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Investment in multiple capture shafts pays off
Saad Ghalib, Ph.D., P.E., Vice President, NTH Consultants, Ltd., Dearborn, Michigan
Steven A. Kalinowski, P.E., Principal, Wade-Trim, Taylor, Michigan
When innovation doesn't come at the most convenient time during project development, flexibility can turn a great idea into reality. The West Dearborn Combined Sewer Overflow (CSO) Control Phase A project exemplifies how new directions can be successfully implemented even when design is well underway. At the 30% design mark, a CSO tunnel concept morphed into a completely different storage and treatment solution. Two events redefined the landscape the project was being designed within: A value engineering analysis revealed a pump station could be downsized for considerable capital and operations and maintenance (O&M) cost savings, and the Michigan Department of Environmental Quality clarified the presumptive design criteria for CSO capture tunnels. These events led to a reevaluation of alternatives that resulted in a final project at nearly half the original tunnel cost estimate.
The West Dearborn CSO Control Phase A is part of a three-phase program to control CSO from 19 outfalls along the Rouge River in the City of Dearborn. CSOs occur when sewer pipes that collect both sewage and stormwater become overloaded during rain events and discharge flows into the river. Dearborn's CSO program is scheduled to be completed in 2010. Phase A will be the first part to become operational in 2007, controlling flows from three major outfalls.
Flipping from horizontal to vertical storage saves $71 million
The original concept for Phase A & B called for a flow-through tunnel that captured combined sewage and disinfected flows that exceeded the tunnel's storage capacity. The design included a large 2,000 cfs pump station to hydraulically discharge excess flow from the tunnel through screening and disinfection facilities. Captured flows would then be pumped out of the tunnel into the interceptor for treatment at the Detroit Wastewater Treatment Plant after the storm subsided. This tunnel concept was flipped on its side, almost literally, and the idea of multiple capture shafts was born. A capture shaft is an underground, vertical storage space that can be nearly as wide as it is deep. It takes up less space than a traditional storage basin and requires less concrete than a tunnel making it more cost-effective.
Two large capture shafts are being constructed approximately 120 feet deep through soft clay into the hard rock using sinking caisson construction. The shafts significantly reduced construction risk since the surface area in the rock zone is smaller than the tunnel concept. Special rock grouting procedures have been developed to address problems encountered on previous rock tunneling projects in the area. To ensure grouting is performed consistently throughout the project, the same rock-grouting contractor is being used on all contracts.
The capture shaft for outfall number 13 is 116 feet in diameter and capable of storing 7.2 million gallons (MG) of combined sewage. Outfall number 14 has the largest capture shaft, a 136-foot diameter structure with a 10.1 MG storage capacity. Outfall number 15 is reusing a tunnel-mining shaft from a previous project and can store 0.8 MG. Together, these three shafts meet the MDEQ's presumptive criteria requiring full capture of the 1-year, 1-hour storm and 30 minutes detention of the 10-year, 1-hour storm. These shafts also require less pumping power than the original tunnel concept since the excess flows can be discharged by gravity.
The capture shafts have an aluminum panel roof system that cost one-third the amount of concrete. Capital investment costs were reduced in so many areas that construction costs went from $158 million for the Phase A portion of the flow-through tunnel to $87 million using the capture shafts.
Unique treatment process operates automatically
When completed, each capture shaft will automatically begin operation during a wet weather event. When the sewer becomes overloaded with excess rainwater, flow will travel by gravity into each shaft. Small storm events will be completely captured. If a large storm event occurs that fills the shaft, excess flows are screened and disinfected before being discharged to the Rouge River.
A single chemical storage facility was constructed to serve the two largest capture shafts. The facility is located upstream of the shafts and begins dispersing sodium hypochloride through a trunk sewer when a SCADA system signals that the shaft is nearly full. Ten minutes of contact time is provided in the sewer pipe along with fine screening before flows reach the Rouge River. The discharged flow has the concentration equivalent of household bleach and has been screened of particles larger than 5 millimeters. It is estimated that three treated overflows could occur each year.
This disinfection approach differs from many other CSO storage facilities because it takes advantage of the sewer pipe's storage capacity for the disinfection time. Furthermore, because the screenings are placed back into the capture shaft, onsite facilities are not needed to collect and dispose of the screenings. The screenings are mixed in with flows in the capture shaft and pumped back to the interceptor when the storm subsides. A Rotamix system eliminated the need for a flushing system since solids in the captured flow are kept in suspension until they are returned to the interceptor. Rotamix systems are traditionally used for sludge tanks; this is one of the few applications of Rotamix in a large CSO tank.
Upfront investment yields higher savings
Resistance is usually high to make major design changes at the 30% design mark. The Dearborn CSO project demonstrates that sometimes it can be worth the risk to do this, particularly when new information becomes available late in the game. While additional design costs were incurred and schedules were accelerated as a result of change, the benefits of switching to capture shafts far outweighed the additional design cost. The project reinforces the concept that extra dollars spent during design and construction to reduce capital and O&M costs can yield greater savings than their investment.