PILOT SCALE TESTS OF A UNIQUE APPROACH FOR BNR UPGRADE OF A SHORT SRT HIGH PURITY OXYGEN SYSTEM AT PIMA COUNTY, AZ

The Ina Road Water Pollution Control Facility (WPCF) is a major wastewater treatment facility of the Pima County Wastewater Management Division (PCWMD). The Ina Road facility includes a 25 MGD UNOX™ High Purity Oxygen System plant which has been in service for approximately 27 years and was designed for secondary treatment only. Future discharge requirements for the facility are expected to include reduction of NH3-N and Total Nitrogen discharges. This presents a significant problem for the Ina Road facility since;the UNOX HPO system hydraulic detention time is 1.8 hours;the existing UNOX HPO system footprint has limited opportunity for expansion, and;the Ina Road WPCF currently processes anaerobically digested biosolids from two treatment plants, the Ina Road WPCF and the Roger Road WPCF (41 MGD) which results in an internal return stream from the solids handling facility which increases the NH3-N concentration into the secondary process by about 13 mg/l.Due to the unique nature of the Ina Road WPCF operating conditions, centrate characteristics, and the potential significant capital cost savings compared to a conventional BNR upgrade approach, a pilot plant demonstration of the InNitri™ Process for biological nutrient removal was operated from October, 2003, through January, 2004. The InNitri™ Process treats the high concentration NH3-N centrate stream in a side stream reactor to grow nitrifying microorganisms and then introduces these nitrifiers as seed into the short SRT secondary treatment reactor. The nitrifier seed supplement in the short SRT secondary treatment system is the key element in maximizing capacity and achieving nitrification in the limited footprint and with the limited hydraulic detention time. The InNitri™ Process requires the addition of a small aeration tank and clarifier for treating the centrate return stream and growing nitrifiers. This additional system is typically about 2 to 5% of the main stream aeration tankage.The pilot program operated in the fall and winter when the wastewater temperature was low. The pilot plant consisted of a multi-stage UNOX™ HPO system with a preceding, staged anoxic zone in a conventional BNR configuration and a complete centrate nitrification system consisting of an aeration basin and secondary clarifier.The pilot study demonstrated the centrate could be completely nitrified starting with a feed NH3- N concentration in excess of 800 mg/l and ending with an effluent concentration of 0 mg/l NH3- N. Operation of the HPO system with an initial anoxic zone was stable and produced effluent CBOD5 concentrations under 10 mg/l at an F/M of about 0.7. When seeded with nitrifiers from the centrate nitrification system, the HPO system removed an average of 20 mg/l of NH3-N. When seeding was stopped for several days, the HPO system nitrification performance declined, indicating that the addition of seed nitrifiers was maintaining the nitrification reaction under the operating conditions.