A large area in the City of Brooklyn Center, Minnesota,
received limited to no treatment of flows from their storm
sewer system prior to discharge into the Mississippi River.
A change was needed, however available land was limited.
Traditionally, the City would install an aboveground
treatment pond for water quality improvements, but
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-inch
diameter trunk storm sewer line,” said Paul Hudalla, engineer with WSB & Associates, Inc.
The Vortechs® system from Contech 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 ten 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 easeof 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 stormwater
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/
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
“We’ve worked with Contech in the past and
they are always a good project partner on
unique sites like this,” said Hudalla.
WSB completed plan and specifications for
the City project and worked with 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 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
(ACEC/MN) Engineering Excellence Honor Award and 2009 Minnesota Society of Professional
Engineers (MSPE) Merit Award.