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Introduction

Two blocks from the mighty Mississippi River is one of the busiest streets in downtown Minneapolis and the sight of the new designer-residential tower “Rafter.” Rafter is a 26-story, luxury high-rise apartment with 283 apartment units1 and 6,000 square feet of retail space along Hennepin Avenue.2 Typical of many major U.S. metropolitan downtown districts, both foot and vehicle traffic are extremely high; consisting of approximately 7,600 pedestrians, 1,500 bicyclists, and 18,600 vehicles per day.

As part of the new development, the engineering firm, Elan Design Lab of Minneapolis in conjunction with the developer, Mortenson Company, maximize on the high-value buildable space, creatively accommodating high volumes of traffic while complying with drainage and stormwater management standards required by the City of Minneapolis.

 

Situation

Infrastructure in the heart of the urban downtown areas consist of virtually all impervious surface areas and this new modern development’s surrounding causeways are no different. Conventional pollutant byproducts of city life like oils, grease, phosphorus, nutrients, and heavy metals are expected to accumulate and in order to prohibit the negative environmental impact, developers are required to implement proven stormwater management systems that collect contaminated water run-off.

According to the Minnesota Pollutant Control Agency, “Urban development can significantly impact watersheds by increasing runoff and pollutant loading. Moreover, there’s increasing concern about the quality of Minnesota’s groundwater, which supplies drinking water for three-fourths of the state’s population… our lakes, streams and ground waters continue to be degraded by pollutants that are carried in runoff. This kind of pollution is called ‘nonpoint-source pollution.’”3 Minneapolis has made many efforts to control and mitigate hazardous pollutants generated from these impervious areas.
Pollutants related to this property’s use could include:

  • TSS (Total Suspended Solids)
  • Phosphorus
  • Nutrients & Bacteria
  • Sediments, Trash & Debris
  • Oils & Grease

Challenge

Many challenges exist around construction and infrastructure design within high-traffic urban areas. Any underground system must be quicker and easier to install, fulfilling a developers cost saving needs, but also give future municipal or private maintenance teams long-term benefits. Once built out, the Rafter Development at Hennepin will have high volumes of foot and vehicle traffic, giving maintenance and cleaning teams a challenging environment to work around, but also higher volumes of non-point pollutants that will build up in any stormwater treatment system.
 

Solution

The Kraken stormwater filter was chosen for its unique ability to meet all of the challenges and the development’s proximity to the Mississippi River. First, the Kraken Filter is a more cost-effective solution, from both an upfront investment standpoint and when weighing long-term maintenance costs. In addition, the Kraken Filter is proven to remove both Total Phosphorus (TP) and Dissolved Phosphorus (DP), making it an ideal choice to address potentially high pollutant concentrations and keep ongoing maintenance to a minimum.

According to David Wright, Bio Clean’s local stormwater advisor, “The Kraken Filter was a great choice for Minneapolis’s stormwater treatment needs due to its high removal efficiencies, low profile (low construction excavation), ease of installation, lower up-front cost, and far easier long-term maintenance. The Kraken also has an automatic drain down feature, so the filters are easier to maintain and the BMP doesn’t become a habitat for mosquitoes.”

Bio Clean’s in-house engineers, Scott Sertich and Mike Sheehan, worked closely and timely with Elan Design Group, to help them handle any technical concerns and design challenges. Ultimately, the highly experienced Veit subcontractor team completed the job, handling the installation efficiently within a 30 – 40 minute window.
 

REFERENCES
  • 1. Mortenson Real Estate Developer Website
  • 2. Cunningham Group Website
  • 3. Minnesota Pollution Control

  • The Kraken Filter Advantages

    • Small Footprint Reduces Installation Costs
    • 89% TSS Removal
    • 72% Phosphorus Removal
    • No Deep Sump Chamber Reduces Costs
    • Long-lasting filters, 3-5 years
    • System drains dry after storm events
    • Built-in pretreatment removes floatables, hydrocarbons and trash
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