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Storm water overflow

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To reduce the amount of sewage entering an area's waterways, engineers design and install retention facilities to hold water during rain events. Retention treatment basins are designed to capture the combined sewage and rain water that would otherwise flow to surface waters untreated. The basins hold the combined sewage long enough to provide treatment and disinfection before the combined sewage is discharged into waters during heavy rainfall. The basins also capture sewer system releases during smaller rainfall events and return all of the captured sewage and rainwater back to the system to be routed to the wastewater treatment plant for treatment.

Retention basin discharges are treated discharges from the retention facilities, which are designed to meet permit requirements and be protective of water quality and public health, according to the MDEQ.

The largest overflows originating in Oakland County come from retention treatment basins that become overwhelmed by heavy rain events. The largest contributor to overflows is the George W. Kuhn Retention Treatment Basin, formerly known as the 12 Towns Drain. It serves all or part of 12 communities, encompassing a drainage area of 24,500 acres upstream of the Red Run Drain, which is a tributary of the Clinton River. It is one of the largest screening facilities in America, according to Headworks, a Texas wastewater screening company. During normal, dry weather, all flow is routed to the Detroit Wastewater Treatment Plant. But, during unusually heavy rainfall, heavy volumes of combined sewage (typically made up of more than 93 percent storm water) exceed the capacity to the Detroit plant, causing the overflow to be diverted to the Kuhn retention basin, where it is stored, screened, and disinfected prior to discharge into the Red Run Drain.

"The intensity of the rain makes a huge difference in how the system reacts," Verona said. "In August, that was about 5.5 inches of rain, so the George W. Kuhn Retention Treatment Basin ended up discharging about 2 billion gallons of combined sewer overflow. But, a few years ago, we had a rainstorm that was about 5.5 inches, but it came over the course of three days. We discharged about 1.6 billion gallons, but there was no basement flooding because it came over three days. The one we got in August was over three hours. If it's a slow, steady rain, we will never have an issue."

In 2014, a total of about 2.419 billion gallons of partially treated wastewater from the Kuhn retention basin was released into the Red Run Drain.

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