UPDATES: October 2010

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October 2010 (vol. 5, issue 5)

Retrofitting Wet Ponds with Iron-Enhanced Filter Trenches

Contributed by Ross Bintner, City of Prior Lake, MN; and Andy Erickson, St. Anthony Falls Laboratory, University of Minnesota

Funded by the Prior-Lake Spring Lake Watershed District, Scott County Watershed Management Organization, and the City of Prior Lake

Nutrients (phosphorus and nitrogen) in excess can cause nuisance algae blooms that generate negative aesthetic and eutrophic conditions in receiving lakes and rivers. In temperate fresh water, dissolved phosphorus is the limiting nutrient and exists in the form of phosphates (HXPO4) contributed to urban stormwater from sources such as lawn fertilizers, leaf litter, grass clippings, unfertilized soils, detergents, and rainfall, among others. A recent study of nationwide monitoring data reports that the fraction of dissolved phosphorus (phosphates) to total phosphorus is approximately 44% (median values) (Pitt et al., 2005).

While most stormwater treatment practices can capture particulate phosphorus through settling or filtration, very few practices have a mechanism to consistently capture dissolved phosphorus over the life-cycle of a treatment practice. Wet detention basins, in particular, are typically designed to capture greater than 80% total suspended solids and, on average, can achieve an estimated 60% total phosphorus load reduction but do little to remove dissolved contaminants from stormwater. Because dissolved phosphorus has a higher bioavailability factor than particulate forms (Sharpley et al., 1992), removing only particulate fractions from stormwater only minimally reduces phosphorus bioavailability. To capture dissolved phosphorus, a chemical adsorption or precipitation process must be added to stormwater treatment practices. Adding steel wool to a sand filter has been shown to capture a significant amount of dissolved phosphorus (Erickson et al., 2007) because dissolved phosphorus binds to steel wool by strong surface adsorption to iron oxide (rust) as the steel wool rusts.

The City of Prior Lake in conjunction with the University of Minnesota is developing a novel enhancement to wet detention basins that has the potential to significantly increase dissolved phosphorus capture in both urban and agricultural environments. A proposal submitted by the City of Prior Lake and the University of Minnesota has been accepted by the Minnesota Pollution Control Agency (MPCA) to study these trenches with rigorous long-term monitoring of natural rainfall-runoff events over a 3-year period. This trial study aims to show how IESF trenches can be retrofitted into existing wet detention basins or installed with newly constructed basins at a significant cost savings compared to other conventional stormwater treatment practices. If successful, this concept will provide local units of government with a new, cost-effective, tool to significantly reduce dissolved phosphorus concentration in stormwater.

The May 2009 UPDATES Newsletter reported on an iron-enhanced sand filter that was installed to capture dissolved phosphorus in Maplewood, Minnesota using approximately 5% iron filings by weight mixed with standard concrete sand. Iron-enhanced filtration trenches are another application of this concept and have been installed and tested in Prior Lake, MN. Two prototype iron-enhanced sand filtration (IESF) systems have been retrofit onto existing wet detention ponds. This prototype connects the under drain from IESF trenches to the outlet structure of an existing wet detention pond and creates a filter volume that is slowly drained down between storm events. This ‘stormwater polishing’ technique has been shown to remove both total and dissolved phosphorus from stormwater runoff by filtration of particulate phosphorus and adsorption of dissolved phosphorus. To evaluate the cost efficiency of the system, construction and maintenance costs are being tracked and associated with modeled pond efficiency combined with IESF trench performance data.

A field application of iron-enhanced sand filtration (IESF) was installed in Prior Lake, Minnesota, USA in February 2010 and is shown in the site photo (Figure 1). In this application, two iron-enhanced filtration trenches were installed along the perimeter of wet detention basins (positioned end to end longitudinally). During rainfall events, stormwater flows into the wet detention pond and increases the water level such that stormwater begins to flow over the surface of the iron-enhanced filter trenches and into the media. The stormwater flows through the mix of iron and sand to a perforated pipe underdrain where it is captured and conveyed to the outlet structure of the wet detention basin. The IESF system is set below the weir overflow, setting up a filter volume in the pond. For small rainfall events which are less than the filter volume, all the stormwater passes through the IESF trenches. For large rainfall events, the water level in the wet detention basin overflows the filter volume where the outlet structure and a portion of the stormwater runoff bypasses the sand filters. When the water level drops below the outlet structure, the remaining stormwater is filtered by the IESF trenches to capture dissolved phosphorus. Four protoype IESF trenches were installed in Prior Lake, MN in February of 2010 at two separate sites, and began treating stormwater in March and May 2010, respectively.

Figure 1:Site photograph of iron-enhanced sand filter trenches installed along the perimeter of a wet detention pond in Prior Lake, MN.

Two IESF trenches are each approximately 40 feet (12 meters) long, 4 feet (1.2 meters) wide, and 2 feet (0.6 meters) deep. In Figure 1, the nearer cleanout pipe protruding vertically out of the trench is the approximate boundary between the two trenches. The trench in the foreground is mixed with approximately 7% iron filings and the trench in the background is mixed with approximately 12% iron filings. The trenches are installed along the perimeter of a wet detention basin (shown in Figure 1). The drainage area for the wet detention basin is approximately 15 acres in size and composed of suburban residential land use. The wet detention basin can capture approximately 1.4 foot of water depth (0.21 ac-ft) on top of the pond before overflow occurs.

Another IESF trench was installed in a similar fashion on another pond with approximately 12% iron filings and 18% iron filings (not pictured). The hydraulics of this system are such that the outlets of the underdrains in the pond outlet structure are continuously underwater (due to backwater). The flow rate through this system is considerably slower than the system pictured in Figure 1 and described above which may be due to the combined effects of the backwater conditions and increased iron filings concentration.

With funding from the Prior-Lake Spring Lake Watershed District, Scott County Watershed Management Organization, and the City of Prior Lake, The University of Minnesota has conducted field testing to measure the performance of the iron-enhanced filtration trenches for capturing dissolved phosphorus. Thus far, four tests have been completed under various conditions as listed in Table 1 and Table 2. Two of these tests (July 1 and July 13) were conducted with synthetic stormwater in which a nearby fire hydrant was turned on and city water was allowed to enter the upstream catch basin that emptied into the wet pond and IESF trenches. As this synthetic stormwater (no pollutants were added) entered the wet pond, the water elevation increased such that stormwater already stored within the wet pond was displaced and flowed into the IESF trenches. Samples were collected within the wet pond just before the water flowed into the IESF trenches and samples were collected from the underdrain outlets. Flow rated was measured at the outlets of the underdrains.

Table 1: Testing results for an iron-enhanced filtration trench mixed with 7% iron filings

Test Date July 1 July 13 August 11 August 12
Influent Flow-weighted Phosphorus EMC (mg/L) 0.032 0.026 0.101 0.077
Effluent Flow-weighted Phosphorus EMC (mg/L) 0.023 0.013 0.016 0.021
Flow-weighted Phosphorus EMC Efficiency (%) 28.6% 51.5% 84.2% 72.6%
Influent Phosphorus Load (mg) 1,456 544 205 74
Effluent Phosphorus Load (mg) 1,041 264 32 20
Phosphorus Load Efficiency (%) 28.6% 51.5% 84.2% 72.6%

The other two tests (August 11 and August 12) occurred immediately following a natural rainfall event in which the wet detention pond was filled with natural stormwater. Samples again were collected within the wet pond just before the water flowed into the IESF trenches and samples were collected from the underdrain outlets. Flow rated was measured at the outlets of the underdrains.

Table 2: Testing results for an iron-enhanced filtration trench mixed with 12% iron filings

Test Date July 1 July 13 August 11 August 12
Influent Flow-weighted Phosphorus EMC (mg/L) 0.033 0.026 0.101 0.077
Effluent Flow-weighted Phosphorus EMC (mg/L) 0.013 0.003 0.015 0.017
Flow-weighted Phosphorus EMC Efficiency (%) 61.4% 88.2% 84.8% 77.4%
Influent Phosphorus Load (mg) 443 263 45 16
Effluent Phosphorus Load (mg) 171 31 6.8 3.5
Phosphorus Load Efficiency (%) 61.4% 88.2% 84.8% 77.4%

It is evident when comparing the influent event mean concentrations (EMCs) in Tables 1 and 2 that the synthetic stormwater had approximately 50 to 75% less dissolved phosphorus concentration than the natural stormwater events. This is likely due to algae within the pond converting dissolved phosphorus to particulate phosphorus between storm events. The influent dissolved phosphorus EMC for these tests varied from approximately 0.026 to 0.101 mg/L but the effluent EMC was consistently between 0.003 and 0.021 mg/L. The dissolved phosphorus removal efficiency therefore varied between approximately 29% and 88% but for most tests (only excluding July 1) capture is greater than 50%.

It is evident that an increase in iron concentration results in an increase in phosphorus capture (when comparing the results from Tables 1 and 2) as was observed in the laboratory experiments. While this is consistent with the applied model, it is far from being a best practice. Significant study is still needed to optimize for IESF cost efficiency because large iron concentrations and backwater conditions have resulted in smaller flow rates.

In 2011, 6-10 new IESF systems will be installed in and around existing wet detention basins throughout the City of Prior Lake, MN (the City). The City currently owns over 135 wet detention basins and stormwater from the City and surrounding areas discharges to nutrient-impaired waterbodies including Upper Prior Lake and Spring Lake (Draft Report: Spring Lake – Upper Prior Lake Nutrient TMDL). Many of these IESF trenches will be installed concurrently with scheduled maintenance and hydraulic modification resulting in cost savings compared to installing these or other stormwater Best Management Practices (BMPs) individually. A 319 proposal submitted by the City of Prior Lake and the University of Minnesota has been accepted by the Minnesota Pollution Control Agency (MPCA) to study these trenches with rigorous long-term monitoring of natural rainfall-runoff events over a 3-year period. The study will quantify phosphorus capture performance over the range of flow and phosphorus concentration values found in natural runoff and to estimate the change in performance, especially over time. The results will be used to develop design (details and specifications) and maintenance recommendations optimized for cost efficiency, estimate the longevity of these new systems, and scientifically determine a potential pollutant load reduction credit.

Integrating this new technology into the stormwater treatment system will significantly reduce the dissolved phosphorus concentration discharged to sensitive downstream waterbodies and has the potential to nearly double the treatment capacity of the existing system. After optimization for pollutant removal effectiveness and cost efficiency, widespread implementation of IESF trenches into existing and newly-constructed wet detention basins could result in a significant, cost effective reduction of dissolved phosphorus load entering wetlands, rivers, and lakes in Minnesota and beyond.

Link to the Final Report, SAFL Project Report No. 549, "Performance Assessment of an Iron-Enhanced Sand Filtration Trench for Capturing Dissolved Phosphorus."


  • Erickson, A.J., J.S. Gulliver and P.T. Weiss, 2007. Enhanced sand filtration for storm water phosphorus removal. Journal of Environmental Engineering-Asce, 133: 485-497.
  • Pitt, R., A. Maestre, R. Morquecho, T. Brown, T. Schueler, K. Cappiella and P. Sturm, 2005. Evaluation of NPDES Phase 1 Municipal Stormwater Monitoring Data. University of Alabama and the Center for Watershed Protection.
  • Sharpley, A.N., S.J. Smith, O.R. Jones, W.A. Berg and G.A. Coleman, 1992. The Transport of Bioavailable Phosphorus in Agricultural Runoff. Journal of Environmental Quality, 21: 30-35.


We want to hear from you!!!

Let us know your thoughts, experiences, and questions by posting a comment. To get you thinking, here are a few questions: 

  • Do you think iron-enhanced filtration trenches would work in your area? 
  • What concerns or reservations do you have about installing iron-enhanced trenches?