Preliminary results indicate this iron-enhanced surface sand filter is reducing the dissolved phosphorus concentration to or below the detection limit of 0.01 milligrams/liter. (The stones were added for aesthetics.) Images: University of Minnesota
THREE MORE USES FOR THE MINNESOTA FILTER: Iron-enhanced filtration in weirs, roadside drainage ditches, and rain gardens. Compost provides nutrients for plants, but increases phosphorus concentrations and captures dissolved metals. A two-stage bioretention system mitigates this by capturing sediment and metals before removing dissolved phosphorus in the runoff or from the compost above. To adequately oxygenate the filter section, make sure the underdrain's bottom end is open to the air. Diagrams: University of Minnesota
Last summer, the City of Vadnais Heights replaced an existing weir with one based on the Minnesota Filter. Rain fills a wet detention basin and begins to flow through a permeable wall made of vertical posts and horizontal composite planks. Permeable socks 4 feet long and 8 inches in diameter filled with iron filings and concrete sand are installed between the planks. The design treats all low-flow events as well as stored runoff. Performance measurements are planned for the 2011 field season.
A horizontal-flow filter would intercept runoff as it flows through roadside ditches, allowing particles to settle to the swale's bottom and capturing dissolved phosphorus and other pollutants as the stormwater trickles through a mix of gravel, sand, and iron.