Calculating credits for bioretention

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Credit refers to the quantity of stormwater or pollutant reduction achieved either by an individual BMP or cumulatively with multiple BMPs. Stormwater credits are a tool for local stormwater authorities who are interested in

• providing incentives to site developers to encourage the preservation of natural areas and the reduction of the volume of stormwater runoff being conveyed to a best management practice (BMP);
• complying with permit requirements, including antidegradation (see [1]; [2]);
• meeting the MIDS performance goal; or
• meeting or complying with water quality objectives, including Total Maximum Daily Load (TMDL) Wasteload Allocations (WLAs).

This page provides a discussion of how biofiltration practices can achieve stormwater credits.

Description

Biofiltration, commonly termed bioretention with underdrains, is primarily a stormwater quality control practice. Some water quantity reduction can be achieved through infiltration below the underdrain, particularly if the underdrain is raised above the bottom of the Best Management Practice (BMP), and through evapotranspiration.

Typically biofiltration consists of an engineered soil layer designed to treat stormwater runoff via filtration through plant and soil media, and evapotranspiration from plants. Pretreatment is REQUIRED for all biofiltration facilities to settle particulates before entering the system. Biofiltration includes an underdrain layer to collect the filtered runoff for downstream discharge. Other common components may include a stone aggregate layer to allow for increased retention storage and an impermeable liner on the bottom or sides of the facility if located near buildings, subgrade utilities, or in karst formations. Biofiltration is a versatile stormwater treatment method applicable to all types of settings such as landscaping islands, cul-de-sacs, parking lot margins, commercial setbacks, open space, rooftop drainage, and streetscapes.

Warning: Pretreatment is REQUIRED for bioretention practices

Pollutant removal mechanisms

Biofiltration has one of the highest nutrient and pollutant removal efficiencies of any BMP (Mid-America Regional Council and American Public Works Association Manual of Best Management PracticeBMPs for Stormwater Quality, 2012). Biofiltration pollutant removal primarily occurs through filtering by the engineered soil media and vegetation. Biofiltration also provides pollutant removal and volume reduction through evaporation, infiltration, transpiration, biological and microbiological uptake, and soil adsorption; the extent of these benefits is highly dependent on site specific conditions and design. In addition to phosphorus and total suspended solids (TSS), which are discussed in greater detail below, biofiltration treats a wide variety of other pollutants.

Removal of phosphorus is dependent on the engineered media. Media mixes with high organic matter content typically leach phosphorus and can therefore contribute to water quality degradation. The Manual provides a detailed discussion of media mixes, including information on phosphorus retention.

Location in the treatment train

Stormwater treatment trains are multiple Best Management Practice (BMPs) that work together to minimize the volume of stormwater runoff, remove pollutants, and reduce the rate of stormwater runoff being discharged to Minnesota wetlands, lakes and streams. Under the Treatment Train approach, stormwater management begins with simple methods that prevent pollution from accumulating on the land surface, followed by methods that minimize the volume of runoff generated, and is completed by BMPs that reduce the pollutant concentration and/or volume of stormwater runoff. Biofiltration facilities are typically located in upland areas of the stormwater treatment train, controlling stormwater runoff close to the source.

Calculating credits

Biofiltration generates credits for Total Suspended Solids (TSS) and Total Phosphorus (TP). Biofiltration does not substantially reduce the volume of runoff but may qualify for a partial volume credit as a result of evapotranspiration, infiltration occurring through the sidewalls above the underdrain, and infiltration below the underdrain piping. Biofiltration is also effective at reducing concentrations of other pollutants including nitrogen, metals, bacteria, and hydrocarbons. This article does not provide information on calculating credits for pollutants other than TSS and TP, but references are provided that may be useful for calculating credits for other pollutants.

Assumptions and approach

Schematic illustrating the water quality volume (WQV) for a biofiltration BMP. The WQV equals the water ponded above the medai and below the overflow point in the BMP.

In developing the credit calculations, it is assumed the bioretention practice is properly designed, constructed, and maintained in accordance with the Minnesota Stormwater Manual. If any of these assumptions are is not valid, the credit will be reduced. See the appropriate article within the bioretention section of the Manual.

In the following discussion, the kerplunk method is assumed in calculating volume and pollutant reductions. This method assumes the water quality volume (WQV) is delivered instantaneously to the BMP. The WQV is stored as water ponded above the filter media and below the overflow point in the BMP. The WQV can vary depending on the stormwater management objective(s). For construction stormwater, the water quality volume is 1 inch off new impervious surface. For MIDS, the WQV is 1.1 inches.

In reality, some water will infiltrate through the bottom and sidewalls of the BMP as a rain event proceeds. The kerplunk method therefore may underestimate actual volume and pollutant losses.

Volume credit calculations

Schematic illustrating the different water loss terms for a biofiltration BMP with an underdrain at the bottom.

Schematic illustrating the different water loss terms for a biofiltration BMP with a raised underdrain.

Volume credits for biofiltration are available only if the BMP permanently removes a portion of the stormwater runoff via infiltration through sidewalls or beneath the underdrain piping, or through evapotranspiration. These credits are assumed to be instantaneous values based on the design capacity of the BMP for a specific storm event. Instantaneous volume reduction, also termed event based volume reduction, can be converted to annual volume reduction percentages using the MIDS calculator or other appropriate modeling tools.

Volume credits for biofiltration basins with underdrains are calculated by a combination of infiltration through the unlined sides and bottom of the basin, the volume loss through evapotranspiration (ET), and the retention volume below the underdrain, if applicable (this is based on the assumption that this stored water will infiltrate into the underlying soil). The main design variables impacting the volume credits include whether the underdrain is elevated above the native soils and if an impermeable liner on the sides or bottom of the basin is used. Other design variables include surface area at overflow, media top surface area, underdrain location, and basin bottom locations, total depth of media, soil water holding capacity and media porosity, and infiltration rate of underlying soils.

Information: For the following equations, units typically used are given and unit correction factors are based on those units

The volume credit (V) for biofiltration basins with underdrains, in cubic feet, is given by

\( V = V_{inf_b} + V_{inf_s} + V_{ET} + V_U \)

where:

• V_infb = volume of infiltration through the bottom of the basin (cubic feet);
• V_infs = volume of infiltration through the sides of the basin (cubic feet);
• V_ET = volume reduction due to evapotranspiration (cubic feet); and
• V_U = volume of water stored beneath the underdrain that will infiltrate into the underlying soil (cubic feet).

Volume credits for infiltration through the bottom of the basin (V_{inf_b}) are accounted for only if the bottom of the basin is not lined. As long as water continues to draw down, some infiltration will occur through the bottom of the BMP. However, It is assumed that when an underdrain is included in the installation, the majority of water will be filtered through the media and exit through the underdrain. Because of this, the drawdown time is likely to be short. Volume credit for infiltration through the bottom of the basin is given by

\( V_{inf_B} = I_R/12 * A_B * DDT \)

where

• I_R = design infiltration rate of underlying soil (inches per hour);
• A_B = surface area at the bottom of the basin (square feet); and
• DDT = drawdown time for ponded water (hours).

If the kerplunk method is being applied and runoff water is delivered instantaneously to the BMP, the design infiltration rate (I_r) will equal the saturated hydraulic conductivity. The drawdown time is typically a maximum of 48 hours, which is designed to be protective of plants grown in the media. The Construction Stormwater permit requires drawdown within 48 hours and recommends 24 hours when discharges are to a trout stream.

Volume credit for infiltration through the sides of the basin is accounted for only if the sides of the basin are not lined with an impermeable liner. Volume credit for infiltration through the sides of the basin is given by

\( V_{inf_s} = I_R/12 * (A_O - A_U) * DDT \)

where

• A_O = the surface area at the overflow (square feet); and
• A_U = the surface area at the underdrain (square feet).

This equation assumes water will infiltrate through the entire sideslope area during the period when water is being drawn down. This is not the case, however, since the water level will decline in the BMP. The MIDS calculator assumes a linear drop in water level and thus divides the right hand term in the above equation by 2.

Volume credit for media storage capacity below the underdrain (V_U) is accounted for only if the underdrain is elevated above the native soils. Volume credit for media storage capacity below the underdrain is given by

\( V_U = (A_U + A_B)/2 * n * D_U \)

where

• A_B = surface area at the bottom of the media (square feet);
• n = media porosity (cubic feet per cubic foot); and
• D_U = the depth of media below the underdrain (feet).

Per the kerplunk method, this is an instantaneous volume. This will somewhat overestimate actual storage when the majority of water is being captured by the underdrains. This equation also assumes this stored water will infiltrate into the underlying soil. Thus, using the entire porosity will overestimate the actual drainage. The MIDS calculator assumes water between the soil porosity and field capacity will infiltrate into the underlying soil.

The volume of water lost through ET is assumed to be the smaller of two calculated values: potential ET and measured ET. Potential ET (ET_pot) is equal to the amount of water stored in the basin between field capacity and the wilting point. ET_pot is converted to ET by multiplying by a factor of 0.5. Measured ET (ET_mea) is the amount of water lost to ET as measured using available data and is assumed to be 0.2 inches/day. ET is considered to occur over a period equal to the drawdown time of the basin. Volume credit for evapotranspiration is given by the lesser of

\( ET_{mea} = 0.2/12 * A * 0.5 * t \) \( ET_{pot} = D * A * C_S \)

where

• t = time over which ET is occurring (hours);
• D = depth being considered (feet);
• A = area being considered (square feet); and
• C_S = soil water available for ET.

Provided soil water content is greater than the wilting point, ET will continually occur during the non-frozen period. However, because the above volume calculations are event based, t will be equal to the time between rain events. In the MIDS calculator, a value of 3 days is used because this is the average number of days between precipitation events. ET will occur over the entire media depth. D may therefore be set equal to the media depth (D_M). In this case, the value for A would be the average area through the entire depth of the media. The MIDS calculator limits ET to the area above the underdrain. If infiltration is being computed through the bottom and sidewalls of the basin, then C_S would be or field capacity minus the wilting point of soils (cubic feet per cubic foot) since water above the field capacity would infiltrate (or go to an underdrain).

The volume of water passing through underdrains can be determined by subtracting the volume loss (V) from the volume of water instantaneously captured by the BMP. No volume reduction credit is given for filtered stormwater that exits through the underdrain, but the volume of filtered water can be used in the calculation of pollutant removal credits through filtration.

The volume reduction credit (V) can be converted to an annual volume if desired. This conversion can be generated using the MIDS calculator or other appropriate modeling techniques. The MIDS calculator obtains the percentage annual volume reduction through performance curves developed from multiple modeling scenarios using the volume reduction capacity for biofiltration, the infiltration rate of the underlying soils, and the contributing watershed size and imperviousness.

Total suspended solids credit calculations

TSS reduction credits correspond with volume reduction through infiltration and filtration of water captured by the biofiltration basin and are given by

\( M_{TSS} = M_{TSS_i} + M_{TSS_f} \)

where

• M_TSS = TSS removal (pounds);
• M_{TSS_i} = TSS removal from infiltrated water (pounds); and
• M_{TSS_f} = TSS removal from filtered water (pounds).

Pollutant removal for infiltrated water is assumed to be 100 percent. The mass of pollutant removed through infiltration, in pounds, is given by

\( M_{TSS_i} = 0.0000624 * (V_{inf_b} + V_{inf_s} + V_U) * EMC_{TSS} \)

where

• EMC_TSS is the event mean TSS concentration in runoff water entering the BMP (milligrams per liter).

The EMC_TSS entering the BMP is a function of the contributing land use and treatment by upstream tributary BMPs.

Removal for the filtered portion is less than 100 percent. The Stormwater Manual provides a recommended value of 85 percent removal for filtered water, while the MIDS calculator provides a value of 65 percent. Alternate justified percentages for TSS removal can be used if proven to be applicable to the BMP design.

The mass of pollutant removed through filtration, in pounds, is given by

\( M_{TSS_f} = 0.0000624 * (V_{total} - (V_{inf_b} + V_{inf_s} + V_U)) * EMC_{TSS} * R_{TSS} \)

where

• V_total is the total volume of water captured by the BMP (cubic feet); and
• R_TSS is the TSS pollutant removal percentage for filtered runoff. Alternate justified percentages for TSS removal, including direct monitoring data, can be used if proven to be applicable to BMP credit calculation.

The above calculations may be applied on an event or annual basis using the appropriate units.

Total phosphorus credit calculations

Total phosphorus (TP) reduction credits correspond with volume reduction through infiltration and filtration of water captured by the biofiltration basin and are given by

\( M_{TP} = M_{TP_i} + M_{TP_f} \)

where

• M_TP = TP removal (pounds);
• M_{TP_i} = TP removal from infiltrated water (pounds); and
• M_{TP_f} = TP removal from filtered water (pounds).

Pollutant removal for infiltrated water is assumed to be 100 percent. The mass of pollutant removed through infiltration, in pounds, is given by

\( M_{TP_i} = 0.0000624 * (V_{inf_b} + V_{inf_s} + V_U) * EMC_{TP} \)

where

• EMC_TP is the event mean TP concentration in runoff water entering the BMP (milligrams per liter).

The EMC_TP entering the BMP is a function of the contributing land use and treatment by upstream tributary BMPs.

The filtration credit for TP in biofiltration with underdrains assumes removal rates based on the soil media mix used and the presence or absence of amendments. Soil mixes with more than 30 mg/kg phosphorus (P) content are likely to leach phosphorus and do not qualify for a water quality credit. If the soil phosphorus concentration is less than 30 mg/kg, the mass of phosphorus removed through filtration, in pounds, is given by

\( M_{TP_f} = 0.0000624 * (V_{total} - (V_{inf_b} + V_{inf_s} + V_U)) * EMC_{TP} * R_{TP} \)

Information: Soil mixes C and D are assumed to contain less than 30 mg/kg of phosphorus and therefore do not require testing

Again, assuming the phosphorus content in the media is less than 30 milligrams per kilogram, the removal efficiency (R_TP) provided in the Stormwater Manual is a function of the fraction of phosphorus that is in particulate or dissolved form, the depth of the media, and the presence or absence of soil amendments. For the purpose of calculating credits it can be assumed that TP in storm water runoff consists of 55 percent particulate phosphorus (PP) and 45 percent dissolved phosphorus (DP). The removal efficiency for particulate phosphorus is 80 percent. The removal efficiency for dissolved phosphorus is 20 percent if the media depth is 2 feet or greater. The efficiency decreases by 1 percent for each 0.1 foot decrease in media thickness below 2 feet. If a soil amendment is added to the BMP design, an additional 40 percent credit is applied to dissolved phosphorus. Thus, the overall removal efficiency, (R_TP), expressed as a percent removal of total phosphorus, is given by

\( R_{TP} = [0.8 * 0.55] + [0.45 * ((0.2 * (D_{MU_{max=2}})/2) + 0.40_{if amendment is used})] * 100 \)

where

• the first term on the right side of the equation represents the removal of particulate phosphorus;
• the second term on the right side of the equation represents the removal of dissolved phosphorus; and
• D_MUmax=2 = the media depth above the underdrain, up to a maximum of 2 feet.

Methods for calculating credits

This section provides specific information on generating and calculating credits from biofiltration for volume, Total Suspended Solids (TSS) and Total Phosphorus (TP). Stormwater runoff volume and pollution reductions (“credits”) may be calculated using one of the following methods:

1. Quantifying volume and pollution reductions based on accepted hydrologic models
2. MIDS Calculator approach
3. Quantifying volume and pollution reductions based on values reported in literature
4. Quantifying volume and pollution reductions based on field measurements

Credits based on models

Users may opt to use a water quality model or calculator to compute volume, TSS and/or TP pollutant removal for the purpose of determining credits for biofiltration. The available models described in the following sections are commonly used by water resource professionals, but are not explicitly endorsed or required by the Minnesota Pollution Control Agency.

Use of models or calculators for the purpose of computing pollutant removal credits should be supported by detailed documentation, including:

• Model name and version
• Date of analysis
• Person or organization conducting analysis
• Detailed summary of input data
• Calibration and verification information
• Detailed summary of output data

The following table lists water quantity and water quality models that are commonly used by water resource professionals to predict the hydrologic, hydraulic, and/or pollutant removal capabilities of a single or multiple stormwater BMPs. The table can be used to guide a user in selecting the most appropriate model for computing volume, TSS, and/or TP removal for biofiltration BMPs.

MIDS Calculator

Download the MIDS Calculator

The Minimal Impact Design Standards (MIDS) best management practice (BMP) calculator is a tool used to determine stormwater runoff volume and pollutant reduction capabilities of various low impact development (LID) BMPs. The MIDS calculator estimates the stormwater runoff volume reductions for various BMPs and annual pollutant load reductions for total phosphorus (including a breakdown between particulate and dissolved phosphorus) and total suspended solids (TSS). The calculator was intended for use on individual development sites, though capable modelers could modify its use for larger applications.

The MIDS calculator is designed in Microsoft Excel with a graphical user interface (GUI), packaged as a windows application, used to organize input parameters. The Excel spreadsheet conducts the calculations and stores parameters, while the GUI provides a platform that allows the user to enter data and presents results in a user-friendly manner.

Detailed guidance has been developed for all BMPs in the calculator, including biofiltration. An overview of individual input parameters and workflows is presented in the MIDS Calculator User Documentation.

Credits based on reported literature values

A simplified approach to computing a credit would be to apply a reduction value found in literature to the pollutant mass load or concentration (EMC) of the biofiltration device. A more detailed explanation of the differences between mass load reductions and concentration (EMC) reductions can be found on the pollutant removal page of this WIKI (here). Designers may use the pollutant reduction values reported in this WIKI (here) or may research values from other databases and published literature. Designers who opt for this approach should:

• Select the median value from pollutant reduction databases that report a range of reductions, such as from the International BMP Database.
• Select a pollutant removal reduction from literature that studied a biofiltration device with site characteristics and climate similar to the device being considered for credits.
• When using data from an individual study, review the article to determine that the design principles of the studied biofiltration are close to the design recommendations for Minnesota, as described in this WIKI, and/or by a local permitting agency.
• Preference should be given to literature that has been published in a peer-reviewed publication.

Credits based on field measurements

In the event that credit is being calculated for an existing Biofiltration installation, field measurements may be made in lieu of the desktop calculation models/calculators discussed. It is HIGHLY RECOMMENDED that field measurements be obtained by following the methods described in the University of Minnesota Assessment of Stormwater Best Management Practices Manual.

Other pollutants

According to the International BMP Database, studies have shown Biofiltration is effective at reducing concentrations of several pollutants, including solids, bacteria, metals, and nutrients. The National BMP Database provides an overview of BMP performance studies in relation to various pollutant categories and constituents that were monitored for each BMP. The report notes that effectiveness and range of unit treatment processes can vary greatly depending on BMP design and location. Table 3-4 shows a list of the pollutants for the monitored Biofiltration facilities where the median of the data points and the 95% confidence interval about the median show decreases in effluent pollutant loads. Pollutant removal percentages for Biofiltration BMPS can also be found on the WIKI page.