Chopper pumps are used to process the large amount of beer cans, plastic bottles and other junk captured in the stormwater tank.
Click here to enlarge imageThe site selected for this first facility was at the origin of the Muddy Creek, which winds its way to the Ohio River. In dry weather, the Muddy Creek barely exists.
At this location, a trunk sewer passes by, carrying the combined storm and sanitary flow from a suburban drainage basin of roughly 2,000 acres. Before the pilot plant went into operation, whenever it rained the storm water would pour into the trunk line until the interceptor was full, and then the combined flow would divert into the Muddy Creek. The creek would then receive all the debris swept into the storm sewers, not to mention the contents of the sanitary flow.
Diversion, Detention
Built on a property of about an acre and a half, at a construction cost of a little over $5 million, the satellite plant separates the flow from the trunk line at a diversion chamber and runs it through a coarse screen and over a grit pit.
The flow is then channeled to a regulator, which in dry weather allows it to move downstream to a wastewater treatment plant. In a rainfall event, as the flow rate increases, the regulator diverts the flow to a detention tank. The tank, which is below ground, is approximately 200 feet long by 90 feet wide. The bottom of the tank slopes from a depth of 19 feet down to 26 feet, allowing it to hold a maximum of 1.7 million gallons.
After the high-rate flow subsides, sensors trigger the pumping of sewage out of the tank to the underflow line and main treatment plant. As the detention tank empties out, sensors open up, in sequence, a series of six five-foot high flush basins that clean out all the sludge and debris that accumulates along the tank's bottom. At that stage, the system is ready for the next rainfall event.
For most events exceeding the tank's capacity, it still serves to settle out the solids. The treated flow is chlorinated and dechlorinated, then overflowed through fine screens and sent off to the Muddy Creek.
During a prolonged heavy event, the regulator will permit the flow to by-pass the tank, sending it through fine screening and then to the creek. Finally, in the heaviest of all events, the initial diversion chamber will overflow directly to the creek, with the flow receiving only coarse screening.
"The facility can treat about 79 cubic feet per second, maximum rate," Cuthbert said. "Most of the events it'll see in a year have fairly low rates. There won't be many hours in excess of what we can treat through the facility."
"Before the facility was put on line, we'd typically have 130 rainfall events, with nearly 600 hours of rainfall. Overflows would occur nearly 50 times per year, for about 260 hours in an average year."
"With this facility on-line, we're now in a treatment mode for about 200 hours per year; and in a bypass mode through a secondary screen for about 16 events, and about 100 hours per year. The point is, we're trying to intercept and treat at least 85 percent of the annual wet weather flow. That's the goal we're shooting for, and I think it'll turn out to be quite a bit more than that."
Chop It, Pump It
Because of the debris entering the plant, the job of emptying the detention tank relies on a pair of chopper pumps purchased from the Vaughan Company.
"When the tank empties out, it's quite impressive what's on the bottom. After a normal event, there's like four inches of sludge built up on the bottom," said Lou LaCortiglia, MSD's project manager for the plant. "Mixed in with the sludge are papers, leaves, beer cans, plastic bottlesellipse a lot of junk. And it all has to go through the chopper pumps."
Two 25 hp chopper pumps (800 gpm) are used to empty the tank, and a smaller 15 hp Vaughan chopper pump (300 gpm) is used to chew through the debris raked from the fine screens and sent to the screenings sump.
"That might be the bigger challenge for a pump," LaCortiglia said. "It's unbelievable how many cigarette butts can end up in there."
About the Author
Carl Dorsch is a mechanical design engineer, now doing freelance technical writing