Iron-enhanced sand filters performance and maintenance meta-analysis

Project overview

Excess phosphorus in lakes, ponds, and streams produces negative environmental outcomes and negatively impacts human health. For this reason, stormwater system owners (often, local governmental units) are required by the State of Minnesota to reduce the amount of phosphorus leaving stormwater systems and entering waterbodies. Many stormwater management agencies use iron-enhanced sand filters (IESFs) to meet these phosphorus reduction goals. However, the only way to assess the effectiveness of an IESF currently is to sample the water entering and leaving the filter system to measure the amount of phosphorus removed. While the information gained by this costly monitoring process can indicate when an IESF is failing to perform as designed, it does not indicate why the IESF is failing or what to do to restore the filter’s function. Without knowing which construction methods or maintenance actions lead to effective phosphorus removal, stormwater managers and engineers lack the knowledge required to make sure these systems actually perform as intended.

This research project aimed to fill that knowledge gap, identifying factors that can be used to predict how much longer an IESF is likely to continue removing soluble reactive phosphorus (SRP), which is in the dissolved form, and phosphorus that is bound to suspended sediments (particulate phosphorus). The amount of impervious area in the watershed, age of the filter, and surface area of the filter was used to predict the amount of total phosphorus (dissolved plus particulate phosphorus) capacity remaining. Construction methods and maintenance actions that are likely to extend an IESF’s lifespan were also recommended. 

Research questions

  • Is it possible to develop a method for measuring the remaining phosphorus adsorption capacity of an IESF?
  • How can we predict the lifespan of an IESF?
  • Which construction methods and maintenance protocols are recommended to ensure IESFs perform as intended for as long as possible?

Research findings

  • The most accurate way to measure the remaining SRP adsorption capacity of an IESF is to test a sample of the filter media in a laboratory setting. However, these tests are not a standard procedure at commercial laboratories.
  • An IESF’s remaining phosphorus removal capacity can be predicted by calculating the volume of water treated, the amount of impervious area in the watershed, age of the filter, and surface area of the filter. Depth treated (the volume of water treated divided by the surface area of the filter) was used to predict the amount of SRP removal capacity remaining. The amount of impervious area in the watershed, age of the filter, and surface area of the filter was used to predict the amount of total phosphorus (SRP plus particulate phosphorus) capacity remaining. Graphs and equations were developed, which a stormwater manager can use to estimate how much longer an IESF is likely to effectively remove both SRP and total phosphorus (TP).
  • This project produced recommendations for construction methods and maintenance protocols to increase the effective lifespan of an IESF. These recommendations include the following:
    • To ensure iron is evenly distributed throughout the media, hire an experienced particle mixing contractor to prepare the media mix.
    • Grade the filter’s surface so that water is evenly distributed over the total filter area.
    • Rake the top inch of the IESF to remove all organic material and dispose of it at a separate location.
    • Thick vegetation growing on the filter’s surface could indicate that the media is no longer removing SRP. If you see plants growing on the IESF, test the media for SRP retention.
    • In the autumn, remove all plants. Otherwise, they will supply phosphorus to the filter.
    • Don’t allow the IESF to remain inundated for multiple days in a row. Flooding can produce anaerobic conditions within the media, which will cause much of the phosphorus retained in the filter to be released. 

Key innovations/contributions

This project identified factors that predict or extend the remaining effective lifespan of an IESF.

What does this mean for Minnesota?

With a clearer understanding of how design and maintenance choices impact performance, stormwater practitioners will be better equipped to design and operate iron-enhanced sand filters effectively. When IESFs perform effectively, phosphorus is captured, resulting result in cleaner lakes and rivers. This helps local governments meet their stormwater treatment requirements, improves environmental quality, and prevents negative impacts to human health. Improved design and maintenance choices also lead to more efficient use of staff time and budgetary resources.  

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