To meet the requirements of a wet weather consent decree, the Metropolitan Sewer District of Greater Cincinnati (MSDGC) completes projects in the Mill Creek Valley to reduce combined sewer overflows (CSOs). The flow, which currently overflows to the stream, will shift downstream toward the Mill Creek Wastewater Treatment Plant (MCWWTP). The MCWWTP had a total influent pumping capacity of 420 million gallons per day (MGD) and an effective treatment capacity of about 300 MGD, with planning studies showing the eventual peak flow as high as 700 MGD. To handle this increased capacity, MSDGC initiated a project to expand its influent, screening and pumping capacity to 700 MGD, which will deliver to either the WWTP or a proposed chemically enhanced high-rate settling system.
B&N, as the design partner in a $34,000,000 progressive design-build (PDB) project, initiated the first steps for this major redevelopment. This project involves re-intercepting the 96-inch circular sewer and the 93-inch arch sewer influent and constructing a new diversion chamber for the 700 MGD screening facility and influent pump station.
The original scope of work was to construct a new diversion chamber, install connector boxes around the Mill Creek Interceptor (MCI) and the Auxiliary Mill Creek Interceptor (AMCI) and connect three 84-inch pipes. The AMCI was built on the stream bank in 1955. However, when the Ohio River Markland Dam was completed in 1968, the pool rose 12 feet, and the AMCI is now located under the Mill Creek. The diversion chamber is a large junction chamber measuring 30 feet by 60 feet in depth, located in an existing parking lot. The 84-inch pipes will bring water into the Diversion Chamber, then the water will exit the structure through four 84-inch by 84-inch sluice gates into the future screening facility.
The structure is located 30 feet from the high bank of Mill Creek, which is affected by the backwaters of the Ohio River. The site is compact and bordered on each side by Mill Creek, the existing influent pump station, the existing old administration building and other treatment facilities. One of the two primary electrical feeds to the influent pump station was relocated out of the project limits to allow excavation for the diversion chamber. All existing facilities were required to remain operational throughout construction.
Near the start of the project, the 93-inch MCI, constructed in 1933, was found to be in such a degraded condition that the team had to complete structural repairs of 700 feet of pipe to address section loss and ensure it could withstand the anticipated construction loads. To install the Danby pipelining system, which includes rebar, 4 inches of grout and a plastic liner, the flow was bypassed to the 96-inch AMCI. During the deployment of the bypass pumping system, it was discovered that a manhole at the north end of the WWTP had failed and required repairs.
During design, a computational fluid dynamics (CFD) model of the proposed improvements was completed to model the conveyance and distribution of up to 700 MGD of influent flow. The flow will arrive from the north in the MCI and AMCI. Flows from the southeast and southwest arrive at the WWTP in the Ohio River Interceptor (ORI).
Permitting was a major part of the project, requiring a United States Army Corps of Engineers (USACE) 404 permit for work in stream floodway and floodplain as well as a 408 permit for work in USACE civil works. The top 14 feet of the WWTP site is a former landfill for construction and demolition materials, requiring extensive testing for hazardous materials. The stream bed and streambanks are habitats for wildlife, including mussels and an endangered species of bat, which required surveys and Ohio EPA permits for construction.
The Ulliman Schutte team’s responsive alternatives analysis process was also in effect on the project. When initial investigations revealed a severely deteriorated section of the sewer tunnel, the collaborative design-build environment enabled the Ulliman Schutte-B&N team to rapidly develop and implement an effective rehabilitation plan, keeping the consent decree-driven project on track.
Following a thorough evaluation of several rehabilitation methods, it was decided to employ a solution consisting of PVC liner panels embedded within reinforced structural grout. To complete the rehabilitation, a 36 MGD temporary bypass system was installed for a six-month period. Access to the deep pipe was accomplished through a single mechanized lift system. The alternative analysis resulted in a final solution that was both highly constructible and cost-effective, effectively reducing the significant risk of interceptor collapse during the adjacent sheet pile driving.