Treatment in BMP’s and LID’s
Excerpt from the EPA manual Storm Water Management Model Reference Manual Volume III – Water Quality (PDF) which can be found here
Management of stormwater quality is usually performed through a combination of so-called “best management practices” (BMPs) and a form of hydrologic source control popularly known as “low impact development” (LID). Treatment of stormwater runoff, either by natural means or by engineered devices, can occur at both the source of the generated runoff or at locations within the conveyance network. Source treatment through LID is discussed in the next chapter. This section describes how SWMM models treatment applied to flows already captured and transported within a conveyance system.
Table 5-1, adapted from Huber et al. (2006), categorizes the different unit treatment processes used by various types of conveyance system BMPs. Ideally one would like to model these processes at a fundamental level, to be able to estimate pollutant removal based on physical design parameters, hydraulic variables, and intrinsic chemical properties and reaction rates. With a few exceptions, the state of our knowledge does not permit this, at least within the scope of a general purpose stormwater management model like SWMM. Instead one has to rely on empirical relationships developed from site-specific monitoring data.
Strecker et al. (2001) discuss the challenges of using monitoring data to develop consistent estimates of BMP effectiveness and pollutant removal. The International Stormwater BMP Database (www.bmpdatabase.org) provides a comprehensive compilation of BMP performance data from over 500 BMP studies on 17 different categories of BMPs and LID practices. It is continually updated with new data contributed by the stormwater management community. Table 5-2 lists the median influent and effluent event mean concentrations (EMCs) for a variety of BMP categories and pollutants that were compiled from this database. The cells highlighted in yellow indicate that a statistically significant removal of the pollutant was achieved by the BMP category. A summary of the median removal percentages of several common pollutants treated by filtration, ponds, and wetlands published in the Minnesota Stormwater Manual is listed in Table 5-3. Most of these percentages are consistent with those inferred from median EMC numbers in the BMP database table 5-2.
Table 5-1 Treatment processes used by various types of BMPs
|Sedimentation||Gravitational settling of suspended particles from the water column.||Ponds, wetlands, vaults, and tanks.|
|Flotation||Separation of particulates with a specific gravity less than water (e.g., trash, oil and grease).||Oil-water separators, density separators, dissolved-air flotation.|
|Filtration||Removal of particulates by passing water through a porous medium like sand, gravel, soil, etc.||Sand filters, screens, and bar racks.|
|Infiltration||Allowing captured runoff to infiltrate into the ground reducing both runoff volume and loadings of particulates and dissolved nutrients and heavy metals.||Infiltration basins, ponds, and constructed wetlands.|
|Adsorption||Binding of contaminants to clay particles, vegetation or certain filter media.||Infiltration systems, sand filters with iron oxide, constructed wetlands.|
|Biological Uptake and Conversion||Uptake of nutrients by aquatic plants and microorganisms; conversion of organics to less harmful compounds by bacteria and other organisms.||Ponds and wetlands.|
|Chemical Treatment||Chemicals used to promote settling and filtration. Disinfectants used to treat combined sewer overflows.||Ponds, wetlands, rapid mixing devices.|
|Natural Degradation (volatilization, hydrolysis, photolysis)||Chemical decomposition or conversion to a gaseous state by natural processes.||Ponds and wetlands.|
|Hydrodynamic Separation||Uses the physics of flowing water to create a swirling vortex to remove both settleable particulates and flotables.||Swirl concentrators, secondary current devices, oil-water separators.|
Table 5-2 Median inlet and outlet EMCs for selected stormwater treatment practices
|Pollutant||Media Filtration||Detention Basin||Retention Pond||Wetland Basin||Manufactured Device|
|F. Coliform, #/100mL||1350||542||1480||1030||1920||707||13000||6140||2210||2750|
|Total P, mg/L||0.18||0.09||0.28||0.22||0.30||0.13||0.13||0.08||0.19||0.12|
|Total N, mg/L||1.06||0.82||1.40||2.37||1.83||1.28||1.14||1.19||2.27||2.22|
Source: International Stormwater BMP Database, “International Stormwater Best Management Practices (BMP) Database Pollutant Category Summary Statistical Addendum: TSS, Bacteria, Nutrients, and Metals”, July 2012 (www.bmpdatabase.org).
Table 5-3 Median pollutant removal percentages for select stormwater BMPs
|Total Suspended Solids||85||84||73|
|Zinc and Copper||50||70||70|
Source: Minnesota Stormwater Manual (http://stormwater.pca.state.mn.us).