This stormwater manual contains information on pollutant removal for a limited number of pollutants. A detailed discussion and links to this information are provided below. Users should be aware that the 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. Pollutant removal is a function of many factors, including design, construction, and maintenance of the BMP, quality of incoming stormwater, time of year, and so on. We have included additional links to information on pollutant removal.
The following table provides a summary of median pollutant removal for BMPs. Note that in 2017 the MPCA conducted a review of crdit information from other documents for comparison with the Minnesota Stormwater Manual and Minimal Impact Design Standards. results of that review can be seen at this link: File:Credit analysis.docx
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.
Link to this table
|Biofiltration and Tree trench/tree box with underdrain||80||link to table||link to table||link to table||50||35||95||80|
|Iron enhanced sand filter||85||77||91||60||35||50||80||80|
|Dry swale||68||link to table||link to table||link to table||35||0||80||80|
|Constructed wet ponds4||84||50||91||0||30||70||60||80|
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
Several databases and data compilations were reviewed and compared to develop expected performance measures for Total Suspended Solids (TSS) and Total Phosphorus (TP) for the following categories of BMPs:
The following studies were used to develop these performance measures.
BMP performance can be evaluated in several ways, the most common of which are pollutant outflow concentration and pollutant percent removal.
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.
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.
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.
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.
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.
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
1MnDOT values for mean TSS removal=65% and mean TP removal=52%
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.
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
1MnDOT values for mean TSS removal=68% and mean TP removal=42%
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.
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
1MnDOT values for mean TSS removal=68% and mean TP removal=41%
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).
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
1MnDOT values for mean TSS removal=82% and mean TP removal=46%
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.
Link to this table
|Particulate phosphorus||Dissolved phosphorus|
|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?
TP removal credit
|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?
2. Does the system include approved P-sorbing soil amendments2?
1Other widely accepted soil P tests may be used. Note: a basic conversion of test results may be necessary
2Acceptable P sorption amendments include
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.
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: