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Revision as of 12:22, 12 June 2022

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photo of a rain garden
A rain garden in a residential development. Photo courtesy of Katherine Sullivan.

Bioretention is a terrestrial-based (up-land as opposed to wetland) water quality and water quantity control process. Bioretention employs a simplistic, site-integrated design that provides opportunity for runoff infiltration, filtration, storage, and water uptake by vegetation.

Bioretention areas are suitable stormwater treatment practices for all land uses, as long as the contributing drainage area is appropriate for the size of the facility. Common bioretention opportunities include landscaping islands, cul-de-sacs, parking lot margins, commercial setbacks, open space, rooftop drainage and street-scapes (i.e., between the curb and sidewalk). Bioretention, when designed with an underdrain and liner, is also a good design option for treating potential stormwater hotspots (PSHs). Bioretention is extremely versatile because of its ability to be incorporated into landscaped areas. The versatility of the practice also allows for bioretention areas to be frequently employed as stormwater retrofits.

Function within stormwater treatment train

Unlike end-of-pipe BMPs, bioretention facilities are typically shallow depressions located in upland areas of a treatment train. The strategic, uniform distribution of bioretention facilities across a development site results in smaller, more manageable subwatersheds, and thus, will help in controlling runoff close to the source where it is generated (Prince George’s County Bioretention Manual, 2002). Bioretention facilities are designed to function by essentially mimicking certain physical, chemical, and biological processes that occur in the natural environment. Depending upon the design of a facility, different processes can be maximized or minimized depending on the type of pollutant loading expected (Prince George’s County, 2002).

Green Infrastructure: bioretention facilities are designed to mimic a site's natural hydrology

MPCA permit applicability

One of the goals of this Manual is to facilitate understanding of and compliance with the MPCA Construction General Permit (CGP), which includes design and performance standards for permanent stormwater management systems. Standards for various categories of stormwater management practices must be applied in all projects in which at least one acre of new impervious area is being created.

For regulatory purposes, bioinfiltration practices fall under Infiltration systems described in the CGP. Biofiltration practices fall under Filtration systems of the permit. If used in combination with other practices, credit for combined stormwater treatment can be given. Due to the statewide prevalence of the MPCA permit, design guidance is presented with the assumption that the permit does apply. Also, although it is expected that in many cases the bioretention practice will be used in combination with other practices, standards are described for the case in which it is a stand-alone practice.

There are situations, particularly retrofit projects, in which a bioretention practice is constructed without being subject to the conditions of the MPCA permit. While compliance with the permit is not required in these cases, the standards it establishes can provide valuable design guidance to the user. It is also important to note that additional and potentially more stringent design requirements may apply for a particular bioretention practice, depending on where it is situated both jurisdictionally and within the surrounding landscape.

Retrofit suitability

The ability to use bioretention as a retrofit often depends on the age of development within a subwatershed. Subwatersheds that have been developed over the last few decades often present many bioretention opportunities because of open spaces created by modern setback, screening and landscaping requirements in local zoning and building codes. However, not every open area will be a good candidate for bioretention due to limitations associated with existing inverts of the storm drain system and the need to tie the underdrain from the bioretention area (for practices requiring an underdrain) into the storm drain system. In general, 4 to 6 feet of elevation above this invert or use of an upturned elbow is needed to drive stormwater through the proposed bioretention area.

Special receiving waters suitability

The table below provides guidance regarding the use of bioretention practices in areas upstream of special receiving waters. Note that the suitability of a bioretention practice depends on whether the practice has an underdrain (i.e. filtration vs. infiltration practice).

Infiltration and filtration bmp1 design restrictions for special waters and watersheds. See also Sensitive waters and other receiving waters.
Link to this table

BMP Group receiving water
A Lakes B Trout Waters C Drinking Water2 D Wetlands E Impaired Waters
Infiltration RECOMMENDED RECOMMENDED NOT RECOMMENDED if potential stormwater pollution sources evident RECOMMENDED RECOMMENDED unless target TMDL pollutant is a soluble nutrient or chloride
Filtration Some variations NOT RECOMMENDED due to poor phosphorus removal, combined with other treatments RECOMMENDED RECOMMENDED ACCEPTABLE RECOMMENDED for non-nutrient impairments

1Filtration practices include green roofs, bmps with an underdrain, or other practices that do not infiltrate water and rely primarily on filtration for treatment.
2 Applies to groundwater drinking water source areas only; use the lakes category to define BMP design restrictions for surface water drinking supplies


It is Highly Recommended that bioretention practices be designed offline. Offline facilities are defined by the flow path through the facility. Any facility that utilizes the same entrance and exit flow path upon reaching pooling capacity is considered an offline facility.

Cold climate suitability

Studies conducted since the 2008 version of this manual indicate the difference between summer and winter performance of bioretention systems is not substantial, even on sites with severe winters (Shanstrom, 2012; Davidson, et al., 2008; Dietz and Clausen, 2006; Kahn et al., 2012; LeFevre et al., 2009; Roseen et al., 2009; Toronto and Region Conservation (TRCA), 2008). Davidson et al. (2008) provide several recommendations for bioretention systems in cold climates. These recommendations are consistent with design recommendations in the Minnesota Stormwater Manual.

Water quantity treatment

High-flow bypass systems are utilized to safely discharge stormwater when bioretention cells fill and reach their maximum ponding depth. This will occur during storms exceeding the Water Quality Volume design storm. There are typically three types of high-flow bypass systems which are split into two categories: offline and online. Whenever possible, offline designs are preferable, as they reduce the potential for internal erosion in the bioretention cell. Offline facilities are defined by the flow path through the bioretention cell. Any facility that utilizes the same entrance and exit point upon reaching maximum ponding depth is considered an offline system. This is typically achieved with a curb cut set at the intended elevation of maximum ponding or through the use of some other upstream diversion, which results in flow bypass down the gutter when the cell has filled. This type of bypass is often simple to utilize in retrofit situations (commercial and transportation applications) where existing drainage infrastructure is present.

Where offline designs are not achievable, it is Highly Recommended that bioretention practices be designed to route high flows on the shortest flow path across the cell to avoid scour in the bioretention practice. The overflow location should be placed as close as practicable to the inlet(s). No matter the bypass design, energy dissipation should always be provided at the inlet(s) to avoid high flow velocity and associated turbulence that can re-suspended particulates and cause erosion in the bioretention cell.

Two types of online bypass systems may be used. The first option is to utilize an internal drainage inlet. Concrete box drop structures may be used to provide an overflow for bioretention cells; however, they should be located away from the inlet(s) to provide an elongated flow path and prevent short-circuiting. These internal drainage structures may be tied into the existing drainage infrastructure, which is an attractive benefit in commercial applications. When using these high-flow bypass devices, it is critical to set the brink-of-overflow elevation properly, otherwise the cell will not function properly when construction is complete. In a tree-shrub-mulch cell, the internal drainage inlets should have a system of screens to prevent loss of mulch. These overflow devices should be designed to safely pass the design discharge.

A second option is to use a broad crested or compound weir in the berm of the bioretention cell to convey overflow. This will typically be the best option in residential, institutional, and rural bioretention applications, where the overflow can tie in to an existing surface conveyance ( swale or ditch). Weir structures may be constructed of pressure-treated lumber, cast-in-place concrete, or precast concrete. The invert of the weir should be set at the intended brink-of-overflow elevation. This type of bypass structure should be designed to non-erosively bypass the design discharge.

In limited cases, a bioretention practice may be able to accommodate the channel protection volume, Vcp, in either an offline or online configuration, and in general they do provide some (albeit limited) storage volume. Bioretention can help reduce detention requirements for a site by providing elongated flow paths, longer times of concentration, and volumetric losses from infiltration and evapotranspiration. Experience and modeling analysis have shown that bioretention can be used for stormwater management quantity control when facilities are distributed throughout a site to reduce runoff and maintain the pre-existing time of concentration. This effort can be incorporated into the site hydrologic analysis. Generally, however, it is Highly Recommended that in order to meet site water quantity or peak discharge criteria, another structural control (e.g. detention) be used in conjunction with a bioretention area.

No matter the type of overflow device used, it is important that the designer provide non-erosive flow velocities at the outlet point to reduce downstream erosion. During the 10-year or 25-year storm (depending on local drainage criteria), discharge velocity should be kept below 4 feet per second for grassed channels. Erosion control matting or rock should be specified if higher velocities are expected.

Water quality treatment

Bioretention can be designed as an effective infiltration / recharge practice, particularly when parent soils have high permeability (> ~ 0.5 inches per hour). Where soils are not favorable, a rock infiltration gallery can be used to promote slow infiltration / recharge of stored water.

Bioretention is an excellent stormwater treatment practice due to the variety of pollutant removal mechanisms including vegetative filtering, settling, evaporation, infiltration, transpiration, biological and microbiological uptake, and soil adsorption. Pollutant removal and effluent concentration data for select parameters are provided in the two tables below.

Caution: The information in the tables below will be updated in summer of 2014. Ranges will be provided rather than a single number because the data are highly variable.
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Schematic of pollutant removal mechanism swale with underdrain
Schematic of pollutant removal mechanisms for a dry swale with an underdrain.

This stormwater manual contains information on pollutant removal for a limited number of pollutants. Links to more detailed or additional information are provided below. Users should be aware there is considerable information on pollutant removal in the literature and on-going research on the topic of pollutant removal by stormwater best management practices (BMPs). The information in this manual is therefore subject to change and often reflects ranges found in the literature rather than specific values. To see updates to credits, see the page called Updates to credits and pollutant removal values.

The following table provides a summary of median pollutant removal for BMPs. These represent recommended values to be used in models, loading estimates, etc. However, pollutant removal is a function of many factors, including design, construction, and maintenance of the BMP; quality of incoming stormwater; time of year; rainfall and watershed characteristics; and so on. The user is encouraged to read the section called Factors affecting pollutant removal.

Median pollutant removal percentages for several stormwater BMPs. Sources. More detailed information and ranges of values can be found in other locations in this manual, as indicated in the table. NSD - not sufficient data. NOTE: Some filtration bmps, such as biofiltration, provide some infiltration. The values for filtration practices in this table are for filtered water.
Link to this table

Practice TSS TP PP DP TN Metals1 Bacteria Hydrocarbons
Infiltration2 3 3 3 3 3 3 3 3
Biofiltration and Tree trench/tree box with underdrain 80 link to table link to table link to table 50 35 95 80
Sand filter 85 50 85 0 35 80 50 80
Iron enhanced sand filter 85 65 or 746 85 40 or 606 35 80 50 80
Dry swale (no check dams) 68 link to table link to table link to table 35 80 0 80
Wet swale (no check dams) 35 0 0 0 15 35 35 NSD
Constructed wet ponds4, 5 84 50 or 685 84 8 or 485 30 60 70 80
Constructed wetlands 73 38 69 0 30 60 70 80
Permeable pavement (with underdrain) 74 41 74 0 NSD NSD NSD NSD
Green roofs 85 0 0 0 NSD NSD NSD NSD
Vegetated (grass) filter 68 0 0 0 NSD NSD NSD NSD
Harvest and reuse Removal is 100% for captured water that is infiltrated. For water captured and routed to another practice, use the removal values for that practice.

TSS=Total suspended solids, TP=Total phosphorus, PP=Particulate phosphorus, DP=Dissolved phosphorus, TN=Total nitrogen
1Data for metals is based on the average of data for zinc and copper
2BMPs designed to infiltrate stormwater runoff, such as infiltration basin/trench, bioinfiltration, permeable pavement with no underdrain, tree trenches with no underdrain, and BMPs with raised underdrains.
3Pollutant removal is 100 percent for the volume infiltrated, 0 for water bypassing the BMP. For filtered water, see values for other BMPs in the table.
4Dry ponds do not receive credit for volume or pollutant removal
5Removal is for Design Level 2. If an iron-enhanced pond bench is included, an additional 40 percent credit is given for dissolved phosphorus. Use the lower values if no iron bench exists and the higher value if an iron bench exists.
6Lower values are for Tier 1 design. Higher values are for Tier 2 design.


Factors affecting pollutant removal

Pollutant removal is affected by several factors. Some of these are briefly discussed below.

  • Influent concentration. Pollutant removal, as a percent, typically increase as pollutant concentration increases, up to some limiting value. At low influent concentrations, pollutant removal may be low since their is likely a limiting concentration below which pollutants can not be effectively removed (often called an irreducible concentration).
  • Maintenance. Pollutant removal decreases if bmps are not properly maintained.
  • Design. Practices that are undersized typically remove less pollutant, as a percent of influent, while oversized practices may remove more pollutant.
  • Characteristics of the pollutant. Bmps typically rely on a specific pollutant removal mechanism. Pollutants in runoff are typically more effectively treated by one mechanism versus another. For example, filtration practices remove sediment and associated pollutants effectively when pollutants are is suspension, but do no effectively remove dissolved pollutants.

Links to tables in the Minnesota Stormwater Manual

Below are links in this manual to information on pollutants removal. Note that the table shown above represents recommended median values, while information in the following links may show ranges of data or information from other sources. The information in these tables therefore represents additional information that can be used for your specific situation rather than the recommended values shown in the table above.

By pollutant

By BMP


Links to credit sections in the Minnesota Stormwater Manual

Links to additional information on pollutant removal

2008 study on recommended approach to estimating pollutant removal for TSS and TP

In 2008 we conducted a study of several databases and data compilations to develop expected performance measures for Total Suspended Solids (TSS) and Total Phosphorus (TP) for the following categories of BMPs:

  • bioretention;
  • filtration;
  • infiltration;
  • stormwater ponds; and
  • stormwater wetlands.

This study provided information across a range of designs. We've utilized this information to develop recommended values for pollutant removal across this range of design levels. However, this information should be used with caution as it is somewhat dated now.

The following studies were used to develop these performance measures.

  • International Stormwater Best Management Practices (BMP) Database: This project began in 1996 under a cooperative agreement between the American Society of Civil Engineers (ASCE) and the U.S. Environmental Protection Agency (USEPA). It now has additional support and funding from the Water Environment Research Foundation (WERF), ASCE Environmental and Water Resources Institute, Federal Highway Administration, and the American Public Works Association (APWA). Wright Water Engineers, Inc. and GeoSyntec Consultants maintain and operate the database clearinghouse and web page. Data from well screened BMP studies from throughout the world continues to be entered. Data were downloaded from the website in February 2007 and were summarized for this current evaluation. This database will hereafter be referred to as the ASCE/EPA database.
  • Stormwater Assessment Monitoring and Performance Program (SWAMP): SWAMP operated from 1995 to 2003. This program was an initiative of the Government of Canada’s Great Lakes Sustainability Fund, the Ontario Ministry of Environment and Energy, the Toronto and Region Conservation Authority, and the Municipal Engineer’s Association. This program is attractive to Minnesota because of the attention paid to cold climate conditions. The SWAMP report (SWAMP 2005) summarizes BMP monitoring data, but does not contain raw data. This program will hereafter be referred to as the SWAMP program.
  • National Pollutant Removal Performance Database for Stormwater Treatment Practices, 2nd Edition, March 2000, Center for Watershed Protection (CWP). This report (Winer 2000) summarizes BMP monitoring data, but does not contain raw data. This report will hereafter be referred to as the CWP report.
  • The Cost and Effectiveness of Stormwater Management Practices: Report prepared by authors from the University of Minnesota and Valparaiso University and published by the Minnesota Department of Transportation (Mn/DOT) in 2005. This report uses several BMP studies and examines performance relative to costs. This report cited as (Weiss et al. 2005) will be referred to as the Mn/DOT report.

Recommended Performance Measures

BMP performance can be evaluated in several ways, the most common of which are pollutant outflow concentration and pollutant percent removal.

  • Pollutant outflow concentration is expressed as the pollutant concentration in the BMP outflow.
  • Pollutant percent removal (or removal efficiency) is expressed as the percent change in pollutant load at the outflow relative to the inflow.
  • Pollutant percent removal (or removal efficiency) is expressed as the percent change in pollutant concentration of the outflow relative to the inflow.

The first of these measures, pollutant outflow concentration, is the best measure of water quality. It provides an indication of how closely stormwater runoff is to meeting water quality standards, which are concentration based.

The second measure, percent removal expressed as a load, is the most widely used approach. This measure is most applicable when attempting to quantify load reductions, such as for assessing compliance with total maximum daily load requirements. However, caution must be used when applying this method. Pollutant removal efficiencies typically are greater when pollutant concentrations are higher. For example, using data from the ASCE/EPA database, at low influent TSS concentrations (less than 100 milligrams per liter), percent removals in stormwater ponds range from 0 percent to almost 100 percent. At higher influent concentration (greater than 100 milligrams per liter), percent removals are greater than 80 percent.

Caution: In taking a credit for a BMP based on percent pollutant removal, be aware that removal efficiency is a function of pollutant concentrations in stormwater entering the BMP

The third measure, percent removal based on pollutant concentration, is not recommended. When percent removal is based solely on concentrations, water volume is ignored, when in fact it could be markedly influencing the performance of the BMP. For example, if a large volume of heavily concentrated pollution is entering a bioretention BMP and much of the water is infiltrating into the ground, the overall load will be greatly reduced, yet not show up as such if the outflow concentration remains high.

Inflow concentration should be examined and considered in the analysis. If inflow concentration is already quite low, and if pollutant percent removal is also low, the low percent removal may partly be due to the fact that additional pollutant removal is not currently technologically possible, a concept known as the “irreducible concentration.”

Another important factor that must be incorporated into every BMP performance assessment is the limitation to only water that actually flows into and through the treatment system. Any flow that exceeds the design specifications for the BMP should be by-passed or diverted and not included in the treatment efficiency calculations. Instead, this flow should be routed to a receiving water as “untreated” or preferably routed to another BMP for subsequent treatment.

BMP Performance

The majority of the BMP performance data was taken from the ASCE/EPA database, the CWP report, and the Mn/DOT report. These studies were comprehensive data gathering efforts and analyses with a high level of quality control. However, the data are not all directly comparable due to different statistics being presented in the different sources. Medians and interquartile ranges of outflow concentrations were available in the ASCE/EPA database, and percent reductions (based on event mean concentrations) were calculated using the database. In the CWP report, the medians were presented in tables, but the interquartile ranges were estimated from graphic presentations of the data. Percent removal data in the CWP report are based on either load or concentration, but that distinction for each BMP was not noted. The Mn/DOT study only reported means.

In the percent removal data, the 75th percentile represents the high tier of BMP performance, the median represents the middle tier, and the 25th percentile represents the low tier. The opposite is true for the outflow concentration data, in that the 25th percentile represents the high tier of expected BMP performance, since low outflow concentration suggests high performance, and high outflow concentration suggests low performance.

The comparisons presented here are from a visual examination of the data; they do not represent statistically significant differences among the groups, which was not possible to test due to the fact that we did not have all of the raw data. Data from bioretention devices and infiltration practices are not included in the following figures due to a lack of evaluation data, but they are discussed in the appropriate section below.

All BMPs combined

When using these data to estimate the performance of a specific BMP, the estimate should be selected based on the design elements shown in the figures for each BMP. If the installation shows neither positive nor negative elements as listed in the design elements figures, the median (50th percentile) should be used. For example, in a stormwater pond, the outflow TSS concentration that could be expected under most conditions would be about 15 milligrams per liter. If there are positive design elements, it would be lowered to approximately 11 milligrams per liter, and if there were negative design elements, the expected outflow concentration would be raised to approximately 30 milligrams per liter. Data from the Mn/DOT report are presented in the tables in this section, even though they present only means and not interquartile ranges. The Mn/DOT study incorporated some of the same studies that the CWP database includes; it is therefore presented simply to show its value compared to the currently reported ASCE/EPA and CWP values.

  • Median TSS outflow concentrations ranged from 13 to 26 milligrams per liter and median/mean percent removals ranged from 54 percent to 88 percent.
  • The lowest TSS outflow concentration and highest percent removal of TSS were achieved by media filters, at approximately 10 milligrams per liter and 85 percent, respectively.
  • Percent removal in stormwater ponds was similar to that in media filters, but the interquartile range was larger in the pond data.
  • TSS percent removal in grass filters differed between the CWP and ASCE/EPA databases, with median percent removals at 81 percent and 54 percent, respectively, and with interquartile ranges not overlapping. However, the TSS interquartile ranges of the two databases showed large overlap in the outflow concentration data. Because of the number of studies and scrutiny placed on the character of the data collection, use of the ASCE/EPA database is preferred.
  • Grass filters performed the poorest among the BMP categories for TP treatment, with median outflow concentrations ranging from 0.19 to 0.29 milligrams per liter, and median/mean percent removals ranging from 35 to 41 percent.

Stormwater ponds

schematic of design elements affecting stormwater pond performance
Schematic of design elements affecting stormwater pond performance

Information on TSS and TP outflow concentrations and removal percentages for stormwater pond practices are shown in the figure to the right and the table below. Median outflow concentrations were 13 and 17 milligrams per liter for TSS and 0.13 and 0.11 milligrams per liter for total phosphorus for the ASCE/EPA and the CWP studies, respectively. Median removal percentages for the two studies were 88 and 80 percent for TSS and 48 and 51 percent for total phosphorus. For standard designs that meet recommended specifications, TSS removal is estimated at 84 percent and TP removal is 50 percent. If 55 percent of TP was in particulate form, the expected removal would be 46 percent. This is close to but less than the median value of 50 percent. Stormwater ponds are generally not considered to be effective at retaining dissolved phosphorus.

Information: For standard designs that meet recommended specifications, TSS removal is estimated at 84 percent and TP removal is 50 percent

Greater removal percentages may be applied when the design water quality volume is exceeded by more than 25 percent, wet extended detention or multiple pond design is used, off-line design is used, the flow path is greater than 1.5:1, sediment forebays are utilized at major inflows, or wetland elements cover at least 10 percent of the surface area of the pond. Lower pollutant removal occurs when the full water quality volume is not provided, a single cell design is used, when the pond intersects groundwater, when the flow path is less than 1:1, when on-line design is utilized, or when the wetland surface area to contributing area is less than 2 percent.

Stormwater pond outflow concentrations and pollutant removals.
Link to this table

Source
TSS outflow concentration (mg/L) at a given Percentile
TP outflow concentration (mg/L) at a given Percentile
25th 50th 75th 25th 50th 75th
ASCE/EPA 11 13 25 0.08 0.13 0.20
CWP 11 17 34 0.07 0.11 0.17
Source
TSS Percent Removal at a given Percentile1
TP Percent Removal at a given Percentile1
25th 50th 75th 25th 50th 75th
ASCE/EPA 60 88 94 29 48 69
CWP 60 80 86 39 51 76

1MnDOT values for mean TSS removal=65% and mean TP removal=52%


Stormwater wetlands

schematic of design elements affecting stormwater wetland performance
Schematic of design elements affecting stormwater wetland performance

Information on TSS and TP outflow concentrations and removal percentages for stormwater wetland practices are shown in the figure to the right and the table below. Median outflow concentrations were 15 and 22 milligrams per liter for TSS and 0.13 and 0.20 milligrams per liter for total phosphorus for the ASCE/EPA and the CWP studies, respectively. Median removal percentages for the two studies were 70 and 76 percent for TSS and 27 and 49 percent for total phosphorus. For standard designs that meet recommended specifications, TSS removal is estimated at 73 percent and TP removal is 38 percent. If 55 percent of TP was in particulate form, the expected removal would be 40 percent. This is close to the median value of 38 percent. Stormwater wetlands are not considered to be effective at removing dissolved phosphorus and in some cases may contribute to dissolved phosphorus loads.

Information: For standard designs that meet recommended specifications, TSS removal is estimated at 73 percent and TP removal is 38 percent

Greater removal percentages may be applied when the design water quality volume is exceeded by more than 25 percent, a pond wetland or multiple cell design is utilized, or a wooded wetland design is used. Lower pollutant removal occurs when the full water quality volume is not provided, when no forebay or pretreatment features are used, when the wetland intersects groundwater, when the flow path is less than 1:1, when no wetland planting plan is specified, or when the wetland surface area to contributing area is less than 1.5 percent.

Stormwater wetland outflow concentrations and pollutant removals.
Link to this table

Source
TSS outflow concentration (mg/L) at a given Percentile
TP outflow concentration (mg/L) at a given Percentile
25th 50th 75th 25th 50th 75th
ASCE/EPA 9 15 26 0.07 0.13 0.25
CWP 11 22 33 0.12 0.20 0.25
Source
TSS Percent Removal at a given Percentile1
TP Percent Removal at a given Percentile1
25th 50th 75th 25th 50th 75th
ASCE/EPA 28 70 75 16 27 32
CWP 49 76 86 23 49 76

1MnDOT values for mean TSS removal=68% and mean TP removal=42%


Filtration practice (grass filters/swales)

schematic of design elements affecting grass filter performance
Schematic of design elements affecting grass filter performance

Information on TSS and TP outflow concentrations and removal percentages for grass filtration practices (swales) are shown in the figure to the right and the table below. Median outflow concentrations were 26 and 14 milligrams per liter for TSS and 0.29 and 0.19 milligrams per liter for total phosphorus for the ASCE/EPA and the CWP studies, respectively. Median removal percentages for the two studies were 54 and 81 percent for TSS and -35 and 34 percent for total phosphorus. For standard designs that meet recommended specifications, TSS removal is estimated at 68 percent and TP removal is 0 percent. Note that the results for TP are highly variable, particularly for the ASCE/EPA study.

Information: For standard designs that meet recommended specifications, TSS removal is estimated at 68 percent and TP removal is 0 percent

Greater removal percentages may be applied when the design water quality volume is exceeded by more than 25 percent. Removal of phosphorus can be achieved if amendments such as iron are added to the filter media (see Iron enhanced sand filter (Minnesota Filter)). Lower pollutant removal occurs when the full water quality volume is not provided, when on-line design is used with no storm bypass, when no pretreatment is used, when swale slopes are too great or small, when swale side slopes are too steep, or when soil infiltration rates are low.

Stormwater filtration practice (grass filters/swales) outflow concentrations and pollutant removals.
Link to this table

Source
TSS outflow concentration (mg/L) at a given Percentile
TP outflow concentration (mg/L) at a given Percentile
25th 50th 75th 25th 50th 75th
ASCE/EPA 18 26 53 0.19 0.29 0.46
CWP 8 14 35 0.11 0.19 0.26
Source
TSS Percent Removal at a given Percentile1
TP Percent Removal at a given Percentile1
25th 50th 75th 25th 50th 75th
ASCE/EPA 8 54 70 -121 -35 15
CWP 70 81 86 19 34 54

1MnDOT values for mean TSS removal=68% and mean TP removal=41%


Filtration practice (media filters/sand filters/peat mixed with sand/other)

schematic of design elements affecting filter media performance
Schematic of design elements affecting filter media performance

Information on TSS and TP outflow concentrations and removal percentages for filtration practices are shown in the figure to the right and the table below. Median outflow concentrations were 8 and 11 milligrams per liter for TSS and 0.11 and 0.10 milligrams per liter for total phosphorus for the ASCE/EPA and the CWP studies, respectively. Median removal percentages for the two studies were 83 and 86 percent for TSS and 35 and 59 percent for total phosphorus. For standard designs that meet recommended specifications, TSS removal is estimated at 85 percent and TP removal is 47 percent. Assuming that all the phosphorus is particulate, this corresponds with 55 percent of the total phosphorus being associated with sediment (0.85 * 0.55 * 100 percent = 47 percent).

Information: For standard designs that meet recommended specifications, TSS removal is estimated at 85 percent and TP removal is 47 percent

Greater removal percentages may be applied when the design water quality volume is exceeded by more than 25 percent. Greater removal of phosphorus can be achieved if amendments such as iron are added to the filter media (see Iron enhanced sand filter (Minnesota Filter)). Lower pollutant removal occurs when the full water quality volume is not provided, when on-line design is used with no storm bypass, when dry pretreatment is used, or when the BMP is not adequately maintained.

Stormwater filtration practice (media filters; includes sand filters, peat mixed with sand, and other) outflow concentrations and pollutant removals.
Link to this table

Source
TSS outflow concentration (mg/L) at a given Percentile
TP outflow concentration (mg/L) at a given Percentile
25th 50th 75th 25th 50th 75th
ASCE/EPA 6 8 12 0.08 0.11 0.16
CWP 5 11 16 0.06 0.10 0.19
Source
TSS Percent Removal at a given Percentile1
TP Percent Removal at a given Percentile1
25th 50th 75th 25th 50th 75th
ASCE/EPA 73 83 87 15 35 47
CWP 80 86 92 41 59 65

1MnDOT values for mean TSS removal=82% and mean TP removal=46%


Bioretention

Performance data from bioretention devices are less available than data for the other BMP types. Since inflow often does not enter bioretention devices through a channel, it is difficult to monitor. More importantly, bioretention devices are often designed to infiltrate stormwater, and therefore do not always overflow. In a USGS study on the effects on water quality of rain gardens in the Twin Cities metropolitan area, two of the five studied rain gardens did not overflow at all during the study’s time period (Tornes, 2005) and therefore retained all of the incoming TSS and TP loads. At sites where overflow did occur, the pollutant concentrations in the outflow were generally lower than the concentrations in the inflow. Effluent concentrations from bioretention BMPs with an underdrain range from 0.04 to 0.35 milligrams per liter with a mean of 0.16 milligrams per liter. However, since volumes were not monitored it was not possible to estimate what percent of the pollutant loads were retained. TSS was not monitored in that study, and TSS data for bioretention devices are not included in the CWP report; therefore only TP values were presented.

In a study on the Burnsville rain gardens in the Twin Cities Metropolitan area, there was an 82 percent reduction in annual stormwater runoff over a two-year monitoring period, with a greater than 95 percent reduction in volume for many storms (Yetka and Leuthold, 2005). Other local data, from the H.B. Fuller Company bioretention system, show a 73 percent reduction in stormwater volume, a 94 percent reduction in particulates, and a 70 percent reduction in TP. However, the soluble fraction of phosphorus in the runoff increased by 70 percent (Langer, 1997). Interpretation of the performance data presented here for bioretention is somewhat inconclusive due to the methods used and the low number of documented studies. Bioretention devices are highly effective at removing TSS and TP loads when they infiltrate the majority of the volume of stormwater runoff events.

Phosphorus removal percentages for bioretention BMPs can be calculated from the following table.

Phosphorus credits for bioretention systems with an underdrain. This includes tree trenches and dry swales.
Link to this table

Particulate phosphorus (PP) Dissolved phosphorus (DP)
Is Media Mix C or D being used or, if using a mix other than C or D, is the media phosphorus content 30 mg/kg or less per the Mehlich 3 (or equivalent) test1?
  • If yes, particulate credit = 100% of the particulate fraction (assumed to be 55% of total P)
  • If no or unknown, particulate credit = 0%

The assumption of 55 percent particulate phosphorus and 45 percent dissolved phosphorus is likely inaccurate for certain land uses, such as industrial, transportation, and some commercial areas. Studies indicate particulate phosphorus comprises a greater percent of total phosphorus in these land uses. It may therefore be appropriate to modify the above equation with locally derived ratios for particulate and dissolved phosphorus. For more information on fractionation of phosphorus in stormwater runoff, link here.


Example PP removal credit

  • Particulate fraction (55% of TP) * removal rate for that fraction (80% for bioretention and tree trench and 68% for swale main channel) = 0.55 * 0.80 = 0.44 or 44% for bioretention and tree trench (with underdrain) and 0.55 * 0.68 or 36% for swale main channel (with underdrain)
  • If particulate is 75% of TP, removal = 0.75 * 0.80 or 60% for bioretention and tree trench (with underdrain) and 0.75 * 0.68 or 51% for dry swale
 1. Is Media Mix C or D being used or, if using a mix other than C or D, is the media phosphorus content 30 mg/kg or less per the Mehlich 3 (or equivalent) test1?
  • If yes, credit as a % (up to a maximum of 20%) = 20 * (depth of media above underdrain, in feet/2)
  • If no or unknown, credit = 0%

2. Does the system include approved P-sorbing soil amendments2?

  • If yes, additional 40% credit


Example DP removal credit

  • DP removal if dissolved credit is 20% = Dissolved fraction (45%) * removal rate for that fraction (20%) = 0.09 or 9 percent
  • Adjust DP removal if depth is less than 2 feet. Example depth = 1 foot then DP removal = 0.45 * 10% * 1/2 = 0.045
  • Adjust DP removal if dissolved credit is higher due to use of P-sorbing soil amendments
  • Adjust if the fraction (percentage) of DP is different than 45%

The assumption of 55 percent particulate phosphorus and 45 percent dissolved phosphorus is likely inaccurate for certain land uses, such as industrial, transportation, and some commercial areas. Studies indicate particulate phosphorus comprises a greater percent of total phosphorus in these land uses. It may therefore be appropriate to modify the above equation with locally derived ratios for particulate and dissolved phosphorus. For more information on fractionation of phosphorus in stormwater runoff, link here.

TP removal = PP removal + DP removal

1Other widely accepted soil P tests may be used. Note: a basic conversion of test results may be necessary
2Acceptable P sorption amendments include

  • 5% by volume elemental iron filings above IWS or elevated underdrain
  • minimum 5% by volume sorptive media above IWS or elevated underdrain
  • minimum 5% by weight water treatment residuals (WTR) to a depth of at least 10 cm
  • other P sorptive amendments with supporting third party research results showing P reduction for at least 20 year lifespan, P credit commensurate with research results


Infiltration

Due to similar difficulties as those that exist with monitoring bioretention systems, there are few data available demonstrating the load reductions or outflow concentrations of larger-scale infiltration practices such as infiltration trenches. Few sampling programs collect infiltrating water that flows through an infiltration system. For properly designed, operated, and maintained infiltration systems, all water routed into them should be “removed” from stormwater flow, resulting in 100 percent efficiency relative to volume and pollutant reduction. For this reason, performance tables similar to those above would only reflect this performance. This logic assumes that stormwater is the beneficiary of any infiltration system, but ignores the fact that pollution, if any remains after the internal workings of the infiltration BMP itself, is being transferred into the shallow groundwater system. Good monitoring data on the groundwater impact of infiltrating stormwater are rare, but there are efforts underway today to document this, so future Manual revisions should be able to include some data updates.Properly designed infiltration systems will accommodate a design volume based on the required water quality volume. Excess water must be by-passed and diverted to another BMP so that the design infiltration occurs within 48 hours if under state regulation, or generally within 72 hours under certain local and watershed regulations. In no case should the by-passed volume be included in the pollutant removal calculation. Data that are reported in performance literature for infiltration systems, unless reporting 100 percent effectiveness for surface water or documenting outflow water downward, are not accurately representing behavior, or are representing the excess flow (overflow) from a system. Design specifications should prevent putting excess water beyond that which will infiltrate within the given time frame. Any excess should be diverted away from the infiltration system and reported as inflow to another treatment device.

Extreme caution must be exercised and serious planning undertaken to assure that no highly contaminating material is routed into these BMPs. Of particular concern are toxic organics (gasoline, solvents) and high levels of chloride.

Other Factors Influencing Performance

Even though the ASCE/EPA database is the most comprehensive collection of BMP performance data, there are not enough data points in the database to statistically examine the effect that design parameters have on BMP performance (ASCE/EPA, 2000). The design parameters listed in the BMP performance figures are derived from best professional judgment. There are many other factors that affect the performance of BMPs that have not been discussed in the previous sections. There is little in-depth statistical analysis that can be done for these other factors due to the lack of reported information on them. Nevertheless, it can be stated with certainty that they do have an impact on BMP performance and should, therefore, be considered when designing a practice. Some of these factors include the following:

  • geographic location;
  • watershed configuration (e.g. size, slope, shape and depth to bedrock or water table);
  • uncertainty of hydrologic measurements (e.g. initial losses due to landscape retention, variable runoff coefficients, location of rainfall data recorders and extent of connected imperviousness);
  • BMP design criteria (e.g. ratio of permanent and temporary storage to drainage area, storage configuration, drawdown time, length-to-width and bathymetric ratios, soil permeability);
  • BMP maintenance; and
  • climate.

References


Typical pollutant effluent concentrations, in milligrams per liter, for bioretention BMPs. Source Winer, 2000..
Link to this table

Practice TSS TP TN Cu Zn
Bioretention 11 0.3 1.11 0.007 0.040

1 Assumed values based on filtering practices


Early bioretention facilities were designed to provide water quality benefits by controlling the “first flush” event. Using highly permeable planting soils and an underdrain creates a high-rate biofilter, which can treat 90 to 95 percent (or higher) of the total annual volume of rainfall/runoff, depending on the design.

Limitations

Bioretention practices have been widely utilized for the past decade. Data suggests that these practices, when properly designed, constructed and maintained, perform well over long periods of time. However, design, construction and maintenance of these practices can be complex. In particular, maintenance personnel may need additional instruction on routine Operation and Maintenance requirements.

References

  • Davidson, J.D., M. Isensee, C. Coudron, T. Bistodeau, N.J. LeFevre, and G. Oberts. 2008. Recommendations to Optimize Hydrologic Bioretention Performance for Cold Climates. WERF Project 04-DEC-13SG.
  • Dietz, M.E. and Clausen, J.C. 2006. Saturation to improve pollutant retention in a rain garden. Environmental Science and Technology. Vol. 40. No. 4. pp. 1335-1340.
  • Kahn, U.T., C. Valeo, A. Chu, and B. van Duin. 2012. Bioretention cell efficacy in cold climates: Part 2 — water quality performance. Canadian Journal of Civil Engineering. 39(11):1222-1233.
  • LeFevre, N.J., J. D. Davidson, and G. L. Oberts. 2009. Bioretention of Simulated Snowmelt: Cold Climate Performance and Design Criteria. Proceedings of the 14th Conference on Cold Regions Engineering.
  • Roseen, R.M., Ballestro, T.P., Houle, J.J., Avelleneda, P., Briggs, J., Fowler, G., and Wildey, R. 2009. Seasonal Performance Variations for Storm-Water Management Systems in Cold Climate Conditions. Journal of Environmental Engineering. Vol. 135. No. 3. pp. 128-137.
  • Toronto and Region Conservation (TRCA). 2008. Performance Evaluation of Permeable Pavement and a Bioretention Swale, Seneca College, King City, Ontario. Prepared under the Sustainable Technologies Evaluation Program (STEP). Toronto, Ontario.


Related pages

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