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Few regions in the country today can rival the growth occurring in Phoenix and its surrounding areas. A combination of warm, dry climate and unfettered economic opportunity has resulted in a population that more than doubled between 1970 and 2004 and has positioned the city as the sixth most populous in the U.S.
While that growth has understandably strained the city’s infrastructure, its wastewater treatment effort has kept pace through a combination of results-driven equipment upgrades—particularly its solids pumping function—and a renewed focus on equipment maintenance. The 91st Avenue Wastewater Treatment Facility handles in excess of 150 million gal per day (mgd) of sewage, processes more than 2 mgd of sludge, and removes 100 dry tons per day of processed “cake” for sanitary landfill disposal.
Five Model KSP-45 Schwing Bioset cake pumps have helped the 91st Avenue Wastewater Treatment Facility keep up with population growth in the Phoenix area.
No More Lagoons
On a complex that spans more than a square mile, the solids processing area at the 91st Avenue facility represents the final stop in wastewater’s journey from source to disposal. According to Cecil Chandler, the site’s operations and management technician, the plant’s approach to handling solids has come a long way since coming online.
“By treatment plant standards, this part of the plant is not that old,” Chandler said. “Up until 1996, we were taking waste directly from the digesters out to more than 100 lagoons we had in the desert and drying it for disposal. In 1996 we made the switch to the process we use today: taking sludge—both primary and waste-activated sludge—routing it to digesters, dewatering it, running it through a centrifuge and sending it to cake pumps for movement to a hopper and subsequent disposal in a sanitary landfill.”
Chandler notes that, until several years ago, that same material from the hoppers that is landfilled today was used as a soil additive on non-food crops such as alfalfa and cotton.
“It was really an ideal use for that material,” Chandler said. “But current laws have been tightened to eliminate any risk to area groundwater, so now it is simply spread out at a sanitary landfill and covered on a daily basis.”
Having Your Cake…
The cake pumps, five Schwing Bioset Model KSP-45 units, have proven an integral part of the process, meeting demanding needs in a round-the-clock operation at 91st Avenue.
“Those cake pumps run 24/7, year in and year out,” Chandler said. “The maximum stroke we have them running at is 9.5 strokes per minute with each stroke pushing out 11 gal of sludge. On average we will run three of the five pumps, but in winter, when ‘Snowbirds’ boost the local population, we often have to run all five. The initial installation in 1996 utilized 6-in.-diameter pipe for the system; about two years ago that was upgraded to 9-in.-diameter piping to increase efficiency. In both situations, they have been extremely productive and reliable for us.”
Chandler feels the hydraulic design of the cake pumps lends itself particularly well to their type of application: one in which the flow rate can vary greatly. As part of the normal procurement process, however, his department looked into alternative pump technology and in doing so, confirmed that the pumps they have in place are best-suited for the job.
“We experimented with a progressive cavity (PC) pump, but it didn’t fit our application,” Chandler said.
Maintaining an Edge
While the hydraulic cake pumps have answered the needs for Chandler and his group, since coming on board in 2000, he has instituted some maintenance-based changes to maximize pump productivity and minimize the risk of downtime. In addition, he has made some changes to components on the pumps themselves, which have resulted in a measurable cost savings.
Chandler worked with Schwing Bioset to have each cake pump retrofitted to include a “slip ring,” or pipeline lubrication system. This feature injects a thin film of water that separates viscous and sticky materials from the inner wall of the pipeline. Doing so reduces friction loss in the pipeline and lowers pipeline operating pressure—in some cases by more than 50%.
Looking Ahead at 91st Avenue
With no letup in sight for growth in the Phoenix area, the 91st Avenue facility is poised for growth of its own and, in fact, has several expansion projects underway. According to Chandler, his area is making changes to meet the challenges ahead.
“We are currently a 200-mgd plant and, when all is said and done we will easily be at double that capacity. In our department, we originally had four Schwing Bioset cake pumps but we added a fifth one when we converted one of our thickeners to a dewatering machine and added two thickeners. We are currently in the process of doing the same thing to two more, bringing our total of thickeners to seven. This is a challenging application, but through it all we have seen excellent performance from the cake pumps—we’re confident that this is the best equipment available for that type of work.”