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

This page provides pollutant removal credits guidance on determining credits for manufactured treatment devices (mtds).

Overview

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.

Definition and abbreviations

A manufactured treatment device is a pre-fabricated stormwater treatment structure utilizing settling, 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.

• TP – total 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. They are not considered pretreatment practices. Many devices approved through the TAPE program would be considered pretreatment practices in Minnesota and are not credited for pollutant removal in Minnesota since pretreatment is required for primary treatment practices. For more information on pretreatment devices, link here.

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 analysis

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 data for all storm events for each device. Not all data are included in TAPE’s analysis. For example, for TAPE analysis, 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 lower confidence limit (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 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

Total phosphorus credits

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.

The discussion in this section provides an overview of the methodology for deriving credits and a discussion of factors that potentially affect pollutant removal for mtds.

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

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

Methodology for phosphorus credits

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. Note the conditions applicable for each tier and device are described below in the individual sections for each device.

• Tier 1 applies a 50 percent reduction for TP. This is the TAPE-approved value.
• Tier 2 is based on an assessment of 95% lower confidence limits (LCLs) for the device. Since the LCL varies when all data are considered compared to just data used by TAPE, we typically select the more conservative value, though this may be adjusted based on additional analysis and professional judgement. To receive this credit the device must meet Tier 1 conditions and any additional conditions described below in the individual sections for each device.
• Tier 3 is based on an assessment of medians for the device. Since the medians vary when all data are considered compared to just data used by TAPE, we typically select the more conservative value, though this may be adjusted based on additional analysis and professional judgement. To receive this credit the device must meet Tier 1 and Tier 2 conditions and any additional conditions described below in the individual sections for each device.

Phosphorus analysis

Phosphorus may be divided into particulate (PP) and dissolved (DP) forms. 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.

Some mtds, such as those utilizing only filtration or settling as the treatment mechanism, treat only PP, while some devices may include 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.

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

The adjacent table summarizes data from two MTDs, from the Minnesota Stormwater Manual, from Capitol Region Watershed District (CRWD) in Minnesota, and from a study by Fairbairn. The data show that DP:TP ratios vary from 0.14 for the CRWD 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.

TP removal as a function of inflow TP concentration

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
Filterra	I	0.09	0.01		0.16
UpFlo	I	0.11	0.01		0.13
Modular wetland	I	ND
Phosphosorb	II	0.28
Manual - residential	II	0.325		0.20	0.43
Manual - commercial	II	0.200	0.07		0.25
CRWD	II	0.304	0.042	0.053	0.23
Fairbairn	II	0.16	0.68	0.78	0.43

¹When only OP data was available, we estimated DP assuming OP is 70 percent of DP

Predicted (equation 1) vs observed TP removal for two MTDs.

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. It is unclear how these relationships affect the applicability of equation 1. To determine the applicability of equation 1, we conducted the following.

• Plotted expected TP removal based on equation 1 versus observed TP removal. Equation 1 is applicable if the resulting plot is linear and reasonably 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. 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.

• Phosphosorb 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

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

• Elevated concentrations of DP relative to TP. The data suggests runoff with 20 percent or more DP may show reduced TP removal, unless a device has been designed to remove DP.
• TP inflow concentrations below about 0.15 mg/L may result in reduced TP removal.
• There is insufficient data to determine if particle size plays an important role in TP removal, though the literature suggests runoff with a high percentage of clay-sized particles may result in reduced TP removal.

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	Particle size (microns)
NURP50 (nationwide)	8
NURP90 (nationwide)	90
Selbig mixed (WI)	95
Selbig parking (WI)	32
Selbig arterial streets and roof (WI)	43
Selbig feeder streets (WI)	80
Selbig residential (WI)	80
Gonclaves and van Seters	13.7 (4.2 – 31.1)
Kim and Sansalone	100 (40-400)
Sansalone	520
MRSC (WA)	120
EPA (nationwide)	8
Kellner (MO)	59
Filterra	50
Phosphosorb	100
UpFlo	11
Modular wetland	15-45
Median	54.5
Mean	81.7

The results indicate the four mtds fall within acceptable particle size distributions, though the PSD for the UpFlo and one of the modular wetland systems had finer PSDs while the Phosphosorb had a a coarser PSD.

The following conclusions were drawn from the 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, though 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.

Calculating annual volume treated by a device

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.

• Translators
• Modeling
• Monitoring
• Devices approved in similar climatic conditions
• The device is downstream of a retention device that ensures a specified volume is treated by the device

Credits

Credits for manufactured treatment devices

References

• Selbig
• Zhao