Stormwater UPDATES - October 2025

Performance and Maintenance of Sand Filters Amended to Retain Phosphate

UPDATES: October 2025 | Volume 20, Issue 2

Contributed by John Gulliver, Levi Burrows, Peter Weiss and Andy Erickson

The Challenge

Iron enhanced sand filters (IESFs, Figure 1) are composed of 5 to 8% iron filings mixed into sand and are often used in the upper Midwest to capture soluble reactive phosphate (SRP), which is dissolved in stormwater runoff. An IESF is a final “polishing” step before discharge of stormwater into lakes, streams, and rivers. First successfully developed in 2005 (Erickson, et al. 2007, 2012) and installed in Minnesota in 2009 with a Barr Engineering design, we now have up to fifteen years of IESF implementation, but little knowledge of how to best optimize their performance and longevity. Questions, like what is a reasonable lifetime of the IESF media, what maintenance and maintenance frequency is required, and is the spent media toxic are all uninvestigated. We set out to provide some answers to these questions. 

images of iron enhanced sand filters
Figure 1. Examples of Iron Enhanced Sand Filters. Top: Traditional filter basin arrangement. Bottom: Pond-perimeter IESF filter trench. Photos courtesy Barr Engineering and Andy Erickson.
What We Did

The Minnesota Stormwater Research Council funded a project at the St. Anthony Falls Laboratory, University of Minnesota, to investigate the unknowns surrounding IESFs. We first solicited as much current information on the performance of existing IESFs as possible and analyzed it to determine if answers could be found in the existing dataset. We felt that a good metric could be the volume of runoff treated divided by the filter surface area, resulting in the runoff depth treated. The treated depth would be compared with SRP and total phosphorus (TP) retained within the media, as determined from inflow and outflow concentrations. In addition, we also collected media samples from IESFs to 1) investigate correlations between the TP retention of IESFs to the media iron concentration as determined by an ICP-OES and 2) conduct a second test on the media, which determined its phosphorus retention capacity remaining. Both metrics can possibly be related to SRP and total phosphorus retained within the media as observed in the field. Finally, we used the ICP-OES analysis to determine if the spent media contained toxic metals. We also used the field experience that resulted from our site visits to develop improved operation and maintenance recommendations. 

The available data was insufficient to meet project objectives (5 sites with flow data and 7 sites with SRP retention data). So, we set out to collect our own data at 21 sites where sample collection was feasible. We collected SRP and TP retention data of between 3 and 9 events, at least 10 media samples at each IESF for ICP-OES analysis, and conducted batch studies to estimate capacity remaining (Figure 2). We also developed a hydrologic model to estimate the total depth of runoff treated over the life of an IESF to for investigation of correlations with retention data. We verified the hydrologic model with the fives sites that had recorded volume treated over area of the filter.

images of batch studies with data
Figure 2: Top: Example of surface media batch study set-up; Middle: batch study shaker tables mixing the batch studies; Bottom: example results (Woodcrest IESF) at starting concentrations of 300, 100 and 0 ppb P.
What We Found

The comparison of runoff depth treated to SRP and TP retention capacity is shown in Figures 3 and 4. These figures can be used to estimate the expected life of an IESF and to estimate the current SRP and TP retention of an IESF, respectively.  If the retention estimate is at or below the desired level, one could perform a batch test on the media to determine if there is sufficient capacity remaining. At best, if an IESF is designed and maintained properly, the performance drops off precipitously at 12,000 ft of depth treated, as shown in Figures 3 and 4. 

figure of srp versus depth treated
Figure 3: Relationship between % SRP retention and depth of water treated by IESFs. Pumped and passive indicates sites that were sampled for this report. Named sites were collected from the literature and partners. Outliers are not included in the regression. Vertical error bars represent one standard deviation from the mean SRP retention value for each site. Horizontal error bars represent one standard deviation of the ratio of estimated/measured depths of the sites used for model verification.
total phosphorus versus depth treated
Figure 4: Relationship between % TP retention and depth of water treated by IESFs. Pumped and passive indicates sites that were sampled for this report. Named sites were collected from the literature or partners. Outliers are not included in the regression. Vertical error bars represent one standard deviation from the mean TP retention value for each site. Horizontal error bars represent one standard deviation of the ratio of estimated/measured depths of the sites used for model verification.

Finally, we developed Figure 5 to relate TP retention to a function of the area and age of the IESF, which allows designers to estimate the potential IESF lifespan based on these variables.

measured tp removal versus modeled tp retention
Figure 5: Comparison of measured %TP retention to modeled %TP retention according to Equation 14. Vertical error bars represent one standard deviation for measured %TP retention, if applicable. Horizontal error bars represent the root mean square error of 16% in modeled %TP retention compared to measured. See the full report for details on this equation and its variables.

Additionally, we found that none of the sites tested contained media with metal concentrations above toxic limits. We also developed improved operation and maintenance recommendations based on field observations.

What you can do

If you are designing an IESF and want to make decisions about the proper size of the filter and cost of construction, then use Figure 5.

If you are operating an existing IESF, you can use the results of this research to help make decisions. The process is explained in more detail in Burrows, et al. (2025), but here is a quick guide to the process:

  1. Use Figure 3 or Figure 4 to estimate the SRP and TP retention of an existing IESF.
  2. If retention is less than desired and you want a more refined estimate of SRP retention, test your IESF media through batch studies (Figure 2).
  3. Make decisions about when or whether to replace your IESF media.

References

  • Burrows, L.J., Gulliver, J.S., Erickson, A.J, and Weiss, P.T. (2025). "Iron Enhanced Sand Filters Performance and Maintenance Meta-Analysis." Project Report No. 609, St. Anthony Falls Laboratory, University of Minnesota, Minneapolis, MN. May 2025. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/271303
  • Erickson, A.J., Gulliver, J.S., Weiss, P.T. Capturing phosphates with iron enhanced sand filtration. Water Res. 2012, 46, 3032–3042. http://dx.doi.org/10.1016/j.watres.2012.03.009
  • Erickson, A.J., Gulliver, J.S., Weiss, P.T. Enhanced sand filtration for storm water phosphorus removal. J. Environ. Eng. 2007, 133, 485–497. http://dx.doi.org/10.1061/(ASCE)0733-9372(2007)133:5(485)

 


 

 

Citation: "Stormwater Research at St. Anthony Falls Laboratory." University of Minnesota, St. Anthony Falls Laboratory. Minneapolis, MN. https://stormwater.safl.umn.edu/