TP and TSS credits and guidance for manufactured treatment devices (mtds)

Information: We are in the process of evaluating data and developing credits for the devices discussed on this page. The credits are applicable to Minnesota but should be viewed as draft. Credits may be modified based on additional analysis and information.

Phosphorus removal credits, as a percent of total phosphorus passing through the device
Device	Tier 1	Tier 2	Tier 3
BayFilter w/EMC Media	50	58
BioPod Biofilter with Curb Inlet	50	59
ecoStorm plus	50
Filterra Bioscape	50	59
Filterra System	50	59
FloGard Perk Filter	50	55
Media Filtration System	50
MWS-Linear Modular Wetland	50	54
StormFilter using PhosphoSorb Media	50	64	70
StormGarden Modular Stormwater Bio-filtration System	50	58
The Kraken	50	58
Up-Flo Filter w/Filter Ribbons	50
Jellyfish	50	56

This page provides total phosphorus (TP) and total suspended solid (TSS) removal credits (stormwater credit) for manufactured treatment devices (mtds) in the State of Minnesota. The page includes supporting information, guidance and recommendations, and links to additional information.

Information: Credits refer to the amount of pollutant reduced by treatment with a stormwater best management practice (BMP). For more information on credits, see Overview of stormwater credits

Definition and acronyms

A manufactured treatment device (mtd) is a pre-fabricated stormwater treatment structure utilizing settling (sedimentation), filtration, absorptive/adsorptive materials, vortex separation, vegetative components, and/or other appropriate technology to remove pollutants from stormwater runoff (New Jersey Department of Environmental Protection). MTDs are typically proprietary devices.

Acronyms

• TP – total phosphorus
• PP - particulate phosphorus
• DP – dissolved phosphorus
• OP – orthophosphorus
• TSS – total suspended solids
• TAPE - Technology Assessment Program – Ecology
• TER – Technical Evaluation Report
• GULD - General Use Level Designation
• LCL – Lower confidence limit of the mean (usually the 95% LCL)
• MTD (mtd) – manufactured treatment device

What manufactured treatment devices are credited?

Only Washington State Technology Assessment Protocol - Ecology (TAPE) devices that received General Use Level Designation (GULD) are credited for pollutant reductions. These devices typically are permanent structural practices providing primary treatment for pollutants.

Not all GULD-approved technologies receive pollutant removal credits. Some devices approved through TAPE are considered pretreatment practices in Minnesota. Since pretreatment is required under the Construction Stormwater General Permit (CSW permit), they are not credited for pollutant removal. The credits developed for practices requiring pretreatment such as infiltration and filtration systems already assume an adequate pretreatment system is included (see sections 16.6 and 17.5 of the Construction Stormwater general permit). For more information on pretreatment devices, link here.

Pollutant removal credits are provided for the following devices. Tiered means there are multiple credit levels based on specific criteria, while non-tiered means there is a single value (i.e. just one credit value).

Device name	Manufacturer	TAPE approval date	Total suspended solids	Total phosphorus
BayFilter w/EMC Media	BaySaver Technologies, Inc.	7/10/19	Non-tiered	Tiered
BioPod Biofilter with Curb Inlet	Oldcastle Infrastructure, Inc.	10/28/19	Non-tiered	Tiered
ecoStorm plus	Watertectonics, Inc.	1/9/13	Non-tiered	Non-tiered
Filterra Bioscape	CONTECH Engineered Solutions, LLC.	9/16/19	Non-tiered	Tiered
Filterra System	CONTECH Engineered Solutions, LLC.	6/11/20	Non-tiered	Tiered
FloGard Perk Filter	Oldcastle Infrastructure, Inc.	8/9/18	Non-tiered	Tiered
Media Filtration System	CONTECH Engineered Solutions, LLC.	11/15/16	Non-tiered	Non-tiered
MWS-Linear Modular Wetland	Bio Clean Environmental Services, Inc. (A Forterra Company)	12/16/19	Non-tiered	Tiered
StormFilter using PhosphoSorb Media	CONTECH Engineered Solutions, LLC.	1/2/20	Non-tiered	Tiered
StormFilter using ZPG Media	CONTECH Engineered Solutions, LLC.	4/14/17	Non-tiered	Non-tiered
StormGarden Modular Stormwater Bio-filtration System	Rotondo Environmental Solutions	8/28/19	Non-tiered	Tiered
The Kraken	Bio Clean Environmental Services, Inc. (A Forterra Company)	12/16/19	Non-tiered	Tiered
Up-Flo Filter w/Filter Ribbons	Hydro International	3/5/19	Non-tiered	Non-tiered
Jellyfish	CONTECH Engineered Solutions, LLC.	1/27/21	Non-tiered	Tiered
Tiered means there are multiple credit levels based on specific criteria, while non-tiered means there is a single value (i.e. just one credit value)

TP and TSS credits

To derive credits for GULD-approved mtds, data were compiled from Technical Evaluation Reports (TERs) for Washington State’s Technology Assessment Program – Ecology (TAPE). We compiled and analyzed data for all storm events for each device. A summary of the analysis is provided in the adjacent table. The lower confidence limit (LCL) was derived using Washington State’s bootstrap calculator. Derivation of TP credits for each practice are provided in the table below.

Note the table includes analysis of all data and TAPE analysis. TAPE analysis is based on samples with influent concentrations of 0.1 to 0.5 mg/L for TP. Although concentrations of TP in urban stormwater runoff are typically greater than 0.1 mg/L, analysis of data from the National Stormwater Quality Database indicates about 13% of samples in Minnesota's rainfall region are below this concentration. Only about 2.8% of samples have influent concentrations below 0.05 mg/L. We therefore compared analysis of data with concentrations greater than 0.05 mg/L to the TAPE analysis to determine if performance differed at the lower influent concentrations. This factored into the determination of credits for some devices, as discussed 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 System	0.090	43.5	17	10	69.0	66.8	59.0	78.5	69.2	89.5	86.2	82.8	89.3
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	11	68.6	64.0	59.8	68.6	58.5	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
BioPod Biofilter	0.104	40	13	7	71.7	66.7	59.4	75.4	62.4	83.3	86.5	82.1	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
The Kraken	0.142	53	14	10	74.5	71.9	63.9	83.6	71.3	90.6	87.4	77.5	90.7
Jellyfish Filter	0.338	134	23	18	75.2	73.6	68.2	71.0	66.8	86.2	80.0	74.1	78.4
StormTree biofiltration	0.118	38	27	29	62	57	51	68	61	93	92	90	92
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

In Washington State, TAPE-approved practices receive 50 percent credit for TP and 80 percent credit for TSS. Some devices appear capable of removing higher percentages of these pollutants. We therefore created a tiered credit system in which higher TP removal credits may be given provided certain conditions are met. 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.

Data from the TERs indicate differing performance levels for each device. It is therefore not possible to develop uniform credits across all devices. We therefore chose the following approach to crediting.

• TSS credit: 80 percent. If the LCL exceeds 80 percent, the LCL is used but the credit cannot exceed 85 percent.
• Tier 1 - TP
  • Credit: 50 percent reduction
  • Conditions for receiving credit
    • Inspect manufactured treatment device a minimum of twice during year 1. 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. 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. Determine future inspection and maintenance schedules from findings during the first year of operation.
• Tier 2 - TP
  • Credit: based on an assessment of 95% lower confidence limits (LCLs) or the LCL for TSS.
    • We calculate a LCL for all data with influent concentrations greater than 0.05 mg/L
    • We compare the above value with the TAPE LCL and use the lower of the two values
    • If both LCLs are less than 50%, we calculate a value for TP removal based on the LCL for TSS removal. This value equals the TSS LCL times 0.75, where 0.75 represents the assumed fraction of phosphorus that is in particulate form. If this value exceeds 50%, it is used as the Tier 2 credit.
    • We did not conduct rigorous analysis of data for presence of outliers or significant differences in removal as inflow concentration changed. We used these factors in a non-rigorous manner to inform our recommended credit.
  • Conditions for receiving credit:
    • Must meet Tier 1 conditions.
    • DP should be 25 percent or less of TP. If site specific monitoring data do not exist to verify this, acceptable land uses include commercial land use, industrial land use, transportation land use, highly urban and ultra-urban environments (ultra-urban), or combinations of these. The credit should not be applied in residential land use, to roof runoff, areas with tree canopy or other inputs of organic material, or other areas where dissolved phosphorus is likely to exceed 25 percent of total phosphorus unless site specific monitoring data, collected using appropriate protocols, shows DP:TP ratios are 0.25 or less (link here for discussion of dissolved phosphorus ratios).
    • 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.
• Tier 3 - TP
  • Credit
    • Based on monitoring using appropriate monitoring protocols
    • If a device has a known mechanism for removing dissolved phosphorus, the credit is based on an assumption that 40% of the DP is removed, PP is 75% of TP, and the LCL for TP removal is equal to or greater than the Tier 3 value.
  • Conditions for receiving credit:
    • Must meet Tier 1 and Tier 2 conditions
    • Unless a specific Tier 3 numeric credit is established, demonstrate, via water quality monitoring, that the device meets the recommended removal credit. Monitoring data must be collected using appropriate protocols

Rationale for Tier 2 credits

Tier 2 credits should not be applied when dissolved phosphorus (DP) accounts for more than 25 percent of total phosphorus. Since the dissolved phosphorus concentration in runoff at a particular location will likely not be known, we recommend credits be applied to commercial land use, industrial land use, transportation land use, highly urban and ultra-urban environments (ultra-urban), or combinations of these, and not be applied in residential land use, to roof runoff, or in areas with tree canopy or other inputs of organic material. The rationale for this is based on the following. Monitoring data collected for TAPE certification is insufficient to determine if a device can remove DP, either because DP concentrations are very low or DP was not sampled. Unless a device contains material that will sorb DP (e.g. iron, calcium, aluminum) or has another mechanism for retaining DP (e.g. infiltration, vegetative uptake), we assume DP retention is zero (0). If DP retention is 0, all phosphorus retained is in particulate form (PP). Total phosphorus retention can be calculated by multiplying the PP fraction by the fraction of TSS removal. For a device removing 80% of TSS, PP must be 62.5% of TP to achieve a 50% TP removal. The relationship between TP removal, TSS removal, and PP fraction is shown in the image below. The area shaded in salmon color shows conditions where TP removal is less than 50%, which is the Tier 1 value. The recommendation that PP be 75% or more of TP is therefore conservative, but there is insufficient data for PP:TP ratios for different land uses to support less stringent values. Data from the literature were used to identify land uses that appear to be acceptable for applying the Tier 2 credits. Plot of TP removal as a function of TSS removal and the fraction of TP that is particulate (PP).

The adjacent table provides TSS and TP 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. Link here for information on determining annual volume to apply credit.

To receive the credit(s), follow use conditions for each device, found at the appropriate link below.

• Contech Filterra System

Caution: Total phosphorus credits only apply to the volume of water being treated by a device

Information: Credits listed in the following table, and conditions associated for receiving the credit, are based on analysis of data from TAPE TERs. The credits may therefore be adjusted if additional data is collected, analyzed, and supports modification of the credits.

Credits for manufactured treatment devices. The credits are expressed as a percent removal.
Link to this table

Device	Manufacturer	Total phosphorus (%)			Total suspended solids (%)
		Tier 1 (Basic)	Tier 2	Tier 3
BayFilter w/EMC Media	BaySaver Technologies, Inc	50	60		85
BioPod Biofilter with Curb Inlet	Oldcastle Infrastructure, Inc.	50	60		82
ecoStorm plus	Watertectonics, Inc.	50
Filterra Bioscape	CONTECH Engineered Solutions, LLC.	50	60		82
Filterra System	CONTECH Engineered Solutions, LLC.	50	60		82
FloGard Perk Filter	Oldcastle Infrastructure, Inc.	50	53		80
Media Filtration System	CONTECH Engineered Solutions, LLC.	50
MWS-Linear Modular Wetland	Bio Clean Environmental Services, Inc. (A Forterra Company)	50	54		80
StormFilter using PhosphoSorb Media	CONTECH Engineered Solutions, LLC.	50	60	70	85
Stormfilter using ZPG Media	CONTECH Engineered Solutions, LLC.	50			80
StormGarden Modular Stormwater Bio-filtration System	Environmental Solutions	50	53		83
The Kraken	CONTECH Engineered Solutions, LLC.	50	58		80
Up-Flo Filter w/Filter Ribbons	Hydro International	50			80
Jellyfish	Contech	50	56		80
StormTree biofiltration	StormTree	50	60		85

Derivation of TP and TSS credits for specific mtds

This section provides summaries of TAPE monitoring data for each device.

Contech Filterra

Tier 1 TP = 50%; Tier 2 TP = 59%; no Tier 3 credit; TSS = 82%

• Does the device have a known mechanism for retaining dissolved phosphorus - No
• Does TP removal percentage decrease as influent TP concentration decreases below 0.1 mg/L - Yes
• 95% boot strap LCLs: 52% for all data (22 samples), 58.9% for data with influent concentrations > 0.05 mg/L (17 samples), and 69% for TAPE analyzed data (10 samples)
  • Recommended Tier 2 value is the LCL for all data with inflow concentrations greater than 0.05 mg/L = 59% since this value is lower than the TAPE LCL and represents a lower bound for TP inflow concentrations
  • Because there is no known mechanism for removing DP, no Tier 3 credit is given
• TSS removal credit is the LCL for all data = 82%, since this is lower than the TAPE of 86%

Contech Stormwater Management StormFilter with PhosphoSorb media

Tier 1 TP = 50%; Tier 2 TP = 64%; Tier 3 TP = 70%; TSS = 85%

• Does the device have a known mechanism for retaining dissolved phosphorus - Yes
• Does TP removal percentage decrease as influent TP concentration decreases below 0.1 mg/L - It does not appear removal is affected by inflow concentrations below 0.1 mg/L, though there is one very low removal value that appears to be an outlier
• 95% boot strap LCL is 69.7% for all data and 63.6% for TAPE analyzed data
  • Recommended Tier 2 value is 64% based on the TAPE LCL
  • Recommended Tier 3 value is 70% based on the LCL for all data
• TSS removal credit is the maximum credit of 85% based on an LCL of 86% for all data

Hydro International UpFlo

Tier 1 TP = 50%; No Tier 2 or Tier 3 value recommended; TSS = 80%

• Does the device have a known mechanism for retaining dissolved phosphorus - No
• Does TP removal percentage decrease as influent TP concentration decreases below 0.1 mg/L - Yes
• The 95% boot strap LCL for TAPE-analyzed data was 49.9%. The LCL for all data was 19.1%, but there appeared to be multiple outliers in the dataset. The TAPE approval document noted the small median particle size (11 microns) in runoff to the device, citing multiple times when the device became clogged and required maintenance: "The system was subjected to atypical sediment loading and needed to be serviced after 4 months, or 12.7% of a water year. Monitoring personnel observed similar sediment loading and blinding issues with other systems evaluated at the Test Facility. The runoff from the Test Facility is not expected to be characteristic of other urban runoff applications". We therefore recommend using the TAPE LCL of 50% for Tier 1 TP credit.
• No Tier 2 or Tier 3 credit is given for this device based on data collected for TAPE certification.
• TSS credit is the TAPE default of 80% since the LCL was less than 80%

BioClean MWS-Linear Modular Wetland

Tier 1 TP = 50%; Tier 2 = 54%; No Tier 3 value; TSS = 80%

• Does the device have a known mechanism for retaining dissolved phosphorus - No
• Does TP removal percentage decrease as influent TP concentration decreases below 0.1 mg/L - It does not appear removal percent decreases at low influent concentrations, though there is one value that appears to be an outlier
• The 95% boot strap LCL is 52.7% for all data and 54.1% for TAPE analyzed data. The recommended Tier 2 value is 53%.
• We do not recommend a specific Tier 3 value based on this analysis
• TSS credit is the TAPE default of 80% since the LCL was less than 80%

BaySaver Technologies Inc. BayFilter with EMC Media

Tier 1 TP = 50%; Tier 2 = 58%; no Tier 3 credit; TSS = 80%

• Does the device have a known mechanism for retaining dissolved phosphorus - No
• Does TP removal percentage decrease as influent TP concentration decreases below 0.1 mg/L - Cannot determine this since only one sample was less than 0.1 mg/L
• The 95% boot strap LCL is 59.4% for all data and 58.2% for TAPE analyzed data
  • Recommended Tier 2 value is 58%
  • Because there is no known mechanism for removing DP, no Tier 3 credit is given
• TSS credit is 80%. This is the LCL for all data. There appears to be one outlier for a sample having a low influent concentration. If this outlier is removed, the LCL is 87%.

Rotondo StormGarden Modular Stormwater Bio-filtration System Standard Box Filter

Tier 1 TP = 50%; Tier 2 = 58%; no Tier 3 credit; TSS = 83%

• Does the device have a known mechanism for retaining dissolved phosphorus - No
• Does TP removal percentage decrease as influent TP concentration decreases below 0.1 mg/L - No
• The 95% boot strap LCL is 43.2% for TAPE analyzed data and 51.6% for all data. However in the TAPE-approval document, the reported LCL was 53.4% based on inclusion of data below 0.1 mg/L.
  • We recommend a TP credit of 52% based on the LCL for all data and TAPE's use of a similar data set
  • Because there is no known mechanism for removing DP, no Tier 3 credit is given
• The recommended TSS credit is 83% based on the LCL of 82.8%

OldCastle BioPod with curb inlet

Tier 1 TP = 50%; Tier 2 = 59%; no Tier 3 credit; TSS = 82%

• Does the device have a known mechanism for retaining dissolved phosphorus - No
• Does TP removal percentage decrease as influent TP concentration decreases below 0.1 mg/L - TP removal appears to decrease as influent concentration decreases below 0.1 mg/L
• The 95% boot strap LCL was 62.4% for TAPE-analyzed data and 59.5% for all data
  • The recommended Tier 2 value is 59%
  • Because there is no known mechanism for removing DP, no Tier 3 credit is given
  • The recommended TSS credit is 82% based on the LCL of 82.1%

OldCastle Kristar Flo-gard

Tier 1 TP = 50%; Tier 2 = 55%; No Tier 3 value recommended; TSS = 80%

• Does the device have a known mechanism for retaining dissolved phosphorus - No
• Does TP removal percentage decrease as influent TP concentration decreases below 0.1 mg/L - TP removal appears to decrease as influent concentration decreases below 0.1 mg/L
• The 95% boot strap LCL was 48.8% for TAPE-analyzed data and 50.7% for all data. However, there was one outlier where TP removal was negative. Removing this outlier gives a TAPE-LCL of 60.6% and an overall LCL of 55.4%
  • The recommended Tier 2 value is 55%
  • No Tier 3 value is recommended
• TSS credit is the TAPE default of 80% since the LCL was less than 80%

BioClean Kraken

Tier 1 TP = 50%; Tier 2 = 58%; no Tier 3 credit; TSS = 80%

• Does the device have a known mechanism for retaining dissolved phosphorus - No
• Does TP removal percentage decrease as influent TP concentration decreases below 0.1 mg/L - TP removal appears to decrease as influent concentration decreases below 0.1 mg/L
• The 95% boot strap LCL was 66.3% for TAPE-analyzed data and 63.9% for all data
  • Using a LCL of 77.5% for TSS, the theoretical TP removal is 58%. This is the recommended Tier 2 value
  • Because there is no known mechanism for removing DP, no Tier 3 credit is given
• TSS credit is the TAPE default of 80% since the LCL was less than 80%

Contech Jellyfish

Tier 1 TP = 50%; Tier 2 = 56%; no Tier 3 credit; TSS = 80%

• Does the device have a known mechanism for retaining dissolved phosphorus - No
• Does TP removal percentage decrease as influent TP concentration decreases below 0.1 mg/L - TP removal appears to decrease as influent concentration decreases below 0.1 mg/L
• The 95% boot strap LCL was 66.8% for TAPE-analyzed data and 68.2% for all data
  • Using a LCL of 74.1% for TSS, the theoretical TP removal is 56%. This is the recommended Tier 2 value
  • Because there is no known mechanism for removing DP, no Tier 3 credit is given
  • TSS credit is the TAPE default of 80% since the LCL was less than 80%

Contech Stormfilter using ZPG Media

This device is GULD certified but is not promoted for phosphorus removal in Minnesota. The credits are therefore the TAPE defaults of 50% for TP and 80% for TSS

• Plots of TP removal (%) as a function of influent TP concentrations (mg/L) Click on an image for enlarged view.
• 

  Insufficient data exist to determine if performance decreases with lower influent TP concentrations

• 

  Performance decreases with lower influent TP concentrations

• 

  It is unclear if performance decreases with lower influent TP concentrations

• 

  It is unclear if performance decreases with lower influent TP concentrations

• Plots of TP removal (%) as a function of influent TP concentrations (mg/L) Click on an image for enlarged view.
• 

  Performance decreases with lower influent TP concentrations

• 

  It is unclear if performance decreases with lower influent TP concentrations

• 

  Performance appears to decrease with lower influent TP concentrations

• Plots of TP removal (%) as a function of influent TP concentrations (mg/L) Click on an image for enlarged view.
• 

  Performance appears to decrease with lower influent TP concentrations

• 

  Performance decreases with lower influent TP concentrations

• 

  Performance appears unaffected by lower influent TP concentrations

Analysis of TAPE data to determine applicability for deriving Minnesota credits

Remocal credits only apply to water captured and treated by a stormwater practice. In this example, the credit would not apply to bypass water.

TP removal as a function of OP:TP ratio for two mtds. Data are from TERs for the two devices.

TP removal as a function of inflow TP concentration

Predicted (equation 1) vs observed TP removal for two MTDs. The blue lines represent 1:1 lines passing through the origin.

Monitoring data used to derive the above credits were primarily collected in Washington State. To address the suitability of data from the TAPE program for setting pollutant removal credits, we addressed the following questions.

1. Is the particle size distribution for stormwater runoff similar between the TAPE monitored sites and Minnesota?
2. Can total phosphorus removal be predicted from removal data for TSS and distributions of particulate and dissolved phosphorus in runoff?
3. Is performance affected by phosphorus concentration?

Is the particle size distribution for stormwater runoff similar between the TAPE monitored sites and Minnesota?

Particle size potentially affects the retention of both TSS and phosphorus. If runoff from the TAPE studies has a significantly different particle size than runoff in Minnesota and particle size affects pollutant retention, TAPE data would not be appropriate for setting credits.

The adjacent table provides a summary of particle size distribution for runoff samples collected through the TAPE program and values from the literature. The data from TAPE sampling have a similar median particle size (41.5 microns) as data from the literature (38.3 microns). There is considerably less variation in the TAPE data (stdev = 25.5) compared to the literature values (stdev = 111.6).

Several factors potentially affect particle size distribution in runoff, including the following.

• Soil. Locally, soil type can affect the particle size distribution if source areas contribute to stormwater runoff. This will be more common in areas with significant construction activity and in areas with soils that contribute to runoff (e.g. compacted or non-vegetated soils).
• Land use. Studies suggest residential and mixed areas with residential have greater median particle sizes compared to major roads, parking areas, and commercial areas (Selbig and Bannerman (2012); Zhao (2010).
• First flush. Some studies show an effect of first flush on particle size, while other studies show no effect. Some studies show increased percentage of fines in the first flush, while other studies show the opposite. Differences in particle size distribution during a runoff event are likely associated with the rainfall characteristics, particularly intensity and duration (Degroot and Weiss, 2008).
• Sampling method. DeGroot and Weiss (2008) state "the accuracy of automatic samplers, especially with respect particles larger than 44 to 88 microns". We did not find specific studies that investigated the effect of sampling method on the resulting observed particle size distribution.

The wide variability in particle sizes and limited information on specific land uses for data from the literature make comparisons difficult. Using data from the Selbig studies in Wisconsin, particle size distributions for roads are comparable to particle sizes for the mtds. The majority of TAPE monitoring sites were located in areas where transportation was the primary land use.

Can total phosphorus removal be predicted from removal data for TSS and distributions of particulate and dissolved phosphorus in runoff?

Summary data from the regression Measured TP removal = a + b*predicted TP removal where the predicted TP removal is calculated using Equation 1.
Device	p-value	R2
StormGarden Modular Stormwater Bio-filtration System	< 0.001	0.859
The Kraken	< 0.001	0.858
Up-Flo Filter w/Filter Ribbons	< 0.001	0.880
Jellyfish	< 0.001	0.940

Particle size distributions of mtd effluent were not determined for all devices. Devices having both influent and effluent particle size data showed the devices selectively removed particles larger than silt size.

• Jellyfish: 54% of influent was silt and clay size, 99% of effluent is silt and clay size
• FloGard: 60% of influent was silt and clay size, 94% of effluent was silt and clay size
• Kraken: Sediment sampling showed silt and clay particles comprised 14% of the total sediment captured while making up about 65% of the total sediment in the influent

Preferential capture of coarse material is common to filtering and sedimentation practices. If a device preferentially removes coarser particles and phosphorus retention varies with particle size, a credit based on monitoring data for that device will under- or over-predict phosphorus removal, depending on which particle size fraction phosphorus preferentially sorbs to. For devices that utilize media that does not leach phosphorus, does not retain dissolved phosphorus, and does not selectively remove particle sizes to which phosphorus preferentially sorbs, retention of total phosphorus should correlate with removal of total suspended solids and the fraction of phosphorus that is in particulate form. For example, if a device retains 85% of TSS and does not selectively remove phosphorus (based on selective particle size removal), and if particulate phosphorus is 75% of total phosphorus in runoff, the total phosphorus removal should be 0.85 * 0.75 = 0.6375 or 64%. For this situation, the formula for TP removal as a percent of TP in runoff (TPR) is given by

\( TP_R = PP_F * TSS_R [Eq 1] \)

where PP_F is the fraction of TP in particulate form and TSS_R is the removal percent for TSS.

We plotted predicted total phosphorus removal versus observed removal for four devices which had sufficient data to calculate the particulate fraction (PPF). The adjacent plot indicates that Equation 1 is appropriate for these devices. Summary statistics are provided in the adjacent table.

Dissolved phosphorus concentrations are very low in runoff collected for TAPE certification (median orthophosphate concentration across six devices = 0.011 mg/L). Particulate phosphorus comprised more than 90% of the total phosphorus in runoff. The observed TP removal for TAPE data therefore appear to accurately represent TP removal for runoff containing primarily particulate phosphorus.

How does the distribution of PP and DP affect phosphorus crediting?

Concentrations of dissolved phosphorus concentrations are typically greater than concentrations observed from TAPE monitoring data. In a literature review we conducted, the median concentration of dissolved phosphorus was 0.06 mg/L. Most mtds do not have a mechanism for removing dissolved phosphorus (Phosphosorb is an exception). If DP makes up a higher percentage of TP than what was found in runoff for TAPE certification, then removal credits based on TAPE data would overpredict TP removal.

For devices with OP data, we plotted TP removal as a function of TP concentration.

Phosphorus analysis

Phosphorus may be divided into particulate (PP) and dissolved (DP) forms. Dissolved phosphorus is typically identified as passing through a 0.45 micron filter. For more information on the forms of phosphorus in water, link here. Orthophosphate (OP) is often measured instead of DP. Using data from Capitol Region Watershed District, we estimate OP is about 70 percent of DP.

Some mtds, such as those utilizing only filtration or settling as the treatment mechanism, treat only PP, while some devices may utilize adsorption (e.g. with an amendment), biological uptake (e.g. from vegetation), infiltration, or other mechanisms that can retain 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 (Note the figure illustrates OP:TP ratios rather than DP:TP ratios).

There is limited data on DP concentrations and ratios of DP to TP. Available data from the Midwest indicates DP concentrations in runoff are higher than concentrations observed for TAPE monitoring and vary by land use. Available data also indicates DP:TP ratios vary with land use and are generally higher than ratios observed from TAPE monitoring.

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. Typical TP concentrations in Minnesota runoff are above this 0.15 mg/L threshold.

For devices that utilize media that does not leach phosphorus, does not retain dissolved phosphorus, and does not selectively remove larger particle sizes, retention of total phosphorus should correlate with removal of total suspended solids and the fraction of phosphorus that is in particulate form. For example, if a device retains 85% of TSS and does not selectively remove larger particles, and if particulate phosphorus is 75% of total phosphorus in runoff, the total phosphorus removal should be 0.85*0.75=0.6375 or 64%. The formula for TP removal as a percent of TP in runoff (TP_R) is thus given by

\( TP_R = PP_F * TSS_R [Eq 1] \)

where PP_F is the fraction of TP in particulate form and TSS_R is the removal percent for TSS.

Research indicates that clay size particles sorb more phosphorus than larger particles, though clay particles also desorb phosphorus more readily. To determine the applicability of equation 1, we conducted the following.

• We plotted expected TP removal based on equation 1 versus observed TP removal. Equation 1 is applicable if the resulting plot is linear and close to 1:1.
• We evaluated the TERs to determine if the device selectively removed coarser particles.

Filterra and UpFlo were the only two devices for which we had orthophosphate data, which allows us to calculate PP. For each device we used Equation 1 to calculate the expected TP removal (TPR) and compared it to the observed removal. The resulting plots are shown in the adjacent figure. For Filterra the regression was significant at the 0.01 level (p = 1.9 X 10^-14) with an R² of 0.975. The intercept was fixed at 0, giving a slope of 1.10. For the UpFlo device, the regression was significant at the 0.01 level (p = 1.2 X 10^-10) with an R² of 0.848. The intercept was fixed at 0, giving a slope of 0.977.

The following information was collected from the TERs.

• Stormwater Management StormFilter with PhosphoSorb media removed 78-83 percent of clay- and silt-sized particles, compared to 88 percent overall
• The mean particle size for the UpFlo device was silt-sized (11 microns)
• The mean particle size for the Modular Wetland was silt-sized
• Approximately 60 percent of particles for the Filterra device were silt sized or smaller

Conclusions: Analysis of the phosphorus data for the mtds indicates the following may affect pollutant removal.

• Elevated concentrations of DP relative to TP. Using equation 1 and assuming 80 percent TSS removal and 60 percent TP removal, the particulate fraction is 75 percent assuming no retention or loss of DP by the device.
• TP inflow concentrations below about 0.15 mg/L may result in reduced TP removal. This concentration is typically exceeded in Minnesota runoff.
• Phosphorus removal did not appear to be affected by particle size for the mtds analyzed, though literature suggests runoff with a high percentage of clay-sized particles may result in reduced TP removal. This may account for the observed lower removal for the UpFlo device.

Recommendations

• Tier 2 and Tier 3 credits should not be given if the DP:TP ratio is greater than 0.25. If DP:TP data are not collected, we recommend Tier 2 credit not be given for residential land use, roof runoff, areas with tree canopy or inputs from organic sources, or other areas where dissolved phosphorus is likely to exceed 25 percent of total phosphorus.

Particle size analysis

TAPE testing of mtds includes particle size analysis. TAPE requires a modified ASTM 3977 PSD analysis and requires that runoff have 50% or more silt-size or smaller particles (50 microns or less).

For pollutant removal data from TAPE testing to be applicable to Minnesota, it is important to ensure the particle size distribution in the runoff collected for TAPE certification compares reasonably to Minnesota runoff. We gathered information on particle size distribution of runoff from the literature. The adjacent table summarizes the results. The median particle size from the literature review was 54.5 microns, and the mean was 81.7 microns.

Study	Median particle size (microns)	Primary land use
NURP50	8	nationwide datase7
NURP90	90	nationwide dataset
Selbig	95	mixed
Selbig	32	parking
Selbig	43	arterial streets and roof
Selbig	80	feeder streets
Selbig	80	residential
Gonclaves and van Seters (2012)	23.7	mixed residential
Gonclaves and van Seters	12.5	mixed residential
Gonclaves and van Seters	16	parking lot
Gonclaves and van Seters	15	parking lot
Gonclaves and van Seters	16.4	parking lot
Gonclaves and van Seters	7.8	parking lot
Kim and Sansalone	100 (40-400)
Sansalone et al.	520
MRSC (WA)	120
Penn State (2008)	8	Public works building
Kellner ad Hubbart (MO)	59	mixed
Charters et al.	71.5	roof
Anta et al. mixed	33.5
Literature	Median = 38.3, mean = 71.5, stdev = 111.6
Filterra	50	residential
Stormwater Management StormFilter with PhosphoSorb media	100	transportation (road)
UpFlo	11	transportation (road)
Modular wetland	15-45	parking lot
Baysaver	54	commercial and transportation
Kristar FloGard	60
StormGarden	33	transportation (road)
BioPod	28	transportation (road)
Flo-Gard	60	parking and transportation
Kraken	22	parking and transportation
Jellyfish	58	parking and transportation
Device summary	Median = 41.5, mean = 44.6, stdev = 25.5

The results indicate the four mtds fall within acceptable particle size distributions. The PSD for the UpFlo and one of the modular wetland systems had finer PSDs than typically observed. The Stormwater Management StormFilter with PhosphoSorb media had a a coarser PSD but was shown to remove 78-83 percent of silt- and clay-sized fractions.

The following conclusions were drawn from our literature review.

• The central tendency for particles is in the silt to very fine sand range (approx 60-90 microns).
• There is no specific pattern as to why some studies had smaller average particles compared to others.
• Some studies showed larger particles in snowmelt compared to rainfall runoff and larger particles in residential compared to commercial runoff.
• Season does not appear to affect PSD.
• Land use appears to affect PSD, though the variability in PSDs within a given land use is greater than the variability between different land uses. Residential areas tend to have coarser PSD than parking and commercial areas.
• Particle size appears to have played a role in the performance of some mtds. More specifically, at two sites with about 80% silt size or smaller particles, the devices clogged more quickly and TP removal was subsequently reduced.

Conclusion: The PSDs used to assess mtd performance were in an acceptable range for Minnesota stormwater runoff.

Calculating annual volume treated

The above credits apply to water treated by a device. To determine the quantity of pollutant removed, the volume being treated must be determined. Some devices have some storage built into the system, while others do not.

Pollutant removal is often assumed to represent annual removal based on BMP design. For example, in Minnesota, a device designed to treat the first inch of runoff will typically treat about 85-90 percent of the annual runoff volume from an impervious surface, depending on soils, vegetation, and climate. 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 in Minnesota must be determined. The following are acceptable methods for calculating the average annual volume treated by a device.

• Provide upstream storage. The outlet of the storage device can be sized to keep the discharge rate at the optimal rate for the MTD. An example would be an upstream retention pond designed to capture 90 percent of the annual runoff volume. If all the runoff captured by the pond is treated by the downstream device, the annual treated volume equals the volume captured by the upstream device. If the upstream device provides treatment of a pollutant, this will reduce the pollutant removal efficiency of the downstream device and the pollutant removal credit must be adjusted accordingly.
• Translators. Translators are based on a relationship between volume treated in one location to volume treated in another location. For example, 90 percent annual volume treated in Location A may be equivalent to 80 percent volume treated in Location B. In this example, to treat 90 percent of the annual volume in Location B a BMP would have to be sized up from the same BMP in Location A. Translators have not yet been established for Washington State rainfall and runoff data.
• Modeling. Using Minnesota climate data, a BMP can be sized in a continuous simulation model to determine the percent of total runoff that is captured and treated by a BMP and the runoff that bypasses the BMP.
• Monitoring. Monitoring is based on direct measurement of total runoff volumes and runoff volumes captured and treated by a device. Runoff not captured by a device bypasses the device without treatment. As an example, bypass is determined during TAPE monitoring.
• Devices approved in similar climatic conditions using approved monitoring/testing protocol. Data for devices that have been tested in climate conditions similar to Minnesota can be used to calculate annual volume captured and treated by the device if appropriate protocol have been followed and can be documented. Appropriate protocol are described here.

Protocol for monitoring

As indicated for Tier 2 and Tier 3 credits, monitoring data may be used to verify mtd performance or to characterize stormwater runoff (e.g. to determine DP:TP ratios or particle size distribution). Acceptable protocol include the following.

• Washington State Treatment Assessment Protocol Ecology (TAPE). Note that this document and protocol contains information specific to Washington State and the TAPE program. Applicable items for collecting monitoring data to derive Tier 3 phosphorus credits include the following.
  • Prior to conducting monitoring, the proponent must develop a QAPP that meets TAPE QAPP guidance. The QAPP must include the following elements from the TAPE protocol document (see pages 9-34). The QAPP developed for TAPE GULD certification may be used with appropriate modifications.
    • Background
    • Project description
    • Organization and schedule
    • Quality objectives
    • Experimental design
    • Sampling procedures
    • Measurement procedures
    • Quality control
    • Data management procedures
    • Audits and reports
    • Data verification and validation
    • Data quality assessment
  • Storm event guidelines (Table 5 in the TAPE protocol) are as follows.
  • Total phosphorus (TP) influent concentrations of 0.1 to 0.5 mg/L
  • The following protocol for total dissolved phosphorus (TDS)
    • Influent concentrations of X to X
    • (Field or lab?) filtered samples
    • Laboratory method
  • Sampling for orthophosphate is not acceptable

References

• Anta, J., E. Peña, J. Suárez and J. Cagiao. 2006. A BMP selection process based on the granulometry of runoff solids in a separate urban catchment. Water SA. Vol.32 (3): pp.419-428.
• Charters, F.J., T. A. Cochrane, and A.D. O’Sullivan. 2015. Particle size distribution variance in untreated urban runoff and its implication on treatment selection. Water Research. 85:337-345. https://doi.org/10.1016/j.watres.2015.08.029.
• DeGroot, G., and P. Weiss. 2008. Stormwater particles Sampling – Literature Review. St. Anthony Falls Laboratory.
• Furumai, H., H. Balmer, and M. Boller. 2002. Dynamic behavior of suspended pollutants and particle size distribution in highway runoff. Water Sci. Tech. 46(11-12) 412-418.
• German, J., and Svensson, G. 2002. Metal content and particle size distribution of street sediments and street sweeping waste. Water Sci. Technol., 46: 6-7, 191-198.
• Gonclaves, C., and T.V. Seters. 2012. Characterization of Particle Size Distributions of Runoff from High Impervious Urban Catchments in the Greater Toronto Area. Totonto and Region Conservation. Ontario Ministry of the Environment.
• Kellner, E., J.A. Hubbart, and T. Smith. 2014. Quantifying Urban Land-use Impacts on Suspended Sediment Particle Size Class Distribution. Stormwater.
• Li, Y., M. Kayhanian, and M.K. Stenstrom. 2005. Particle Size Distribution in Highway Runoff. J. Environ Eng. 131:9:1267. DOI:10.1061/(ASCE)0733-9372.
• Municipal Research & Services Center. 1999. Protocol for the Acceptance of Unapproved Stormwater Treatment Technologies for use in the Puget Sound Watershed. American Public Works Association, Washington Chapter – Stormwater, Managers Committee.
• Penn State Harrisburg. 2008. The Terre Hill Concrete Products Terre LeenTM 09 Treatment Device. EPA/600/R-06/136.
• Rinker Materials. 2004.Particle size distribution (psd) in stormwater runoff. Accessed November 8, 2021.
• Selbig, W.R., M. N. Fienen, J. A. Horwatich, and R. T. Bannerman. 2016. The Effect of Particle Size Distribution on the Design of Urban Stormwater Control Measures. Water 2016, 8(1), 17; https://doi.org/10.3390/w8010017.
• Selbig, W.R., and R. T. Bannerman. 2011. Characterizing the Size Distribution of Particles in Urban Stormwater by Use of Fixed-Point Sample-Collection Methods. Open0File Report 2011-1052.
• U.S. Environmental Protection Agency. 1986. Methodology for Analysis of Detention Basins for Control of Urban Runoff Quality. Nonpoint Source Branch, Washington, DC.
• Zhao, H., X. Li, X. Wang, and D. Tian. 2010. Grain size distribution of road-deposited sediment and its contribution to heavy metal pollution in urban runoff in Beijing, China. Journal of Hazardous Materials Volume 183:1-3:203-210