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Contech's StormFilter using PhosphoSorb Media is an engineered media filtration system designed to filter pollutants from stormwater runoff. It is classified as a manufactured treatment device (mtd).

This page provides information on pollutant removal credits for StormFilter using PhosphoSorb Media, links to supporting information, and links to information on system design, inspection, operation, and maintenance.

TP and TSS credits

When conditions described in this section are met, applicable Contech StormFilter using PhosphoSorb Media systems are approved for the following removals for total suspended solids (TSS) and total phosphorus (TP).

  • TSS: 85 percent removal
  • TP: Tier 1
    • Credit: 50 percent removal
    • Conditions for receiving credit
      • Inspect manufactured treatment device a minimum of twice during year 1. Conduct inspections following manufacturer’s instructions/recommendations and any additional instructions listed in this section. Use maintenance indicators provided by manufacturers to determine if maintenance is needed. Maintenance procedures should follow manufacturer’s guidelines. After year 1, follow manufacturers inspection, operation, and maintenance schedule and procedures unless otherwise required or unless year 1 inspections indicate a need for increased inspection and/or operation and maintenance.
      • Comply with appropriate configurations and Use conditions from TAPE.
      • Comply with appropriate configurations and Use conditions
  • TP: Tier 2
    • Credit: 60 percent removal
    • Conditions for receiving credit
      • Must meet Tier 1 conditions.
      • DP should be 25 percent or less of TP. For a discussion of conditions where DP is likely to exceed 25% of TP, link here. TP removal should be adjusted when DP fractions exceed 25%, as described at the link provided above.
      • Inspect manufactured treatment device monthly during the frost free season for the first year of operation after construction. Conduct inspections following manufacturer’s instructions/recommendations. Use maintenance indicators provided by manufacturers to determine if maintenance is needed. Maintenance procedures should follow manufacturer’s guidelines. Determine future inspection and maintenance schedules from findings during the first year of operation.
    • Recommended Tier 3 value = 70%.
      • The practice contains perlite, which contains aluminum oxides that can sorb dissolved phosphorus.
      • Total phosphorus removal by the device in TAPE studies was greater than for other mtds and showed removal of dissolved P (SRP), although DP concentrations were low (typically less than 0.05 mg/L). TP removal may have been influenced by very high TSS influent concentrations (>300 mg/L).
      • In a study conducted at Mitchell Community College in North Carolina (2012), median and mean removal of DP was 69 and 74 percent, respectively. Sample size was 5.
      • In a laboratory study by Lenart et al., the media achieved DP removal of 70% for 1000 pore volumes and 55% for 2000 pore volumes. Influent consisted of 0.5 mg/l Potassium Dihydrogen Phosphate
      • In a study conducted by Ma et al. (2011) and published in the Journal of Irrigation and Drainage Engineering, adsorption isotherms for the engineered media (later incorporated into the Phosphosorb device) indicated significant removal of dissolved phosphorus. Removal rates were approximately 50% up to 500 pore volumes and 70% after 2000 pore volumes. Influent concentration was 0.5 mg/L DP.
      • Conclusion: Studies suggest Phosphorsorb can achieve long-term DP removal of 30 to 50%.
      • Currently mtd TP removal values in the Manual are based on the assumption that particulate P accounts for 75% of TP. Assuming a 30% removal value and a DP fraction of 25%, and with a TP removal of 60% for this device based only on PP removal, the TP removal is 70% (60 + 10).
    • Conditions for receiving credit
      • Must meet Tier 1 and Tier 2 conditions
      • Tier 2 and Tier 3 credits assume PP:TP ratio of 0.75:1 and no dissolved phosphorus removal

For more information on credits, see TP and TSS credits and guidance for manufactured treatment devices (mtds)


For a discussion of StormFilter configurations, link here.

Use conditions

To receive the full credit, StormFilter systems must comply with the following conditions.

  1. Design, assemble, install, operate, and maintain StormFilter systems in accordance with applicable manuals.
  2. The maximum flow rates through the devices are specified below.
    1. 8.35 gpm/cartridge for 12 inch high cartridges
    2. 12.53 gpm/cartridge for 18 inch high cartridges
    3. 18.79 gpm/cartridge for 27 inch high cartridges
  3. Systems are designed to treat a specified water quality volume or annual average runoff volume. The annual average runoff treated can be determined using methods that are specified here.
  4. Each site plan must undergo Contech StormFilter review before approval of the unit for site installation.
  5. StormFilter media shall conform to the specifications described in Specifications and shall be sourced from Contech Engineered Solutions, LLC with no substitutions.
  6. Follow maintenance procedures given in the most recent version of the StormFilter Operation and Maintenance Manual.
  7. Install the StormFilter in such a manner that flows exceeding the maximum StormFilter operating rate are conveyed around the StormFilter media and will not resuspend captured sediment.
  8. Discharges from the StormFilter units shall not cause or contribute to water quality standards violations in receiving waters.

Data analysis

Credits apply only to the water being treated by a device. The applicable treatment volume must be determined using one of the methods described here.

The Tier 1 credit equals the TAPE-approved credits. Tier 2 credit was determined by analyzing water quality data collected at ZigZag, Oregon. The site consisted of transportation land use with 0.063 acres of impervious draining to the device. The system contained one 18-inch StormFilter cartridge with PhosphoSorb media operating at a specific flow rate 1.67 gpm/ft2 or 12.5 gpm per 18-inch tall cartridge.

MPCA compiled data from Appendix D of the report The Stormwater Management StormFilter® PhosphoSorb® at a Specific Flow Rate of 1.67 gpm/ft2 General Use Level Designation Technical Evaluation Report. Below are summary statistics for the device.

  • Median TP inflow - 0.28 mg/L
  • Median TP removal (all data with concentrations > 0.05 mg/L) - 81.5%
  • 95% lower confidence limit TP removal (all data with concentrations > 0.05 mg/L) - 69.9%
  • Median TP removal (data meeting TAPE criteria) - 78.7%
  • 95% lower confidence limit TP removal (data meeting TAPE criteria) - 63.6%
  • Median TSS removal (all data) - 91.6%
  • 95% lower confidence limit TSS removal (all data) - 84.8%

TAPE means and medians represent values meeting TAPE criteria for TP (inflow concentrations of 0.1 to 0.5 mg/L) and TSS (inflow concentrations greater than 20 mg/L). Of the 22 samples, TAPE medians and means represent 16 samples for TP and 17 samples for TSS.

The Tier 3 credit of 70% is based on the following.

  • PP removal of 60%, assuming PP is 75% of TP and TSS removal is 80% (80% * 75% = 60%)
  • DP removal is 40%. Assuming DP is 25% of TP, the DP credit is (40% * 25%) equals 10%
  • TP removal is PP removal plus DP removal equals 60% + 10% = 70%
  • DP removal is based on testing of the phosphosorb media, which contains expanded aluminum (Ma et al., 2011).

Inspection, operation, and maintenance

Meet the conditions specified for the appropriate TP and/or TSS credit.

Meet the conditions specified in StormFilter Inspection and Maintenance Procedures, unless conditions for the appropriate TP and/or TSS credit are more stringent.



  • Ma, J., J. H. Lenhart, K. Tracy. 2011. Orthophosphate Adsorption Equilibrium and Breakthrough on Filtration Media for Storm-Water Runoff Treatment. Journal of Irrigation and Drainage Engineering 137(4):244-250. DOI:10.1061/(ASCE)IR.1943-4774.0000291.

This page was last edited on 15 February 2023, at 16:47.