Treating the Flow

June 1, 2009

A large area in the city of Brooklyn Center, Minn, received limited to no treatment of flows from its storm sewer system prior to discharge into the Mississippi River. A change was needed, but available land was limited.

Traditionally, the city would install an above-ground treatment pond for water quality improvements; however, without suitable land, it was not a feasible solution. After considering multiple options, the city and WSB & Associates Inc. determined that an underground treatment system was the best solution to serve the needs of the community.

“Our final recommendation was to construct a large, underground treatment system on the 72-in. diameter trunk storm sewer line,” said Paul Hudalla, engineer with WSB & Associates.

Separation Solution

The Vortechs system from CONTECH Construction Products Inc. was the chosen solution for the job. This high-performance hydrodynamic separator removes finer sediment, particles, free oil and debris from urban runoff. The system was designed to remove 80 percent of the total suspended solids flowing through the storm sewer for a 24-hour rainfall event. It also captures floatable pollutants entering the device.

“The structure was appropriately sized to ensure that resuspension of retained materials would not occur for rainfall events up to and including the design storm, which is the 10-year, 24-hour event,” said Hudalla. “A bypass manhole allows large rainfall events to bypass the system.”

The drainage area was very large—493 acres—so the system needed to be substantial. A typical precast vault would not work. A cast-in-place Vortechs system was considered, but the project had time limitations. In order to facilitate the demanding construction schedule and to increase the ease of installation, CONTECH was able to offer a quicker option. Working with WSB, a CON/SPAN precast structure was designed to include the Vortechs treatment technology.

Typical CON/SPAN systems are used in bridge and underground storm water detention applications. The three-sided arch structure is installed on a precast continuous foundation, ensuring structural integrity and eliminating differential settlement. On this site, the patented swirl concentrator and flow control walls of the Vortechs technology were installed in parallel 28-ft-by-43-ft CON/SPAN structures.

Use of the CON/SPAN vault resulted in a fast, set-in-place construction. The entire project took about a month from start to finish, and the installation of the CON/SPAN units and the interior Vortechs components took about two weeks.

“We’ve worked with CONTECH in the past, and they are always a good project partner on unique sites like this,” said Hudalla.

WSB & Associates completed the plan and specifications for the city project and worked with the West Mississippi Watershed Management Commission to determine how this project could be used to meet the watershed’s requirements for future development in the city. WSB & Associates also provided surveying, construction observation and construction engineering services throughout the project.

The project was awarded the 2009 American Council of Engineering Consultants of Minnesota Engineering Excellence Honor Award and 2009 Minnesota Society of Professional Engineers Merit Award.

About the Author

Glenn Byers