PERFORMANCE OF STEP-FEED ACTIVATED SLUDGE SYSTEMS WITH ANOXIC SELECTORS

In 1997 the City of Boise, Idaho completed an extensive plant upgrade program at their Lander Street Wastewater Treatment Facility (WWTF). The upgrade focused on providing additional wastewater treatment capacity within the existing WWTF boundary; maintaining a high degree of ammonia nitrification; and investing a reasonable amount in capital and operation and maintenance resources. The WWTF bioreactor process was upgraded from a complete-mix air activated sludge process to an anoxic, step-feed, plug-flow air activated sludge process. Of interest, the plant's bioreactor capacity was increased by over 70 percent using the same aeration basin volume while maintaining nitrification and adding anoxic selectors. This facility has now been operating for over 4 years in this mode, and data are now available to analyze the process under various conditions.This paper will provide a detailed analysis of the most recent 3 years of operation of the Lander Street WWTF. This analysis includes an in depth review of the facility's ability to meet the initial objectives of removing the traditional pollutants of BOD5 and TSS; as well as its nitrification performance, SVI control, nitrogen and phosphorus removal, and a comparison of the performance of the facility with the design objectives. This paper adds substantial information to the limited database of the anoxic, stepfeed, plug-flow air activated sludge process.