The City of Phoenix, Arizona is investigating various disinfection technologies for its 91st Avenue wastewater treatment facility: (1) ultraviolet light (UV), (2) ozone, (3) UV/hydrogen peroxide, (4) ozone/hydrogen peroxide, (5) and UV/ozone. In addition to providing disinfection, the City would like to consider the removal of endocrine disrupting chemicals (EDCs) and personal and pharmaceutical care products (PPCPs) in the treatment technology evaluation. To identify the most economical disinfection system, the evaluation included bench-scale testing of the technologies considered and a year-long water quality monitoring study. This paper presents the results of the benchscale analyses and estimated capital and O&M costs.

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The City of Phoenix (COP) owns and operates the 23 rd Avenue WWTP (63 MGD) whose liquid stream is configured with preliminary treatment, primary treatment, activated sludge with nitrogen removal, effluent filtration, disinfection, and dechlorination. Solids handling includes centrifuge secondary sludge thickening, 15-18 day HRT mesophilic anaerobic digestion, centrifuge dewatering of digested sludge, and land application of the dewatered biosolids. Currently, primary and thickened waste sludges are pumped to four 1.5 MG anaerobic digesters. Prior to December 1999, only primary sludge was feed to the digesters, and waste sludge was diverted to the 91 st Avenue WWTP regional facility.Since the Solids Handling Facility was commissioned during December, 1999, digester foaming at the COP 23 rd Avenue WWTP has caused operational problems ranging from the need for increased operator intervention, to structural damage (digester dome) and reduced solids handling capacity (required for tank protection). Digester foams have been caused by liquid stream filamentous bulking, anaerobic filamentous bulking, liquid stream pass-through events, and digester design and operational issues. Digester foam solutions have included tank and plant upgrades, additional instrumentation, and changes to operational procedures.This case study will look at:• Four different types of foam experienced, and factors that contributed to their formation.• Digester modifications and operational strategies that have prevented, minimized and controlled digester foam blankets.

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In 2001, the City of Phoenix, Arizona (USA) committed to the conversion of the existing high-rate anaerobic digestion process at the 8.9 m(3)/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 (H(2)S) emissions was realised to approximately $310 (USD).