A Systematic Approach to Improved Denitrification - Denver Metro's Experience Meeting Challenging NO<SUB>x</SUB> Limits

Abstract

The Metro Wastewater Reclamation District in Denver, Colorado (Metro) operates the 220-mgd (maximum month) Robert W. Hite Treatment Facility (RWHTF) that includes two separate primary and secondary complexes with combined solids treatment. The two secondary complexes are operated in different modes, with the north secondary complex (NSEC) air activated sludge process being operated in nitrification-denitrification mode, and the south secondary complex high-purity oxygen activated sludge process being operated for carbonaceous BOD removal only. The effluent criteria for discharge, which are applied on the combined RWHTF final effluent, include monthly limits for ammonia (ranging from 9.7 mg/L to 15 mg/L), and a weekly maximum limit of 8.68 mg/L for NO x (nitrates plus nitrites). Weekly maximum NO x limits are relatively rare in the industry, and this one, which took effect on March 1, 2008, posed some unique operating challenges. Foremost among these was the need for increased denitrification efficiency at the NSEC, where all of the nitrogen removal currently occurs. To address this challenge, Metro embarked on a systematic approach to improving both the efficiency and robustness of the denitrification operation at the NSEC. This paper discusses the design and operational strategies implemented at the NSEC as part of this approach, and the resulting improvement in denitrification performance.