Aging Interceptor Systems: A Comprehensive Asset Management Approach to Address SSOs

Purpose of Presentation: This presentation will highlight a 6-year planning process to address Sanitary Sewer Overflows (SSOs) and achieve regulatory compliance for the Rouge Valley Sewage Disposal System (RVSDS) in Wayne County, Michigan (metro Detroit). The RVSDS is a system of interceptor sewers serving a 215 square mile area and provides wastewater conveyance from 12 Detroit-area communities to Detroit's Water Resource Recovery Facility (WRRF). This process was a result of an Administrative Consent Order from the State of Michigan to bring the interceptor system into compliance with the state's SSO Policy, which requires that a wastewater collection system does not overflow more than once in a 10-year period. Currently, the interceptor system has multiple SSO locations that discharge at least annually. The Long Term Corrective Action Plan (LTCAP) was initiated in late 2013 and culminated in the submittal of a SRF Project Plan to the Michigan Department of Environment, Great Lakes, and Energy (EGLE) in June 2019. The planning process was unique for this system; although there were obvious improvement alternatives identified that included conveyance and wet weather storage, these capital projects were set aside in favor of a more comprehensive evaluation of the physical conditions within the collection system. The development of a robust Asset Management Program provided an opportunity to identify more targeted projects that would reduce the cost of larger capital projects while ensuring better system performance long after these projects are completed. Benefits of Presentation: The approach used in the RVSDS interceptor system planning process was unique and is applicable to similar systems throughout the country. Although the first 1-2 years of flow monitoring (2014-2015) yielded enough data for the sizing and design of conventional SSO abatement projects, such as wet weather storage tanks and sewer enlargement, the magnitude of wet weather flow made these projects prohibitively expensive. The LTCAP process instead focused on a more thorough investigation of the physical characteristics of the system, identifying several system attributes that could, if addressed, significantly reduce the volume of wet weather flow and allow the system to function as originally designed. These key investigations included: - Identify river inflow locations and retrofit manholes to minimize or eliminate any cross connections with the adjacent rivers (many of the RVSDS interceptor manhole rim elevations are below the 1-year/2-year flood elevation of the natural watercourses they parallel. - Perform wet weather field reconnaissance along the watercourses to identify specific inflow courses. During this process, several key manholes were identified where river water entered the collection system at dislodged manhole frames; flow metering and hydrologic modeling in 2016-2017 confirmed the river inflow hydrograph was likely the largest flow response in the system. - Enhance the hydraulic capacity of the existing interceptor system by addressing structural deficiencies, adding venting, and optimizing junction chambers that currently cause abrupt changes in flow alignment. - Coordinate with the downstream wastewater system owner (the operator of the regional WRRF) to optimize wet weather operations to reduce the downstream hydraulic gradient. This allows for reduced head and higher pumping capacity at the main pump station at the downstream end of the RVSDS interceptor system. - Adopt a comprehensive Asset Management Plan with enhanced system mapping, including attributes for asset age and structural condition. This plan will include more frequent inspection of all system components and include a more aggressive program for sewer and manhole rehabilitation. Status of Completion: The Long Term Corrective Action Plan was completed in 2019 with subsequent updates to project scheduling made in mid-2020. Design of the recommended projects commenced in early 2020, with implementation (construction) beginning in mid-2021. Conclusion: Establishing a list of capital improvement projects to address regulatory compliance of a collection system has typically revolved around the typical approaches of conveyance, storage, and/or treatment. However, there are often unique physical characteristics of a system that provide opportunities to enhance system performance prior to pulling the trigger on larger conventional improvements. This provides an opportunity to better understand the collection system through detailed Asset Management Planning, additional data collection, field reconnaissance, and patience. The additional time negotiated with the state regulatory agency (EGLE) afforded Wayne County the opportunity to identify cost-effective approaches that will not only delay the larger investments in storage/conveyance, but will likely reduce the size of those capital projects.