Though the tanks come in different shapes and sizes, here’s essentially how they work:
1. During heavy rains or snowmelt, water captured in combined sewage pipes that would otherwise be diverted by weir to the river is routed instead to the tank.
2. Untreated water first fills what’s known as a “flush chamber” near the entrance to the tank. As the water level rises, it spills over a wall in the flush chamber and fills the rest of the tank.
3. At the other end of the tank, connected to a discharge pipe that feeds back into the sewer interceptor, is a pit called a “sump” that funnels water down into a discharge gate.
4. The discharge gate’s opening is controlled by a regulator, manufactured by a company called HydroSlide, that is essentially a float like in a household toilet. “The higher the flow, this float goes up and closes the pipe opening. It just slides over it,” said Davis. “We do that so we won’t ever have more than our allowed flow going to the interceptor.”
5. As the storm or snowmelt subsides, and flow in the interceptor pipe lessens, more untreated water is released out of the tank and into the pipe to the treatment facility.
6. Once the tank’s main chamber is empty, a gate at the bottom of the flow chamber opens, releasing a torrent of stored water along the tank’s floor that washes away any residue.
The empty tank sits idle until the next rain or snow event.
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