Long-Term Projections for Gravity Sewer Renewal Utilizing Weibull Analysis

Union Sanitary District assesses the condition of all sewer pipes in the wastewater collection system once every 6 years. This information is used to determine the amount and cost of current repair, rehabilitation, and replacement needs for sewer pipes, yet does not give the District a long-term understanding of how those costs may change over time. The District wanted to develop projections for sewer repair, rehabilitation, and replacement needs and costs that accounted for deterioration of pipe condition over time. Using Weibull analysis, failure rates were calculated for vitrified clay pipe. Assumptions were made for failure rates of remaining types of pipe which comprised a small portion of the overall system. Long-term projections for sewer repair, rehabilitation, and replacement were generated using the calculated and assumed failure rates. The results provided the District with 50-year projection for miles of sewer repair, rehabilitation, and replacement and associated costs for system renewal. This has given the District an understanding of how capital costs may increase in the coming years and can be used to determine strategies to manage system condition and potential impacts on sewer rates.