In 2001, the City of Phoenix, Arizona (USA) committed to the conversion of the existing high-rate anaerobic digestion process at the 8.9 m3/s 91st Avenue Wastewater Treatment Plant (WWTP) to a multi-phase process (acid/methane, mesophilic/thermophilic) to optimise existing digestion capacity and achieve Class A biosolids. The improved solids reduction is anticipated to double the ammonia load in the sludge dewatering liquor (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 realised to approximately $310 (USD).
Keywords: denitrification, nitrification, odour control, sidestream treatment