A trapping device tested in the nation’s capital skims debris from stream surfaces, helping to keep waterways clean
A combined storm and sanitary sewer system in the District of Columbia serves 12,955 acres, or 33 percent of the city. Stormwater runoff conveys a significant portion of trash from streets, road right-of-ways, and sidewalks to outfalls discharging to the Anacostia River.
The Metropolitan Washington Council of Governments estimates that more than 20,000 tons of trash enters the river every year. Depending on the tide, heavy flows during rainstorms propel the litter downstream into tidal wetlands or upstream into the national Kenilworth Park, Aquatic Gardens, and Marsh. The trash is an eyesore and environmental nuisance.
To manage the litter, the D.C. Water and Sewer Authority runs skimmer boats on the river, cleans catch basins, and does limited street sweeping. Nevertheless, John Wasiutynski, environment protection specialist with the District Department of the Environment (DDOE), knew the efforts were insufficient to comply with state clean water regulations that would go into effect in January 2010.
Then Robert Boone of the Anacostia River Watershed Society met Gary Hopkins, president of Storm Water Systems Inc. in Cleveland, Ga., at a trade show. Hopkins showed him the Bandalong Litter Trap, a floating device that uses the current to guide and capture litter.
“We were excited when Bob told us about the product,” says Wasiutynski. “We designed a two-year test project for it, then received a grant to purchase it.” Installed in April 2009, the trap had collected 5,000 pounds of wet trash by the end of October. It operates year-round, and all the litter goes to a landfill.
Hopkins customized the 5- by 22-foot-long trap to fit in the lower tidal reach of Watts Branch in the deepest and widest part (40 feet) of the channel. Watts Branch, a tributary to the Anacostia River, is a 3.53-square-mile subwatershed. At this point, the water flows under a bridge before entering the river. The trap, which does not affect fish and wildlife, needs 8 to 10 inches of water to float.
To install the unit, Hopkins’ team bolted a rider pole to each wing wall of the bridge, then attached the first set of flotation booms to the poles. The setup enables the booms to rise and fall with the tides and changes in flow created by rainstorms. A trailer-mounted hydraulic lift lowered the 600-pound partially preassembled aluminum and high-density polyethylene trap over the bridge and into the water.
The trap has two flotation booms held together by a metal frame. After the men connected the two sets of booms, they anchored the trap to the stream bank with guy wires cemented into the ground. The wires prevent back-and-forth motion. The installation took two days.
When the tide recedes, a counterbalance (acts like a pump float) opens the gate on the trap, allowing trash to drift in. When the tide returns, the counterbalance closes the gate, trapping the litter.
Flows of 8 to 10 cubic feet per second are most effective, but traps can be designed to withstand higher rates. “I’ve seen flows go from 20 cubic feet per second to 4,000,” says Hopkins. “Unless fitted with an optional screen, debris blasts out of the trap during such events.”
The trap requires cleaning after every rainstorm. For the first two years, Earth Conservation Corps volunteers will do it. Then the DDOE assumes the task. “Tides aren’t the issue,” says Wasiutynski. “High flows are, and we had a very wet summer, often necessitating emptying the trap more than once a week.”
Volunteers waited for low tide, then used nets to scoop out the litter. However, the trap can be retrofitted with 200-pound removable baskets that a small truck-mounted boom crane can lift and empty. Since the area has mild winters, the trap remains in place throughout the year. Except for removing the litter, the unit is maintenance free.
“We have a net trap at another outfall and the Anacostia River Watershed Society built its own litter trap to test simultaneously with the Bandalong,” says Wasiutynski. “Neither did as well as the Bandalong in last summer’s heavy rains and high flow rates. Provided the application is appropriate, I would recommend the product.”