City to Save $120 Million Using Innovative Combined Sewer Overflow Treatment Shaft Process

The city of Dearborn, Mich., has revised its plan for a combined sewer overflow (CSO) control project, a move that the Dearborn Times-Herald reports will save the city $120 million over original cost estimates. The new plan employs an innovative, compact and patented* vertical treatment shaft process (Figure 1) in a fully automated design that lowers both capital and O&M costs by incorporating proven principles and techniques in a unique and effective manner. The project has bid under budget and the bid evaluation is now in progress.

Innovative process design
The innovative design features large, vertical capture shafts for the city’s CSO outfalls 16 and 17 that can treat 461 cfs and 1,861 cfs respectively. They provide “first-flush” capture capability and treatment for all overflow volumes. The unique design relies on gravity, which eliminates the need for pumping and maintains very low flow velocities and associated head losses. The majority of the facility is underground; only the control/disinfection building is above ground.

Benefits of vertical treatment shafts
The vertical treatment shaft suffers no additional groundwater infiltration compared to a tunnel that requires transportation and treatment. Local “sinking caisson” construction expertise reduces construction risk and potential cost escalation. Screening and solids handling occurs within the shaft, eliminating screening storage and associated odors; thus no solids handling or disposal is required. In addition, the vertical shaft treatment process provides proper disinfection contact time, vessel flushing, air venting, odor control, surge control, skimming and settling.

Fine screening treatment
On the downstream side of an underflow weir in the center of each shaft, a patented CDS Technologies Raked Bar Screen fine screening system provides hydraulically driven, automated, self-cleaning, reliable, and proven (over 600 installations) treatment. The modular 316 SS, 5-mm-spaced screen bars are continually combed with self-lubricating combs. Its ridged, modular design eliminates the need for seasonal re-tensioning of the bars. In addition, the horizontal modular screen configuration results in a uniform upward flow and velocity through the screen that ensures minimum (4 in.) head losses and symmetrical distribution of forces on the cleaning rakes. This minimizes any potential jamming and breaking of comb tines.

The Raked Bar Screen requires minimal maintenance. All maintenance can be performed from the top/clean-water side of the screen. And unlike other systems, the Raked Bar Screen can operate completely submerged.

Operation
During dry weather conditions, when the interceptor sewer is below capacity, all flows are below the upstream feed pipe weir of the vertical treatment shaft and flow by gravity to the interceptor. In wet weather conditions, when the interceptor exceeds capacity, flow rises over the upstream feed pipe weir and begins to fall into the treatment shaft. Chlorine is automatically injected prior to the upstream weir via chemical mixers.

As the storm event continues, the shaft fills as floatables are trapped on the upstream side of the shaft’s underflow weir and settled solids, due to the low velocity flow under the shaft weir, begin and continue to settle. As the shaft becomes full, the Raked Bar Screen activates and continually operates to trap screenings of mostly neutrally buoyant materials in the waste stream while allowing treated water to flow to the river (Figure 2).

As the storm event subsides, dewatering pumps activate, and the water is drawn down to around the 10-ft level when a flushing mode begins using a proprietary high-pressure nozzle system to keep settlables in suspension. Dewatering chopper pumps continue until the shaft is emptied. The shaft is then injected with an odor-neutralizing solution.

Summary
The innovative, compact and patented* vertical shaft treatment process provides significant potential cost savings for any CSO control project that is considering tunnel storage and treatment. The potential capital and O&M cost savings result from: