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The City of Phoenix operates two large WWTPs: the 23rd Avenue WWTP (63 MGD, owned by the City of Phoenix, Arizona), and the 91st Avenue Multi-Cities WWTP (180 MGD, owned by the Sub-Regional Operating Group (SROG) cities of Glendale, Mesa, Phoenix, Scottsdale and Tempe, Arizona). Both facilities employ centrifuges for thickening primary and secondary sludges, mesophilic anaerobic digestion, and centrifuges for dewatering digested sludge. Between the two facilities, roughly 650 wet tons/day of 20% total solids class B biosolids are produced, all of which is land applied locally (mileage from WWTPs is between 26 and 88 miles). No long-term biosolids storage is present at either WWTP. The 20% cake is removed daily from sludge hoppers by a contract hauler.

Except for molybdenum, biosolids from the two Phoenix facilities have consistently stayed well below 503 land application pollutant concentration limits, due in large part to effective industrial pretreatment and local limits efforts. Molybdenum concentration in biosolids has a seasonal variation in Phoenix, with the highest concentrations developing in late summer, linked to the use of molybdenum-containing products in cooling towers. During the summer of 2004, preliminary August biosolids molybdenum concentrations at both facilities (76 mg/kg at 23rd Avenue and 74 mg/kg at 91st Avenue) approached the 503 ceiling limit (75 mg/kg), prompting an intensified effort.

This case study will look at:

The multi-organization coordination efforts that are needed to help reduce the risk of land applying biosolids that do not meet 503 pollutant concentration limits, considering that no long-term biosolids storage is available at the WWTPs.

Coordination with local industries, commercial facilities, chemical suppliers, and chemical manufacturers to initiate pollution prevention measures such as product substitution or product reduction to minimize molybdenum discharges to the sewer system.

The progress to date in reducing future molybdenum concentrations.

Document Type: Research Article


Publication date: 2006-01-01

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  • Proceedings of the Water Environment Federation is an archive of papers published in the proceedings of the annual Water Environment Federation® Technical Exhibition and Conference (WEFTEC® ) and specialty conferences held since the year 2000. These proceedings are not peer reviewed.

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