TP and TSS credits and guidance for manufactured treatment devices (mtds)
test This page provides pollutant removal credits guidance on determining credits for manufactured treatment devices (mtds).
Credits are provided for total phosphorus and total suspended solids. Credits refer to the amount of pollutant reduced by treatment with a stormwater best management practice (BMP). For more information on credits, link here.
Only Washington State Technology Assessment Protocol - Ecology (TAPE) devices that received General Use Level Designation (GULD) are credited for pollutant reductions. TAPE-approved devices include devices that are considered pretreatment devices in Minnesota, as well as permanent structural practices not designed specifically for pretreatment. In Minnesota, pretreatment is required for permanent treatment practices and therefore pretreatment devices are not credited for pollutant removal.
Pollutant removal credits are provided for the following devices.
|Device name||Manufacturer||TAPE approval date||Total suspended solids||Total phosphorus|
|BayFilter w/EMC Media||BaySaver Technologies, Inc.||7/10/19||Basic||Basic|
|BioPod Biofilter with Curb Inlet||Oldcastle Infrastructure, Inc.||10/28/19||Basic||Basic|
|Compost-Amended Biofiltration Swale||WSDOT||8/26/13||Basic|
|ecoStorm plus||Watertectonics, Inc.||1/9/13||Basic|
|Filterra Bioscape||CONTECH Engineered Solutions, LLC.||9/16/19||Basic||Basic|
|Filterra System||CONTECH Engineered Solutions, LLC.||6/11/20||Basic||Tiered|
|FloGard Perk Filter||Oldcastle Infrastructure, Inc.||8/9/18||Basic||Basic|
|Media Filter Drain||WSDOT||5/22/14||Basic||Basic|
|Media Filtration System||CONTECH Engineered Solutions, LLC.||11/15/16||Basic|
|MWS-Linear Modular Wetland||Bio Clean Environmental Services, Inc. (A Forterra Company)||12/16/19||Basic||Tiered|
|StormFilter using PhosphoSorb Media||CONTECH Engineered Solutions, LLC.||1/2/20||Basic||Tiered|
|Stormfilter using ZPG Media||CONTECH Engineered Solutions, LLC.||4/14/17||Basic|
|StormGarden Modular Stormwater Bio-filtration System||Environmental Solutions||8/28/19||Basic||Basic|
|The Kraken||Bio Clean Environmental Services, Inc. (A Forterra Company)||12/16/19||Basic||Basic|
|Up-Flo Filter w/Filter Ribbons||Hydro International||3/5/19||Basic||Basic|
Data were compiled from Technical Evaluation Reports (TERs) for Washington State’s Technology Assessment Program – Ecology (TAPE). We compiled data for all storm events for each device. Not all data are included in TAPE’s analysis. For example, inflow concentrations for TP must be 0.1 mg/L or greater. In our assessment, we included both TAPE-generated values and values for all data. A summary of the analysis is provided in the adjacent table. The LCL was derived using Washington State’s bootstrap calculator. Detailed discussions for each practice are provided below.
Summary statistics for data collected for Washington State TAPE GULD certification. NOTE: These data are not to be applied to crediting for mtds in Minnesota. For mtd credits, link here
Link to this table
|Manufactured treatment device||Median inflow TP (mg/L)||Median inflow TSS (mg/L)||TP Removal (%)||TSS removal (%)|
|Number samples||Data with influent > 0.05 mg/L TP||TAPE analysis||All data||TAPE analysis|
|Data with influent > 0.05 mg/L TP||Meeting TAPE criteria||Median||Mean||LCL||TAPE median||TAPE LCL||Median||Mean||LCL||TAPE median|
|Filterra Bioscape - see Filterra System|
|StormFilter using PhosphoSorb Media||0.280||389||17||16||81.5||77.4||69.9||78.7||63.6||91.6||88.1||84.8||89.0|
|Up-Flo Filter w/Filter Ribbons||0.109||29||20||13||50.3||41.5||31.0||57.1||49.9||75||67.6||59.9||77.5|
|MWS-Linear Modular Wetland||0.103||44||24||16||61.3||60.6||52.5||63.3||54.0||81.9||76.6||71.2||82.9|
|BayFilter w/EMC Media1||0.160||53||12||10||68.6||64.0||59.4||68.6||58.2||89.3||86.1||81.12||89.3|
|StormGarden Modular Stormwater Bio-filtration System||0.078||29||20||15||58.9||57.4||52.9||58.9||43.2||89.2||86.0||82.8||91.9|
|BoxlessBioPod Biofilter - see BioPod Biofilter|
|FloGard Perk Filter||0.123||62.5||21||15||66.5||62.3||53.0; 58.6 without outlier||70.7||48.8 (60.6 without outlier)||80.6||74.7||68.5||81.1|
|Media Filtration System||Though this device is GULD certified, it is not used in Minnesota. We therefore did not analyze the data.|
|Stormfilter using ZPG Media||Though this device is GULD certified, it is not used in Minnesota. We therefore did not analyze the data.|
|ecoStorm plus||We have not received data for this device|
|1 Only data meeting TAPE inflow requirements was available.
2 Six outflow concentrations were at the reporting level of 2.5 mg/L; we used half the reporting limit in our analysis
The adjacent table provides phosphorus removal credits for mtds. This credit only applies to the water treated by the device. To calculate annual pollutant mass removal, the removal credit must be multiplied by the volume treated.
|Manufactured treatment device||Total phosphorus||Total suspended solids|
|Tier 1||Tier 2||Tier 3|
A tiered approach to crediting allows flexibility in selecting practices and associated conditions that may affect performance, such as influent water quality, and operation and maintenance. However, data from the TERs indicate differing performance levels for each device. It is therefore not possible to develop uniform credits across all devices. It is not desirable to conduct an in depth analysis of each device. We therefore chose the following approach to crediting.
Phosphorus may generally be divided into particulate phosphorus (PP) and dissolved phosphorus (DP). Dissolved phosphorus is typically identified as phosphorus passing through a 0.45 micron filter. For more information on the forms of phosphorus in water, link here.
TP removal rates are affected by the type of device. MTDs that utilize filtration or settling as the treatment mechanism treat only PP, while some devices may have amendments or other mechanisms that can also treat DP. For devices that do not treat DP, the treatment effectiveness, as a percent removal rate, will decrease as the DP fraction of TP increases. This is illustrated in the adjacent figure for two mtds.
The adjacent table summarizes data from two MTDs, from the Minnesota Stormwater Manual, from Capitol Region Watershed District in Minnesota, and from a study by Fairbairn. The data show that DP:TP ratios vary from 0.14 for the Capitol Region data to about 0.4 for residential land use. The data also show that DP concentrations in the data collected for mtds is low compared to Minnesota data.
We observed that TP removal efficiency decreased as inflow TP concentrations decreased below about 0.15 mg/L. This is illustrated in the adjacent plot. The adjacent table indicates that typical TP concentrations in Minnesota runoff are above this 0.15 mg/L threshold.
Concentrations of TP, OP, and DP for four mtds and Minnesota runoff
Link to this table
|Data source||Rainfall type||TP (mg/L)||OP (mg/L)||DP (mg/L)||DP:TP1|
|Manual - residential||II||0.325||0.20||0.43|
|Manual - commercial||II||0.200||0.07||0.25|
1When only OP data was available, we estimated DP assuming OP is 70 percent of DP
Devices tested and approved in Washington State are required to treat 91% or more of the average annual runoff. Because climatic conditions differ between Washington State and Minnesota, the annual average volume to which the credit applies must be determined. The following are acceptable methods for calculating the average annual volume treated by a device.
Credits for manufactured treatment devices
Although specific guidance and methods for crediting are provided on individual device support documents, an overview of information in these documents is provided here. All devices must meet the following conditions for credit.
This page was last edited on 3 December 2022, at 18:54.