COMBINING SEWER FAILURE CONSEQUENCES WITH CONDITION ASSESSMENT LIKELIHOOD PRODUCES REALISTIC PRIORITIZATION RANKING

Introduction and Background With or without limited operational funding, the general public and regulators expect utilities to effectively manage their organizational activities. This includes implementing business processes that minimize the utility's overall risks by properly prioritizing decisions and expenditures. These business tenets are particularly applicable to a utility's operation, maintenance, and rehabilitation of the sewer conveyance system. However, the utility often finds that the basic data and information needed to support good decisions is incomplete or has varying degrees of reliability. Therefore, whenever possible the utility embarks upon a condition assessment program to improve their information and knowledge about the conveyance system. It is the objective of this paper to describe the development and use of a comprehensive condition assessment methodology and tool that combines the likelihood of a sewer pipe failure with the consequences of a failed sewer pipe if rehabilitation is delayed. Condition Assessment Industry Status Condition assessment and prioritization in the wastewater industry have broad meanings. For instance, to some utilities sewer condition assessment and prioritization means determining only the internal condition of the sewer pipe and subsequently ranking rehabilitation activities. Additionally, the ranking methodologies used to prioritize rehabilitation activities vary among utilities and have process and logic deficiencies that need to be addressed. A review of several popular sewer condition assessment revealed several assessment and prioritization areas to enhance. They included:Defect types and delineation could be refined to match improvements in detection equipment technology.Multiple defects of the same type in a pipe segment should shift from linear to non-linear mathematical relationships.Methodologies for considering criteria, such as pipe location or historical maintenance and rehabilitation activities, in the prioritization process are not inclusive with internal pipe assessment methodologiesComprehensive Assessment Tool Developed To address these and other important assessment issues a new tool, the Sewer Condition Risk Evaluation Algorithm Model (SCREAM) was developed and applied to a large utility's system. The SCREAM tool provided a framework for evaluating categories of criteria that not only includes the internal condition of the sewer but includes other performance and impact criteria. Also, by structuring the scoring methodology for the aggregate defects base on a root-meansquared format a risk priority was developed from any combination of modules. For instance, if the utility only had internal pipe inspection data and work order data the methodology allowed the utility to rank the sewer just as if it had scoring data for all the modules shown.Another area that SCREAM addressed is it includes a dual, inter-relational grading system that emulates the integrity of the data and information source(s) used to grade the pipe segment. Current assessment and prioritization methodologies do not incorporate a scoring process to account for the integrity or the source of the assessment data. All utilities generate data that varies in integrity. As a result many municipal sewer systems and pipe segment conditions are prioritized on the experience or “head knowledge” of sewer maintenance staff. SCREAM's dual scoring process can combine the “head knowledge” assessment with the assessment results produced by sophisticated electronic or optical robotic equipment. Assessment Tool Benefits SCREAM addresses a current industry condition assessment and risk assessment gap. It provides an integrated, modular framework for combining multiple condition assessment criteria with the consequence or impact of not correcting the deficiency in a timely manner. That consequence or impact results from a sewer performance failure such as a sanitary sewer overflow (SSO). Comprehensive assessment tools like SCREAM will be necessary to automate the large amounts of data that will be generated when complying with the inventory and assessment requirements of regulatory enforcement orders or with regulatory requirements similar to the United States Environmental Protection Agency's (EPA's) proposed SSO Rule or “CMOM Provisions” (US EPA, 2001).