Project overview
Excess phosphorus is a water quality problem affecting storm runoff, rivers, and lakes in urban and agricultural settings throughout Minnesota, causing algal blooms, making lakes green, and negatively affecting fish and wildlife. More than 50 iron-enhanced sand filter basins have been installed throughout Minnesota since 2009 to remove phosphorus from water, and many more are scheduled for installation with costs ranging from $30k-$300k each. The physical and hydrological properties of iron-enhanced sand filters are well known, but their chemical properties are not (Table 1), and despite wide success, some iron-enhanced sand filter systems that are currently installed are not working effectively. There is an initiative by designers to secure a local source of iron with consistent chemical characteristics for more consistent filter results and to minimize substantial shipping cost, but any iron source needs to be tested for effectiveness prior to acquisition, and we currently do not know which chemical and mineralogical characteristics of iron are optimal for filter effectiveness and longevity.
Research questions
- What are the most effective minerals for phosphorus trapping?
- Is there a method to screen iron sources prior to acquisition?
- Do the iron minerals in the filter get used up over time?
- Monitoring: Is the water and filter chemistry conducive for effective iron/phosphorus bonding?
Research findings
- Sites should be evaluated before an iron enhanced sand filter (IESF) is installed to make sure there are favorable levels of dissolved oxygen and the pH levels of incoming runoff.
- Magnetic susceptibility (the measure of how much a material can become magnetized) could serve as a practical measure for an IESF’s life span.
- Pretreatment is strongly recommended for an IESF to remove debris, pollutants, and sediments, as they could export iron and phosphorus
Key innovations/contributions
This project helped determine which types of iron are most efficient at removing phosphorus from stormwater runoff when used in an iron-enhanced sand filter.
What does this mean for Minnesota?
IESFs are important pieces of stormwater infrastructure because of their ability to retain and remove pollutants from stormwater. Having a better understanding of them and what makes them effective will allow stormwater practitioners to better maintain these filters, which will help better our waterways.
Reports and Presentations
- Research brief 2017 .pdf
- Final report 2017 .pdf