Process to Estimate Sanitary Sewer Pipe Remaining Life Provides MWWSSB Remedial Decision Information

A condition assessment study for the Water Works and Sanitary Sewer Board of the City of Montgomery’s (MWWSSB’s) West Catoma Interceptor (WCI) was conducted in 1998 and 2003. The WCI was evaluated in 1998 using core sampling, closed-circuit television, and manhole (MH) inspections. Approximately 193 MHs and 65,000 linear feet (LF) of pipe were inspected. A subsequent study conducted in 2003 was initiated to establish a protocol for defining the remaining useful life of the interceptor’s structural constituents, and determine the rate of pipe deterioration in order to schedule rehabilitation, or replacement, of the interceptor. The 2003 study involved core sampling in locations approximately equal to the 1998 locations, and in additional locations. The standard for making interceptor structural constituent useful life projections was based on comparisons of interceptor and MH core samples taken during 1998 to samples taken during 2003. The data were used to estimate hydrogen sulfide (H2S) corrosion since 1998 and to develop reference criteria for the service life of reinforced concrete pipe and cast-in-place MHs (both rectangular and cylindrical). The condition assessment revealed that much of the interceptor had deteriorated at varying levels of severity because of H2S-induced corrosion. Both the 1998 and 2003 investigations provided information that the MWWSSB used to make corrective action decisions. The studies provided baseline criteria describing the theoretical useful life of the interceptor and its structural constituents. Using figures and comparative legends, the present study demonstrates that 72% of the 65,000 LF of interceptor evaluated has an estimated remaining service life ranging from zero to five years. On the basis of this information, the MWWSSB adopted a corrective action implementation plan for the WCI.

Concrete sewer pipes and manholes (MHs) are vulnerable to hydrogen sulfide (H2S) attack and varying degrees of subsequent corrosion during seasonal temperature cycles (Rowe, et al., 2005). The Water Environment Federation/American Society of Civil Engineers (ASCE) manual Existing Sewer Evaluation & Rehabilitation (1994) claims that pipeline evaluation is necessary to identify structural- and corrosion-induced defects, their severity, and the potential consequence for failure. The manual states that two important facets of corrosion evaluation are inspection and the calculation of a sewer’s remaining life expectancy. The ASCE manual Sulfide in Wastewater Collection and Treatment Systems (1989) states that to evaluate sanitary sewer remaining useful life, a system must be established for identifying the structural or corrosion failure mode and the failure state, magnitude, condition, and progress.

There is a paucity of information in the literature describing failure criteria, benchmarking the initiation of failure mode, and replacement scheduling for reinforced concrete pipes (RCPs) and cast-in-place (CIP) reinforced concrete MHs that are located within sanitary sewer collection and conveyance systems. Bradford (2005) described a proactive “Pipe Rating” procedure whereby sanitary sewer collection systems are evaluated based on a four-level rating criteria. However, the platform for the ranking criteria is based on predefined scores related to various defects. The subjective procedure offers no failure based criteria for determining constituent remaining useful life. This paper seeks to establish a standard for defining RCP, circular and rectangular reinforced concrete (RC) MH failure mode and benchmarking. Furthermore, demonstrative data are presented that illustrate the method by which the rate of RCP corrosion-based deterioration, and consequently the collection system remaining useful life, was used by the Water Works and Sanitary Sewer Board of the City of Montgomery (MWWSSB) as a remedial decision making tool.

The MWWSSB owns and maintains the West Catoma Interceptor (WCI). The WCI was constructed in 1976 and contains approximately 65,000 linear feet (LF) of 42-, 48-, 54-, and 66- inch RCP. Approximately 17 million gallons (dry weather flow) of municipal wastewater are conveyed daily to the Catoma Water Pollution Control Plant (WPCP), Montgomery, Alabama. A condition assessment study for the WCI was conducted in 1998. The WCI was evaluated in 1998 using core sampling, closed-circuit television (CCTV), and MH inspections. Approximately 193 MHs and 65,000 LF of pipe were inspected. The 1998 condition assessment study revealed that much of the WCI had deteriorated because of H2S-induced corrosion. A series of subsequent samples was extracted during 2003.

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