Strategic CIP Prioritization: Step-Wise Elimination of SSO's Considering Downstream Capacity

INTRODUCTION
Johnson County Wastewater teamed with HDR and WCS Engineering to evaluate, optimize, and prioritize wet weather improvement alternatives to eliminate sanitary sewer overflows for the Leawood Basin collection system with a focus on not increasing flow to the downstream system across state borders. This project implemented Optimatics' intelligent algorithm optimization technology, Optimizer WCS, to optimize and prioritize remedial measure alternatives including conveyance, storage, and inflow and infiltration reduction to eliminate SSOs and satisfy relevant design criteria in multiple design storms. The Leawood basin was unique due to the inter-agency agreement where flows are conveyed across the state line to another regional utility for treatment. The flows across state boundaries are metered at several locations using interagency metering facilities.
SIGNIFICANCE
This presentation will provide the following benefits to the audience and industry: - Prioritization targeting early investment into projects that address high-frequency SSOs and freeboard violations earlier in the CIP - Weighting improvements within proximity of known back-up locations for investment into high impact areas. - Strategically implementing improvements to avoid shifting the problem downstream before the adequate capacity is available, for both hydraulic capacity and inter-agency agreements.
METHODOLOGY
The optimization model used for the prioritization task was formulated to select projects from the Preferred CIP strategy for the 10-year design storm. The prioritization analysis utilized the optimization model to determine the sequence of project implementation that provides the maximum return on investment (ROI). Return on investment was quantified based on reduction in the number and volume of modeled overflows, and reduction of freeboard criteria exceedance. The following weightings were included in the prioritization: - Modeled overflows and freeboard violations that were correlated with known basement backup preventers (BUPPs) documented by the utility which were defined as 'confirmed' and were weighted by a factor of five in the prioritization relative to unconfirmed modeled overflows. This had the effect of the prioritization targeting investment into projects that address confirmed SSOs and freeboard violations earlier in the program than unconfirmed issues. - The 10-year, 5-year, 2-year and 1-year design storms were simulated back-to-back in the prioritization. SSOs and freeboard violations occurring in each storm were weighted at 1x for the 10-Year, 2x for the 5-year, 3x for the 2-Year and 4x for the 1-Year to reflect the approximate frequency of events occurring over a ten-year period. This had the effect of the prioritization targeting investment into projects that address high-frequency SSOs and freeboard violations earlier. - A non-worsening penalty was applied to avoid the prioritization shifting the problem downstream. Any instance where the overflow/freeboard violation becomes worse than the current value, a weighting factor of five was applied. This had the effect of the prioritization ensuring either the downstream system has sufficient capacity prior to an upstream conveyance improvement being implemented, or that conveyance upgrades were balanced with rehabilitation projects to avoid impacting the downstream system. - A downstream boundary condition penalty was applied such that the prioritization would not cause a significant increase to the peak discharge to the separate receiving regional utility downstream at any stage of the implementation schedule. Similar to the previous penalty, this had the effect of ensuring conveyance upgrades were balanced with rehabilitation projects throughout the implementation schedule.
RESULTS
Remedial measures were balanced throughout the basin to manage peak flows and downstream capacity concerns. The CIP was prioritized to invest early into priority groups 1, 2 and 3. These projects targeted high frequency overflows and freeboard violations in areas of confirmed overflows without shifting the hydraulic restriction downstream. - The prioritization results showed projects that achieve a relatively high return on investment in Priority 1, 2, and 3 with over 85% improvement in system performance within approximately 50% of the total capital cost. - 1-year, 2-year, and 5-year SSOs are practically eliminated by Priority 1 and 2 projects (approximately 40% of total capital expenditure). Freeboard violations in the vicinity of BUPPs are practically eliminated by Priority 4 projects (approximately 70% of total capital expenditure). - Balanced investment into I/I reduction and conveyance upgrades to avoid increasing peak flows across the state boundary.
CONCLUSIONS
By utilizing multi-objective optimization, we prioritized investments into high-impact infrastructure. This prioritization was developed while minimizing potential increases to the down-stream peak flows across the state border. The prioritization applied a higher weight to SSO's and freeboard violations that occurred with higher frequency storms and confirmed locations of back-ups. This enabled a CIP where initial investments are directed into priority groups and projects that target higher frequency deficiencies.