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Iron enhanced sand filter (Minnesota Filter)

Iron-enhanced sand filters are filtration Best Management Practices (BMPs) that incorporate filtration media mixed with iron. The iron removes several dissolved constituents, including phosphate, from stormwater. Iron-enhanced sand filters may be particularly useful for achieving low phosphorus levels needed to improve nutrient impaired waters. Iron-enhanced sand filters could potentially include a wide range of filtration BMPs with the addition of iron; however, iron is not appropriate for all filtration practices due to the potential for iron loss or plugging in low oxygen or persistently inundated filtration practices.

Acknowledgements for iron enhanced sand filters

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photo of an iron enhanced sand filter basin

Iron enhanced sand filter basin, Maplewood, MN. Photo courtesy of Barr Engineering.
Do's and Don'ts of Iron Enhanced Sand Filters (IESFs)
Do's and Don'ts of Iron Enhanced Sand Filters (IESFs)
Don't Do Comments
Don’t assume iron is pure or harmless. Request mineral analysis of iron-based material.
  • Test for iron content. Some iron products are as little as 30% iron.
  • Test for harmful impurities such as Cadmium, Copper, Chromium, etc.)
  • Test for adverse water quality effects (e.g., increase or decrease pH)
Don’t assume iron is reactive with phosphate. Expose iron material to a solution of soluble phosphate and measure adsorption (e.g., batch studies).  
Don’t undersize the IESF for the contributing drainage area. Design the IESF with the appropriate size and dimensions for the contributing area. Undersized IESFs will run out of sorption capacity quickly.
Don’t install an IESF in areas that become inundated. Select locations for IESF that receive intermittent flow, adequately dry out between rainfall events, and such that the outlet is above the downstream high-water level  
Don’t install IESF where the contributing area lacks phosphate in the runoff. Measure the phosphate concentration in the runoff where the IESF is intended to be installed. IESF are most cost-effective when the influent phosphate concentration is > 0.1 mg/L.
Don’t specify too much or too little iron in the IESF media mix. Design IESF with the appropriate amount of iron IESFs are most efficient and successful when the iron content is between 3% and 8% iron by weight. Using > 8% iron by weight can result in iron solidification and reduced hydraulic and sorption capacity.
If mixing with a roto-tiller, don’t assume that the media is well mixed by appearance. If mixing by roto-tiller, mix, mix, and mix 3 to 4 times the appearance of a well-mixed media.  
Don’t ignore inspection and maintenance! Inspect and maintain IESFs.

Recommended Inspection and maintenance of IESF includes:

  • IESFs should be inspected at least annually, though monthly or quarterly inspections are recommended for the first two years to establish necessary maintenance frequencies.
  • Remove accumulated sediment, as needed
  • Remove vegetation, as needed
  • Rake surface with a steel rake to break up oxidized iron clumps or crust
  • For Pond-Perimeter IESF Trenches, remove duckweed, algae, and other organic material weekly, or as buildup occurs
Recommended links:
Design criteria for iron enhanced sand filter
Assessing the performance of iron enhanced sand filter
Operation and maintenance of iron enhanced sand filter
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See a presentation on Multi-Year Performance Analysis of Six TCMA Iron-Enhanced Sand Filters (IESFs), given by Dr. David Fairbairn File:Fairbairn WRC 2020-10-19 (1).pdf