The Annapolis Water Reclamation Facility (WRF), which is owned and operated by the Anne Arundel County Department of Public Works in Annapolis, Maryland, has undergone, over the last ten years, a transition from conventional activated sludge to an Integrated Fixed Film Activated Sludge (IFAS) system for Biological Nutrient Removal (BNR) to, most recently, a dual anoxic zone process for Enhanced Nutrient Removal (ENR) without IFAS. This transition has included bench-scale and full-scale pilot work, full-scale upgrade and operation and finally, further study and modifications to expand the capacity while upgrading to the limit of technology for nitrogen and phosphorus removal. The twelve years of full-scale operating experience with IFAS media has taught some valuable lessons regarding the design, operation and performance of IFAS systems, particularly rope type media systems such as employed at the Annapolis WRF.
The primary objective of this paper is to summarize the lessons learned from this experience focusing on the construction, performance, operation and maintenance issues. The following topics will be addressed using plant operating data to illustrate the points.
The Annapolis Water Reclamation Facility (WRF) was originally a 38,000 m3/d (10 mgd) activated sludge plant designed to meet secondary treatment limits. The plant discharges to the Chesapeake Bay. It has undergone several upgrades in order to meet increasingly more stringent effluent limits requiring nutrient removal. The discharge permit requirements have been driven by the Chesapeake Bay initiatives, which first established as a goal, Biological Nutrient Removal (BNR) levels of treatment or a discharge level of 8 mg/L TN and 2 mg/L TP. Then, nutrient limits based on a waste load allocation were established, which would require effluent limits for TN of 6 mg/L and TP of 1.5 mg/L. Finally Enhanced Nutrient Removal (ENR) levels of treatment were required, which set the discharge limit at 3 mg/L TN and 0.3 mg/L TP. During this period of time the Annapolis WRF was required to expand to 50,000 m3/d (13 mgd).
The initial upgrade to BNR levels of treatment utilized an Integrated Fixed-Film Activated Sludge (IFAS) process, which is a system that incorporated fixed-film media within the conventional activated sludge process. IFAS was selected, after considerable pilot study, because it proved to effectively increase the performance of the plant without having to build additional activated sludge basins. Thus, it was a very cost-effective option for the County.
When the Annapolis WRF had to expand to 50,000 m3/d and upgrade to ENR levels of treatment, there was no longer any other alternative but to build additional reactor volume, either with or without IFAS. At this point the decision was made to abandon the media system and to implement a more conventional process utilizing pre-and post-anoxic zones in strictly a suspended growth system. An IFAS system would have reduced the cost of the upgrade and expansion by requiring less reactor volume to be built, but the cost savings was not enough to overcome the desire to operate a plant with a more conventional hydraulic residence time (HRT). Also, there was some concern regarding the future cost of replacing the existing media.
This paper discusses the performance of the plant with its full complement of media (244,000 m from 1997 to 2005. It references earlier data from full scale pilot operations with various quantities of media from 1993 to 1996. In November 2005, the media was removed from the process during the expansion project, which is nearing completion (final completion October 2006). This experience with the study, design and operation of an IFAS plant through its conversion, but to a conventional activated sludge plant, over a period of approximately 12 years offers a unique opportunity to investigate the unique characteristics of an IFAS system.
A brief description of the wastewater treatment facilities at the Annapolis WRF and history of its upgrades follows.