Innovative Approach to Centrate Nitrification Accomplishes Multiple Goals – Nitrogen Removal and Odor Control

In 2001, the City of Phoenix, Arizona committed to the conversion of the existing high-rate anaerobic digestion process at the 9 m3/s 91st Avenue Wastewater Treatment Plant (WWTP) to a multi-phased process (acid/methane, mesophilic/thermophilic) to optimize existing digestion capacity and achieve Class A biosolids. The improved solids reduction is anticipated to double the ammonia load in the solids dewatering centrate, which is currently treated in the nitrification/denitrification (NdeN) activated sludge facilities. In order to continue to meet the plant total nitrogen discharge limit of 10 mg/L as N, a separate centrate nitrification system was incorporated into the plant's process flow scheme to efficiently manage the high ammonia recycle stream. By discharging the nitrified centrate to the plant headworks, an added benefit of reduced hydrogen sulfide (H2S) emissions was realized. This paper documents the evolution of this venture, from laboratory testing to full-scale design and operation.