In the June issue of SWS, GRAEF engineers Jim Hansen and Bridget Henk wrote about Drexel Town Square, an 86-acre mixed-use development in southeastern Wisconsin. The site boasts a host of storm water management features, including wet detention ponds, a dry detention pond, storm water trees, biofiltration basins and porous pavement.
What the article doesn’t mention is an interesting pollution control feature floating in the middle of a detention pond on the western edge of the development. Three small artificial islands are actively filtering nutrients from the town square’s storm water runoff. Known as Biohaven floating treatment wetlands, the islands are made from recycled PET plastic and are designed to mimic natural wetlands.
Native plants grow in holes drilled into the islands, and the plants’ roots will filter nutrients, microbes and bacteria from the storm water runoff that ends up in the pond. Over time, the bottom of the islands will develop a biofilm from these microbes and bacteria, which will further aid in nutrient reduction. According to Next City, the islands have been shown to remove up to 67% of nitrogen, up to 68% of phosphorus and 75% of suspended solids in storm water.
In addition to their runoff treatment capabilities, the islands also attract wildlife such as birds and pollinating insects, bringing a bit of natural beauty to a pond designed to simply manage storm water runoff.