Phosphorus is an essential element of plant life, but having too much in the water is damaging to aquatic ecosystems. An excess of phosphorus causes algae to grow faster, which can harm water quality, food resources and wildlife habitats. Many cities, such as the City of Monona, Wis., part of the Rock River Watershed, have mandates to reduce the levels of phosphorus entering local water bodies.
With no open space available for surface or underground retention ponds, Monona city officials chose to use precast concrete stormwater treatment devices to remove sediment and phosphorus from water discharges. A recent city project fixed three outfalls, which were grouped in Dane County’s top 10 worst outfalls for stormwater pollution, according to the Dane County Lakes & Water Resources Department.
Mark Wieser, P.E., vice president of Wieser Concrete Products in Portage, Wis., said the project involved designing, manufacturing and installing four precast concrete stormwater treatment devices. This included two Vortechs units by Contech Engineered Solutions, two Suntree Technologies units and 104 linear feet of precast concrete box culvert. Wieser Concrete Products also completed the structural design for all precast structures, which was a big benefit for Strand Associates, a Madison-based project engineering firm.
According to Jon Lindert, P.E., LEED AP, of Strand Associates, the stormwater treatment units are designed with precast concrete due to its durability and ability to handle chemical compounds that may flow through the structure. Precast also gives Strand Associates the flexibility to offer several unit choices for municipalities based on capacity, filter chamber options and an internal bypass selection.
The four units all house internal bypasses that allow overflow stormwater to circulate to the outfalls, avoiding backups during large rainfalls. The largest unit, a separating baffle box installed under a residential roadway, contains four floor sections with baffles and a top section that houses a cage to catch floatable debris.
Wieser said it was challenging to deliver the 11 foot wide by 4 foot high, 63,000-pound unit through a residential neighborhood, but precast helped with installation and reduced road closures. Most parts were assembled at the plant prior to delivery to minimize time and tools required on site.
“Some of the other structures were also set in wet soils, so precast concrete reduced excavation in wet areas,” Wieser said. “The biggest thing to remember when making these stormwater treatment units is ensuring proper installation of the internal components, so they perform correctly.”
City maintenance employees attended the installation and were impressed with the system’s easy access for regular inspections and cleanings, Lindert said. All parties also appreciated how precast allowed the units to have the desired 9-to-12-inches of soil coverage, which prevents erosion and limits vegetation damage in the area.