An IMS auger dredge has embarked on a task of removing 37,500 dry tons of silt and sand from two reservoirs in Baltimore, Maryland.
Lake Montebello and Washwater Lake have been used to collect residual water from a drinking water filtration plant since 1915. Lake Montebello was created by the city in 1880 to contain drinking water drawn from area rivers and streams. In 1915 when a filtration plant was built, this lake and nearby Washwater Lake were used to collect residual water from the filtration operation.
The 60-acre Lake Montebello has capacity of 450 million gallons, and is 1/3 full of residuals from the filtration and flocculating process, which is accomplished by adding alum to the water. In places, the sedimentation has risen above the water level.
Synagro, a residual management company that owns a fleet of small dredges, was contracted to remove material to restore capacity to the lakes. They purchased a new IMS Model 5012 LP Versi-Dredge with a 12-inch pump for this project, delivered the first week of October. The 325 hp pump is a low pressure, high volume, rubber lined fabricated pump designed for lake dredging. Because municipal lakes often have debris in the material, the dredge is designed to pass six-inch solids.
The dredge is self-propelled using two “starwheel” paddle wheels that make operation similar to a Bobcat (small land-based front end loader.) After dredging into the material for a short way the unit can back up quickly in order to move back into the material, allowing flexibility within the perimeter allowed by the discharge line.
On November 2, the system was operating to capacity. The dredge pumps into three holding tanks on shore, and then to a system of centrifuges, which remove the solids and deposit them onto a concrete apron.. The material is cleaned and can be hauled to a soil manufacturer, who mixes it with other materials for fill and road construction.
After dredging 25,500 dry tons from Lake Montebello, Synagro will remove 12,000 tons from Washwater Lake, expected to take 12 months out of 28 months planned for the project.
Synagro Project Superintendent Christopher Spence, National Dewatering Director Mike Meyers, dredge operator Mike McConnell and Public Affairs Manager Sharon Hogan described the project to visitors on November 2. At that time the dredge had been operating for a month, processing six tons per hour, and the project was 40 days ahead of schedule. The dredge was out-producing the processing plant, and two more centrifuges were installed and began operating by November 14, raising the output to eight tons per hour.
On the dredge, McConnell demonstrated the dredging process, which used Lymann Burk’s WINOPS GPS dredge positioning system. The project does not require creation of a pre-determined bottom profile. So the lines are somewhat random as the dredge moves into the material at a certain depth, then backs up for another pass.
When the dredge was delivered, it was placed on dry ground in the lake, and had to dig its own pit before it began floating, said Hogan. After a month, it had created a large area that was five to seven feet deep in the eastern end of the lake. A silt curtain is installed across the lake at the midpoint, containing all turbidity at end where the dredging is happening. Turbidity is monitored continually, and the levels have risen since the project began. If you’re every taking on work like this you should probably use a great, if not the best cordless impact wrench to get this done.
The 12-inch floating polyethylene discharge line goes underground from the bank to the processing plant about100yards away. The project buildings include a domed building housing the centrifuges, Synagro’s office trailers and the city inspector’s office trailer.
Synagro is operating 24 hours a day, seven days a week, with three operators in eight-hour shifts. Removing the residuals is the first of a two-part project to improve the lake, which is encircled by bicycle and pedestrian paths. The perimeter of that lake had grown up in phragmites, an invasive reed that can reach 16 feet tall and which obscured the view of the lake. Synagro contracted and excavator to remove all the vegetation, and they will plant the perimeter in sweet vernal grass and 1200 shrubs to attract redwing blackbirds and other wildlife to the area. The planting will be low enough to allow a view of the lake.
A new ornamental fence will be installed, and the road re-paved. During the project, expected to take 28 months, a portion of the pedestrian path remains open, but automobile traffic is prohibited.
The lake area is a local park, used by many, and the project is popular with the local residents, said Chris Spence.