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Do You Know The Condition of Your Interceptor System?

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Pima County Wastewater Management Department (PCWMD) has recently completed a comprehensive closed-circuit television (CCTV) program covering 230 miles of 15-inch and greater internal diameter interceptors. This program included condition assessment of both lined and unlined reinforced concrete pipe (RCP) as well as sonar review of all siphons within their system. Brown and Caldwell also trained PCWMD staff to perform condition assessment on other pipe products included in this project, such as vitrified clay pipe (VCP) and ductile iron pipe (DIP). This paper will outline the results of the investigation, digital photography, and related highly technical tools that allowed completion of this effort in less than one year. The results and procedures can be translated to any community in the United States.

PCWMD owns and operates eight collection and treatment systems within a 370± square mile sanitary sewerage system service area in eastern Pima County, Arizona. Pima County's wastewater conveyance system includes approximately 3,000 miles of public sanitary sewers, of which approximately 230 miles are considered trunk or interceptor sewers (15-inch and larger internal pipe diameter). These sewers are located in the cities of Tucson and South Tucson, as well as the surrounding communities. The largest of the systems is the Tucson metropolitan system. The metropolitan system conveys flow primarily by gravity to the County's two major wastewater treatment facilities, the Roger Road Wastewater Treatment Plant (41 mgd) and the Ina Road Water Pollution Control Facility (25 mgd with a 12.5 mgd BNR expansion under construction). The metropolitan system dates from 1900 to the present and was built using various pipe materials including Reinforced Concrete Pipe (RCP) (lined and unlined), DIP, VCP, asbestos cement, salt glazed clay, plastic truss, and polyvinyl chloride (PVC). The predominant pipe material is VCP which makes up 90 percent as compared to unlined RCP which makes up 1.6 percent of the overall system. The metropolitan conveyance system presently transports approximately 65 mgd average dry weather flow (ADWF).

To date this is the largest individual CCTV program that has been initiated by any municipal client in Arizona. The program was begun for two primary reasons: 1) a comprehensive investigation beyond limited segments had never been completed, and 2) in anticipation of the pending CMOM regulations. After initiation of the program a sinkhole and related sanitary sewer overflow occurred on a 42-inch unlined RCP interceptor near downtown Tucson. As a result, concern was raised over the integrity of the interceptor system, especially the unlined RCP. This project focused the initial investigation on unlined RCP including related condition assessments.

The project was completed ahead of the one year schedule with many interesting results, including immediate maintenance issues and/or rehabilitation work and planned future rehabilitation work based on the condition assessments.
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Document Type: Research Article

Publication date: 2004-01-01

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