SLUDGE CAKE PUMPING EXPERIENCE AT 91ST AVENUE WASTEWATER TREATMENT PLANT, PHOENIX, ARIZONA

The 91st Avenue Wastewater Treatment Plant (WWTP) in Phoenix, Arizona, dewaters its digested sludge through the use of dewatering centrifuges housed in the plant's Solids Handling Facility (SHF). The centrifuges operate in concert with dedicated piston pumps that convey the dewatered sludge (sludge cake) to storage hoppers for truck hauling to land disposal. A constriction in the overall dewatering capacity of the SHF was realized as a result of the sludge cake pumps limiting the operation of their corresponding centrifuges at full capacity. To eliminate this constriction, the project team decided to replace the sludge cake pumps with either higher capacity units or an alternate sludge cake conveyance system that would permit operation of the centrifuges at their maximum capacity. Different alternatives were investigated and evaluated, including an enclosed belt conveyor, a double round piston pump, and a progressing cavity pump. The progressing cavity pump was the alternative selected to be pilot tested for a period of six months. One of the piston pumps in the SHF was removed to allow full installation of the progressing cavity pump and associated equipment (screw auger, variable frequency drives, control panels, etc.). In addition, a pressure sensor (annular seal type) was installed on the discharge pipe for continuous monitoring of the pump operating pressure.Pump performance and rotor wear were evaluated at the end of the pilot period. Also, a test was conducted, by varying the sludge cake solids content discharged from the centrifuge, to evaluate the effect of sludge cake dryness on the pump operating pressure. An additional test was performed to determine the impact of discharge pipe lubrication on the pump operating pressure.This paper presents a summary of the alternative evaluation that concluded in the recommendation to pilot test a sludge cake progressing cavity pump and discusses the findings and recommendations derived from the pilot test experience.