Ten years ago, Andy Erickson was struggling with the challenge of how to protect lakes and rivers from stormwater pollution. When it rains, water hits rooftops, parking lots and streets, picking up dirt, nutrients, heavy metals and other pollutants along the way. By the time the runoff flows into wetlands, lakes and streams, it is filthy and contaminated. Fish and other wildlife suffer, algae grows thick during the summer, and people can no longer swim, fish and recreate. Stormwater management has improved over the years, but Erickson, a research fellow with the University of MN, St. Anthony Falls Laboratory, felt certain that innovative technologies were needed to help cities and watershed organizations better fight the problem. So, he set up an experiment in which he mixed a variety of common materials with sand and measured how well these enhancements helped the sand to filter dirty water. Surprisingly, he discovered that he could remove up to 85% of the phosphorus (the nutrient that feeds algae in lakes) from a water sample, simply by chopping up steel wool and mixing it in with the sand.
Since 2005, iron-enhanced sand filters have moved from the laboratory into the field, where they are quickly becoming a popular tool in the fight against stormwater pollution. Often, the filters are built around the perimeter of stormwater ponds or filtration basins to enhance their function. Two years ago, the Comfort Lake – Forest Lake Watershed District worked with Washington County and SRF Consulting to install an iron-sand filter as part of a large stormwater treatment project north of Broadway Ave.. As it was originally designed, the sediment pond and filtration basin would have removed 32 pounds per year of phosphorus from runoff that eventually flows through Bixby Park wetlands to the Sunrise River. Adding an iron enhanced sand filter to the basin, enabled it to filter out an additional 5.4 pounds of phosphorus per year.
Iron-sand filters work thanks to a chemical process that makes phosphorus molecules bind to iron in the sand as water passes through. The ability to pull dissolved phosphorus out of water makes iron-sand filters unique compared with other practices like stormwater ponds and sediment basins that can only catch the phosphorus attached to sediment and organic waste. In Stillwater, the Brown’s Creek Watershed District worked with Emmons & Olivier Resources, Inc. to build a settling pond with an iron-sand filter in the Settler’s Glen neighborhood. Approximately 1200 acres drain to the pond, which subsequently flows into a pretreatment cell and then an iron-enhanced sand filter. The pond takes out sediment and other large particles in the water, while the iron-sand filter catches dissolved phosphorus. Altogether, the project removes approximately 118 pounds of phosphorus per year from a tributary stream that flows to Lake McKusick.
The Ramsey-Washington Metro Watershed District used iron-enhanced sand to improve the effectiveness of 55 raingardens it built at the Maplewood Mall as part of a massive stormwater retrofit project completed in 2012. The gardens filter 9 million gallons of polluted runoff from the mall parking lot before it flows through stormwater pipes to Kohlman Lake.
Iron-sand filters can also help to treat stormwater runoff in places with clay soil or high water tables where raingardens don’t work. For example, the city and watershed district worked together to install a filtration basin with an iron-sand filter at the intersection of North Shore Trail and Hayward Ave., just north of Forest Lake, in 2013. The basin keeps 4.7 pounds of phosphorus out of the lake each year.
As iron-enhanced sand filters becomes more common, local water managers and engineers continue to monitor their effectiveness, making modifications when needed to ensure that they are functioning properly. Meanwhile, researchers like Andy Erickson continue to create and innovate, in search of the next great technology for keeping our water clean.