MEMBRANE BIOREACTOR DESIGN: PROBLEMS AND SOLUTIONS FOR A PLANT UPGRADE IN ANTHEM, ARIZONA

In January 2000, Citizens Water Resources of Arizona (CWR) commissioned the design of the Anthem Water Campus - Phase 2 Wastewater Treatment Plant (Phase 2 WWTP). The project was undertaken to increase the plant capacity to meet projected growth and to correct the problems that have been identified during the operation of the Phase 1 WWTP. The Phase 2 WWTP is scheduled for completion in May 2001 and will increase the plant capacity from an average flow of 0.5 mgd to a maximum month flowrate of 1.5 mgd.The Anthem Water Campus, owned and operated by CWR, incorporates a water treatment plant and a wastewater treatment plant into one facility. The water plant is a membrane filtration facility capable of treating water for potable use by the Anthem residential community. The wastewater plant utilizes membrane bioreactor technology to reclaim domestic wastewater generated by the same community. The reclaimed water is subsequently used for golf course irrigation and aquifer recharge.In the early stages of the Phase 2 WWTP expansion design project, a number of operational problems resulting from the design of the Phase 1 WWTP facilities were identified that would need to be addressed for the expansion design. These problems included capacity shortfalls, inconsistent denitrification, difficult dissolved oxygen control, equalization problems, inadequate screenings handling, and labor-intensive membrane cleaning.Many of the process shortcomings were addressed in the Phase 2 WWTP expansion through innovative approaches to the project design. The design addressed the amount of membrane and treatment capacity by evaluating the appropriate membrane flux rates based on relevant operating data and manufacturer's operating data from other facilities. It also incorporated the use of a separate equalization basin to avoid concentration issues and to minimize the amount of membrane required. The design was the first Zenon-based membrane bioreactor to use a split stream mixed liquor recycle (MLR) to allow for the excess recirculation flows to be returned to the aeration basin. In addition, the system was designed with automatic dissolved oxygen and suspended solids control to more accurately regulate the SRT and solids concentrations, and the design included the use of separate membrane basins to allow for in-basin cleaning of the membranes.The Phase 2 Wastewater Treatment Plant expansion design was developed to expand capacity, improve performance, and provide a more operator-friendly facility. The expansion utilized a membrane bioreactor technology similar to the Phase 1 WWTP, but takes advantage of innovative process design features. This paper focuses on the lessons learned from the operational problems experienced with the Phase 1 WWTP and discusses in detail how the Phase 2 expansion design utilized innovative design features to mitigate those shortcomings. The paper also discusses the facts discovered about the process in the course of the design that may be useful to other communities considering membrane bioreactor technology.