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Assessing the performance of iron enhanced sand filter

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.

schematic of an iron enhanced sand filter basin

Iron-enhanced sand filter basin schematic.

Contents

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).

Example soil sampling guidance has been developed by U.S. EPA, MPCA's Remediation program, and the USDA.

Monitoring and sample collection for 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.

If water quality samples are being collected, it is highly recommended to follow rigorous quality assurance-quality control procedures. An example of acceptable sampling and analysis protocol is described here.

Warning: If monitoring is being conducted to calculate and receive pollutant removal credits, sampling and analysis procedures must be approved by the MPCA. The protocol defined for manufactured treatment devices is acceptable, but the proposer may submit other protocol for review by the MPCA. See the Protocol for Monitoring section at TP and TSS credits and guidance for manufactured treatment devices (mtds)

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

Determining the IESF service lifetime

The iron in an IESF has a limited capacity to retain phosphorus. This capacity is affected by several factors, including the following.

  • Design: This includes sizing, the rate of flow through the IESF, contact of inflow water with the media, and incorporation of other features into the design (e.g. compost, which is not recommended)
  • Characteristics of the inflow: This includes concentration of phosphorus, volume, competing ions, and other characteristics such as amount of organic debris
  • Operation and maintenance

There is limited monitoring data to develop specific criteria for estimating the life expectancy of an IESF. Gulliver and Erickson (2022) prepared a memo summarizing analysis of three IESFs (link below). Life expectancy of the three practices was 1200, 2050, and 12,500 feet. A value of 1200 feet is recommended as the lower end for the lifetime of an IESF, though the results suggest this value can be increased with considerations for design. An estimate of the number of years for a practice can be estimated by calculating the annual inflow volume and dividing by the surface area of the practice.

File:Gulliver and Erickson 2022 - Analysis of IESF Monitoring Reports and Data.pdf

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

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