Phoenix Prepares for the New Millennium by Transitioning from Solar Drying to Mechanical Dewatering and Privatized Reuse

The 91st Avenue Wastewater Treatment Plant (WWTP) provides wastewater treatment for the Phoenix (Arizona) metropolitan area. Digested biosolids from the 150 mgd nitrification-denitrification treatment plant had been dried in 180 acres of shallow drying beds until 1998, when dewatering centrifuges were installed. Biosolids are dewatered to 20 percent solids and then solar dried in the drying beds to 60 percent solids or higher. A recent evaluation of the costs and requirements of future solar drying indicated that continuing the current program would be costly and incompatible with future expansion plans for the treatment plant and surrounding area. Bids were solicited to remove and reuse dewatered biosolids that resulted in a decision to eliminate solar drying. This paper discusses the transition from solar drying and the plan for future management of biosolids at the 91st Avenue WWTP.Solar drying in the desert has been a very practical approach in the past, but changing conditions and future needs caused the City of Phoenix to determine the best approach for biosolids management. Issues of encroaching development, odors, air pollution, and reengineering caused the City to transition from solar drying to a more compatible, efficient, and flexible approach using dewatering centrifuges and privatization of dewatered biosolids handling and reuse. Privatized hauling and reuse greatly increases the flexibility to meet changing conditions and future needs for the next 10 years.