Information: Reference to specific commercial products, processes, or services by trade name, trademark, service mark, manufacturer, or otherwise does not constitute or imply endorsement, recommendation, or favoring by the Minnesota Pollution Control Agency.
Information: Information on this page is applicable to the Filterra Bioscape

Contech's Filterra (including Filterra Bioscape) is an engineered bioretention 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 Filterra (including Filterra Bioscape), 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 Filterra (including Filterra Bioscape) systems are approved for the following removals for total suspended solids (TSS) and total phosphorus (TP).

  • TSS: 82 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
  • TP: Tier 2
    • Credit: 59 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.
  • TP: Tier 3 - no Tier 3 credit at this time

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


Applicable configurations (systems) include

  • Filterra Internal Bypass - Pipe (FTIB-P)
  • Filterra Internal Bypass – Curb (FTIB-C)
  • Filterra® Shallow
  • Filterra boxless Bioscape

Use conditions

To receive the full credit, Filterra (including Filterra Bioscape) systems must comply with the following conditions.

  1. Design, assemble, install, operate, and maintain Filterra systems in accordance with applicable Contech Filterra manuals.
  2. The maximum flow rates through the devices are specified below. Use the lowest applicable infiltration rate depending on the pollutant treatment required.
    1. 175 inches per hour for TSS
    2. 100 inches per hour for TP
  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 Filterra review before approval of the unit for site installation.
  5. Filterra media shall conform to the specifications described in Filterra Bioretention System 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 Filterra Operation and Maintenance Manual.
  7. Install the Filterra in such a manner that flows exceeding the maximum Filterra operating rate are conveyed around the Filterra mulch and media and will not resuspend captured sediment.
  8. Discharges from the Filterra units shall not cause or contribute to water quality standards violations in receiving waters.
  9. When applicable, adhere to use conditions provided in Washington State Department of Ecology's report (Ecology's Conditions of Use) GENERAL USE LEVEL DESIGNATION FOR BASIC (TSS), ENHANCED, PHOSPHORUS & OIL TREATMENT. Note that some information in this report is specific to Washington State and not applicable to Minnesota (also see GENERAL USE LEVEL DESIGNATION FOR BASIC (TSS), ENHANCED, PHOSPHORUS & OIL TREATMENT For Contech Engineered Solutions Filterra® BioscapeTM).

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 and Tier 3 credits were determined by analyzing water quality data collected at Bellingham, Washington. The site consisted of residential land use with 0.4 acres of impervious draining to the device. The device was 4 ft by 6.5 ft, with a filter media depth of 1.8 feet and a ponding depth of 0.75 feet.


  • Median TP inflow - 0.090 mg/L
  • Median TP removal (all data) - 67%
  • 95% lower confidence limit TP removal (all data) - 52.2%
  • Median TP removal (data meeting TAPE criteria) - 78.5%
  • 95% lower confidence limit TP removal (data meeting TAPE criteria) - 69.2%
  • Median TSS removal (all data) - 89.5%
  • 95% lower confidence limit TSS removal (all data) - 82.8%

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 10 samples for TP and 18 samples for TSS.

The 95% boot strap LCL is 52% for all data and 69% for TAPE analyzed data. TAPE did not use data when TP inflow concentrations were < 0.1 mg/L. Of 22 samples collected, 12 had TP concentrations < 0.1 mg/L. TP removal decreases as the inflow concentration decreases below 0.1. However, if we consider only TP values > 0.05 mg/L, the LCL is 58.9 percent. TP concentrations of 0.05 mg/L or less are uncommon in Minnesota. Consequently, the recommended Tier 2 value is 59%. The median removal for all data was 67% and for TAPE-analyzed data was 78.5%. We therefore recommend a Tier 3 value of 60%, which meets a commonly employed regulatory threshold.

Phosphorus removal, as a percent of influent TP, decreased as influent TP concentration decreased below 0.1 mg/L and as the ratio of dissolved P:TP increased. For a discussion of effects of these factors on crediting, link here.

To link to the Excel spreadsheet containing water monitoring data click here

Inspection, operation, and maintenance

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

Meet the following conditions, unless conditions for the appropriate TP and/or TSS credit are more stringent.

  • 2 maintenance visits required annually if annual precipitation exceeds 30 inches; one visit annually if annual precipitation is less than 30 inches
  • Spring visit to remove salts and sands and fall visit for sediment and leaves
  • Maintenance includes removing trash, degraded mulch, and accumulated debris from the filter surface and replacing the mulch layer


This page was last edited on 15 February 2023, at 17:07.