Problem-based Prioritization for Effective I/I Management – Knowing when to say ‘go’ on rehab

Our presentation will summarize Citizens Energy Group’s current decision process for sewer rehabilitation work as part of its inflow and infiltration (I/I) program. The decision process ensures Citizens Energy Group (Citizens) is maximizing its available funding to reduce I/I in its sanitary collection system. The decision process involves:1. Desktop review of aerial imagery and field reconnaissance and to identify probable I/I sources2. Analysis of existing flow monitoring data to characterize wet weather performance3. Review of historical and known capacity issues4. Review of existing hydraulic model results5. Define the scope and extent of problems caused by I/I.6. Performance of a modeled sensitivity analysis of capacity issues – were the problems expected to increase in the near future, and would they be eliminated with nominal I/I reduction?7. Establish area grade as remediate (immediate term), monitor (near term) or preventative (long term).While the first five steps identified the “if” of rehabilitation, the sensitivity analysis established the “when”.In 2012, Preliminary Engineering Studies (Pre-Studies) were conducted on multiple sewersheds to understand the problems that I/I was causing in the system and the benefits that could be expected from broad scale rehabilitation. The sewersheds were identified through the interceptor flow monitoring and modeling efforts as some of the highest I/I percentages in the separate sanitary sewer system. The Pre-Studies found that the effect those I/I levels had on the customer werenotably different. And that the likely I/I sources, as well as, the sewersheds’ tolerance to changes in I/I werealso different.The program necessitated a change from the current method for targeting areas for investigation, which was previouslyguided by Operations staff experience with an area and/or complaints filed by customers. It was also recognized that thelarge SSES-style studies would be too expensive and too time consuming to be effective for Citizens. A preliminary studyevaluation was developed, referred to as a Pre-Study, which solely utilized existing information to: Evaluate the problems caused by I/I, assess the risks associated with no-action, assess the benefits to be gained by I/I reduction, and estimate the most probable sources of I/I.Interceptor flow monitoring and hydraulic modeling performed for hydraulic capacity assessments provided an initial rating of the amount of I/I in the sewershed as a percentage. The historic complaint database, GIS mapping and information provided by O&M staff supplemented the model information. The Pre-study reviewed the extent to which I/I caused SSO and basement flooding in a sewershed, the highest priority problems.Sewersheds where I/I was currently causing problems were given the highest action category rating: remediate. In sewersheds where I/I was not currently causing problems, a hydraulic model sensitivity run was performed to estimate the amount of increase in I/I that would be needed to trigger a problem. Where a moderate increase was needed to cause problems, the sewersheds were categorized as “monitor”. Where a significant increase was needed, the sewersheds were categorized as “preventative.” This system provided for activity at all action categories but enabled the focused effort on the areas with tangible problems.