingencies, such as power losses."
Balar designed the new station's building with masonry walls, a standing seam metal roof system and earth tone split-faced block that respond to higher aesthetics than the utilitarian character of the earlier structure. Inside the facility has a split wet/dry pit configuration to facilitate pump maintenance. The 44x15x15-ft footprint has a self-cleaning trench configuration.
The concrete on the 36,000-gal wetwell side was coated with Tnemec Series 120-5003 Vinester F&S, a vinyl ester novolac coating formulated to withstand organic and inorganic acids and sour crude. The trowel-grade liner provides corrosive splash, spillage and fume protection for the structural surfaces and secondary containment. The drypit side received a roller-applied XYPEX waterproofing system. The premium coatings are far more advanced and durable than the tar-like material applied to the previous station.
Mechanical features illustrate even more advancements. A new stainless steel conveyor type screen carries captured solids at the headworks up to a collection dumpster. This feature lessens the impact of solids arriving at the Lucas WWTP. A biological odor control system was installed to pull air from the screen area and through the wetwell. This unit eliminates the offensive odors and prevents them from reaching one of the city’s nearby recreational parks.
Four ITT Flygt Model CT 3356 submersible pumps, recommended by Gulf States Engineering, an ITT Flygt manufacturer's representative, were selected to serve the station's wetwell sump. The 200 hp units are vertically mounted on the dry side of the facility with the suction line extending into the wet-side trench. The city equipped their variable frequency drives (VFDs) with a powerful, new custom SCADA system. The VFDs reduced the size of the wet well portion of the station and should reduce long-term wear and improve efficiency of the equipment. They also improve the peaking factor for the station that normally runs at 2,000 gpm, but can reach 11- to 12 mgd. As explained by the Balar project engineer, the age of the 36 in. diameter concrete force main presented a limiting factor on the pumping force because of the risk of possible blowouts. The station is set to operate fully remote and the SCADA can automatically shut off the entire system if necessary. Two pumps, with 10,000-gpm in combined maximum capacity, normally operate to serve the facility.
Replacing vertical line-shaft pumps with submersibles was a direct response to the city's favorable experience with the highly reliable Flygt units, Mustapha said.
"This dry pit station is an innovation for us," Mustapha noted. "So is the trench-type design which is a first in Shreveport. Stoner Lift Station is our first of the big dry pit stations to apply submersibles, but we [are] going to do another one using them, as well."
The station replacement came just in time, according to participants on the project.
"By the time the project got underway we had only two of the four vertical pumps operational and once under construction we were down to just one," Mustapha said. "The other three had either burned up, had major components fail or been pieced and parted out. Even the remaining unit had begun to display voltage problems, a spokesman with the Maintenance Department emphasized. Louisiana contractor Max Foote Construction had to work fast and remain flexible in completing the changeover to the new station. The challenge was less in the amount of upline storage in the collection system than in the amount of backflow confronting them once they cut the 36 in. force main from the pump station to the Lucas WWTP, according to Ralph Holloway, project manager.
"We planned to tie in the lines at the same time but had to change the original scheme and split the work into two phases," Holloway said. "We used a slide gate to isolate the new from the old station that had nine smaller stations upline from it. We drained down the old station as low as possible, then shut it off and backdrained into the old wetwell. It took an 11-man crew about nine hours to complete the changeover. Fortunately, we had a lot of collection line for storage above it to prevent overflowing."
"This station is our highest example of state-of-the-art," Mustapha added.