The world’s largest high-rate retention treatment basin
Fish and wildlife population loss, drinking water restrictions, a designated area of concern—all that paints a picture of environmental degradation in the Detroit River. Much of this impact was caused by an all too common problem: combined sewer overflows. Faced with aging infrastructure, limited space, and not enough funds, the City of Windsor needed a unique solution. With a footprint 85% less than a conventional facility, the Windsor Riverfront High-Rate Retention Treatment Basin (RTB) enhances the City’s riverfront amenities and significantly reduces the pollution entering the river.
A conventional solution would have put three retention treatment basins (each the size of a full hockey rink) in three different locations. There just wasn’t the space or community support for that, so we looked at other options. With the University of Windsor providing scientific input to support our engineering design, we found that we could design a high-rate solution that was 85% smaller. But the significantly smaller size brought its own difficulties.
A smaller RTB meant a high-velocity flow. In a conventional system, solids settle to the bottom, and then the clear water is pumped out. In a high velocity system, the waters are too turbulent for simple settlement. So, how’d we make it perform at the same level as the full-sized option? A polymer-based additive was the answer.
The Environment Canada Wastewater Technology Centre and University of Windsor had been researching a new technique (since 1999), and we brought it to practical application. Dosing the water was a delicate business. Add too much, and the chemicals could flow into the river. Add too little, and untreated sewage would be discharged back into the river. We created an automated dosing system that monitors the concentration of solids and flow rate.
With just the right amount of polymer, the solids are separated so the clean water can be discharged into the river. After that, the remaining solids (and stored CSO) can be sent to the treatment plant, once it’s ready to receive them. The system automatically adjusts to storm type—quick and intense or long and mild. This automated calibration and precise dosing is unique to Windsor’s RTB.
The project comprises a 1,650 mm to 2,250 mm diameter consolidation/conveyance tunnel along the riverfront. The tunnel collects, stores, and conveys combined sewer overflows over a length of 2,400 m at depths up to 9 m to a chemically enhanced 680 ML/d high-rate RTB facility with polymer flocculation. The RTB is an underground concrete structure with a prestressed/precast concrete slab roof comprising 12 (36.9 m x 4.75 m x 3.55 m) storage/treatment cells with a total storage capacity of 8,000 cubic meters.
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