schematic of an iron enhanced sand filter basin
Iron-enhanced sand filter basin schematic. Click on image to enlarge.

Iron-enhanced sand filters (IESFs) retain solids and associated pollutants by filtering and through adsorption of phosphate (soluble reactive phosphorus) from stormwater. IESFs employ an underdrain. A typical method for assessing the performance of best management practices (bmps) with underdrains is therefore measuring and comparing pollutant concentrations at the influent to the filter and effluent from the underdrain outlet.

Assessment levels

Before developing an assessment program, it is important to have well-defined goals to achieve the desired results. The need for assessment can be determined by permit requirements, voluntary watershed management goals, TMDL ( total maximum daily load) allocation, or protection efforts, among others. Four assessment options are provided below that can be used to assess the performance of a bmp depending on information needs, budgetary constraints, time frames, and legal requirements (see this online manual for detailed descriptions of these assessment levels. (Gulliver, et al. 2010)).

  • Level 1: Visual Inspection. Level 1 inspections consist of a brief site visit to determine whether a stormwater treatment practice is clogged or otherwise impeded. Level 1 inspections can be used to assess hydraulic performance of the IESF but cannot be used to determine phosphorus removal performance. A downloadable checklist can be found at this link.
  • Level 2: Capacity Testing. Level 2 testing consists of a series of point measurements to determine filtration rate. For an IESF, this would involve media sampling for iron, phosphate and total phosphorus concentrations (mg/kg). Although there is currently no metric for relating media concentrations to performance, research is currently being performed to develop this metric.
  • Level 3: Synthetic Runoff Testing. Level 3 testing consists of application of water from a source that has a low sediment concentration (e.g., a fire hydrant, water truck, potable water supply, rooftop rainwater harvesting system) to the practice to simulate a rain event in order to determine drain time. For an IESF, it is recommended to sample for and measure phosphate and total phosphorus concentrations in the water source and the filter effluent at numerous times during the test. If SRP concentration in the water source is below 0.1 mg/L (ppm or parts per million), a phosphate salt, such as potassium phosphate, may need to be added to the source for measurement accuracy.
  • Level 4: Monitoring. Level 4 monitoring consists of flow measurements and sampling of runoff water in response to or between natural rainfall events, and can be used to determine peak flow reduction and pollutant removal efficiency. Monitoring of an IESF should include storm volume discharged through the filter, and SRP and total phosphorus concentrations of the filter influent and effluent.

For information on pollutant crediting see Calculating credits for iron enhanced sand filter or Overview of stormwater credits.

Determination of Appropriate Monitoring Level(s) for a Stormwater Treatment Practice.
Link to this table

Level Should I perform this assessment? Advantages Requirements/limitations Recommended frequency Can this be used to obtain a stormwater credit?
Visual inspection Recommended for all practices Quick and cost-effective Available personnel. Does not necessarily identify causes of poor performance. ≥ 1x / year, at start of rainy season No
Capacity testing If there are suspected filtration rate problems with the practice, or to determine if the media has capacity for phosphorus removal Applicable for practices of all sizes, quickly identify specific areas that require maintenance, less time and expense than monitoring A Modified Philip-Dunne Infiltrometer is recommended for filtration rate testing. Every few years Consult with MPCA (Minnesota Pollution Control Agency) or regulatory agency to determine eligibility
Synthetic runoff testing If there are suspected problems with filtration or pollutant removal, or to establish a baseline condition or baseline performance level Controlled method to accurately measure drawdown time and/or pollutant removal efficiency
  • Testing will be limited by available water supply.
  • Must be able to plug or measure all outflow paths other than filtration.
  • Must be able to continuously measure water surface elevation in practice during test.
  • Recommended for small stormwater treatment practices. May require authorization to conduct testing with targeted pollutants.
Every few years Consult with MPCA or regulatory agency to determine eligibility
Monitoring Goals include obtaining stormwater credits, assessing performance results and life of filter media, or complying with a permit or regulatory agency Most comprehensive assessment technique and measures the response to natural rainfall events
  • Requires the most time, labor, and equipment of the four assessment techniques.
  • Variations in weather, pollutant loading, and flows require correct and simultaneous operation of all instrumentation.
Continuously from construction of the IESF Yes

Filter Media Capacity Testing

Assessing the engineered filter media consists of collection and analysis of filter media for iron, soluble reactive phosphorus and total phosphorus to estimate remaining service life of the media. Current research is designed to enable a relationship between filter media iron and phosphorus in samples and the remaining life of the IESF.

Water sampling: Total phosphorus at the outlet of the iron-sand filter that consistently exceeds 60 to 70 micrograms per liter (mg/L or ppm (parts per million))) may be used as an indicator that the phosphorus binding capacity of the iron-enhanced sand bed has been consumed or that short-circuiting or bypass of the IESF media is occurring. Capacity testing, synthetic runoff testing, or monitoring can be used to determine if short-circuiting is the cause of poor performance. If these concentrations consistently occur, then it is recommended that samples be taken from the iron-sand bed and analyzed for total phosphorus and total iron.

Media: Take two samples of the IESF media at five roughly evenly-spaced locations throughout the media; the first sample at mid-depth and the second towards the bottom of the media, Total phosphorus to total iron ratios that exceed 5 milligrams of phosphorus per gram of elemental iron (Erickson et al., 2007, 2012) indicate the phosphorus binding capacity of the iron-sand bed is exhausted and should be replaced.

Alternatively, IESF media samples can be used in benchtop batch studies (aka jar tests) to determine whether the media has capacity to capture phosphate. Media samples of a known mass (e.g., 10g) can be added to clean, washed jars of known water volume at known phosphate concentration (e.g., 100-300 µg/L), and then mixed for between 10 minutes and up to 24 hours. Then, water samples from the jars can be collected and analyzed for phosphate concentration. From this data, the phosphate capture capacity can be estimated (see Erickson et al., 2018).

Monitoring IESFs

Monitoring is the most comprehensive assessment technique and can be used to assess water volume reduction, peak flow reduction, and pollutant removal efficiency by measuring discharge and pollutant concentration during natural runoff events through flow measurements and water sample analysis. The two types of monitoring recommended for IESFs are as follows:

  1. Flow Monitoring, which consists of measurements of the inflow and outflow for the IESF using techniques described in Water Budget Measurement. The sum of inflows and outflows measured at the IESF inlet(s) and outlet(s) is used for mass balance calculations for Level 4 Monitoring. Flow monitoring allows for calculation of filtration rate, ponding duration, and rate control.
  2. Pollutant Removal, which consists of water sample collection from the IESF inlet(s) and outlet(s) for analysis of concentration of pollutants of concern. Four methods of sampling (in situ, on-site, grab, and automatic) are described in Sampling Methods. Paired with flow monitoring, analysis of pollutant concentrations allows for the calculation of pollutant load removal, as detailed in Data Analysis for Monitoring for both individual storm events and long-term monitoring. The recommended monitoring parameters for IESFs are TSS, soluble reactive phosphorus and total phosphorus.

References and additional information

Use these links to obtain detailed information on the following topics related to BMP performance monitoring:

Additional information on designing a monitoring network and performing field monitoring are found at this link.


Related pages

This page was last edited on 8 December 2022, at 23:17.