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Author: Gary R. Newman; Jim Coughenour and Tom Walz (City of Phoenix)
Date: 3/106
Residuals Conference 2006

Staying ahead of the solids treatment curve has been a continual challenge at the 91st Avenue Wastewater Treatment Plant (91st Ave WWTP) near Phoenix, Arizona. This facility, which has a current capacity of 680 million liter per day (179.25 million gallons per day (mgd)), services the cities of Phoenix, Glendale, Mesa, Scottsdale and Tempe, has been expanded twice over the past 10 years, and future expansions are planned to an ultimate capacity of 1,135 million liter per day (300 mgd) to meet the wastewater treatment needs of the growing population of the Valley of the Sun. Treating primary and waste activated solids from this plant requires approximately 50 percent of the plant's operating budget, occupies an ever increasing share of the available land space for future expansions, and has required millions in capital funds for new facilities for the higher solids loads. To meet this challenge, Brown and Caldwell and the City of Phoenix (agency responsible for operating the 91st Ave WWTP) worked together to address the thickening needs of the current plant and future plant expansions by successfully consolidating thickening operations into one facility – the Solids Thickening Facility (STF) - and implementing centrifugal thickening using machines with the highest throughput capacity available. The result: lower operating costs for solids thickening, improved efficiency in solids treatment, and a single facility designed and constructed to meet the plant's thickening needs through 2025. This paper will describe the key decision points in the project, including conceptual design, centrifuge selection, and detailed design leading to facility start-up in January 2004. Over 20 months of operating data from the completed centrifuge facility, thickening waste activated sludge, will be presented together with comparison data from the plant's dissolved air flotation thickeners.