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[[file:RG pic2.jpg|thumb|300px|alt=pretreatment screen|<font size=3>Pretreatment screen for a residential raingarden. Photo courtesy of Katherine Sullivan.</font size>]]
 
[[file:RG pic2.jpg|thumb|300px|alt=pretreatment screen|<font size=3>Pretreatment screen for a residential raingarden. Photo courtesy of Katherine Sullivan.</font size>]]
  
'''NOTE:'''It is strongly recommended that pretreatment devices be certified through [https://ecology.wa.gov/Regulations-Permits/Guidance-technical-assistance/Stormwater-permittee-guidance-resources/Emerging-stormwater-treatment-technologies Washington State's TAPE] or [http://www.njcat.org/ New Jersey's NJCAT] programs, be modeled using [https://shsam.barr.com/ SHSAM], or follow guidance in this manual for <span title="Pretreatment vegetated filter strips are designed to provide sedimentation and screening (by vegetation) to treat stormwater runoff prior to entering a structural stormwater BMP. Pretreatment vegetated filter strips are especially effective at capturing excess sediment in stormwater runoff by settling solids. Pretreatment vegetated filter strips provide limited (due to size) volume reduction, peak flow reduction, infiltration, and biological treatment. Stormwater management processes not provided in pretreatment vegetated filter strips include filtration and sorption."> [https://stormwater.pca.state.mn.us/index.php?title=Overview_for_pretreatment_vegetated_filter_strips '''vegetated filter strips''']</span> and <span title="An artificial pool of water in front of a larger body of water. The larger body of water may be natural or man-made. Forebays have a number of functions. They may be used upstream of reservoirs to trap sediment and debris (sometimes called a sediment forebay) in order to keep the reservoir clean."> '''[https://stormwater.pca.state.mn.us/index.php?title=Pretreatment_-_Screening_and_straining_devices,_including_forebays forebays]'''</span>.
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'''NOTE:''' It is strongly recommended that pretreatment devices be certified through [https://ecology.wa.gov/Regulations-Permits/Guidance-technical-assistance/Stormwater-permittee-guidance-resources/Emerging-stormwater-treatment-technologies Washington State's TAPE] or [http://www.njcat.org/ New Jersey's NJCAT] programs, be modeled using [https://shsam.barr.com/ SHSAM], or follow guidance in this manual for <span title="Pretreatment vegetated filter strips are designed to provide sedimentation and screening (by vegetation) to treat stormwater runoff prior to entering a structural stormwater BMP. Pretreatment vegetated filter strips are especially effective at capturing excess sediment in stormwater runoff by settling solids. Pretreatment vegetated filter strips provide limited (due to size) volume reduction, peak flow reduction, infiltration, and biological treatment. Stormwater management processes not provided in pretreatment vegetated filter strips include filtration and sorption."> [https://stormwater.pca.state.mn.us/index.php?title=Overview_for_pretreatment_vegetated_filter_strips '''vegetated filter strips''']</span> and <span title="An artificial pool of water in front of a larger body of water. The larger body of water may be natural or man-made. Forebays have a number of functions. They may be used upstream of reservoirs to trap sediment and debris (sometimes called a sediment forebay) in order to keep the reservoir clean."> '''[https://stormwater.pca.state.mn.us/index.php?title=Pretreatment_-_Screening_and_straining_devices,_including_forebays forebays]'''</span>.
  
 
It is important to tailor a <span title="Pretreatment reduces maintenance and prolongs the lifespan of structural stormwater BMPs by removing trash, debris, organic materials, coarse sediments, and associated pollutants prior to entering structural stormwater BMPs. Implementing pretreatment devices also improves aesthetics by capturing debris in focused or hidden areas. Pretreatment practices include settling devices, screens, and pretreatment vegetated filter strips."> [https://stormwater.pca.state.mn.us/index.php?title=Pretreatment '''pretreatment''']</span> practice to the specific site and type of receiving <span title="A stationary and permanent BMP that is designed, constructed and operated to prevent or reduce the discharge of pollutants in stormwater"> '''structural stormwater BMP'''</span>. Many factors influence the choice of pretreatment practices, including but not limited to: (1) contributing area characteristics (<span title="The total drainage area, including pervious and impervious surfaces, contributing to a BMP"> '''[https://stormwater.pca.state.mn.us/index.php?title=Contributing_drainage_area_to_stormwater_BMPs contributing drainage area]'''</span>, <span title="A subset of impervious cover, which is directly connected to a drainage system or a water body via continuous impervious surfaces."> '''connected imperviousness'''</span>, land uses, soils, slopes, dominant vegetation, source controls in place, and availability of public lands ); (2) existing infrastructure (above and below ground); and (3) type of structural stormwater BMP with <span title="Infiltration Best Management Practices (BMPs) treat urban stormwater runoff as it flows through a filtering medium and into underlying soil, where it may eventually percolate into groundwater. The filtering media is typically coarse-textured and may contain organic material, as in the case of bioinfiltration BMPs."> [https://stormwater.pca.state.mn.us/index.php?title=Stormwater_infiltration_Best_Management_Practices '''infiltration''']</span>/<span title="Filtration Best Management Practices (BMPs) treat urban stormwater runoff as it flows through a filtering medium, such as sand or an organic material. They are generally used on small drainage areas (5 acres or less) and are primarily designed for pollutant removal. They are effective at removing total suspended solids (TSS), particulate phosphorus, metals, and most organics. They are less effective for soluble pollutants such as dissolved phosphorus, chloride, and nitrate."> [https://stormwater.pca.state.mn.us/index.php?title=Filtration '''filtration''']</span> BMPs having that have the greatest need for removal of solids prior to treatment areas. The following section describes the specific strengths, weaknesses, and applicability of pretreatment screens.
 
It is important to tailor a <span title="Pretreatment reduces maintenance and prolongs the lifespan of structural stormwater BMPs by removing trash, debris, organic materials, coarse sediments, and associated pollutants prior to entering structural stormwater BMPs. Implementing pretreatment devices also improves aesthetics by capturing debris in focused or hidden areas. Pretreatment practices include settling devices, screens, and pretreatment vegetated filter strips."> [https://stormwater.pca.state.mn.us/index.php?title=Pretreatment '''pretreatment''']</span> practice to the specific site and type of receiving <span title="A stationary and permanent BMP that is designed, constructed and operated to prevent or reduce the discharge of pollutants in stormwater"> '''structural stormwater BMP'''</span>. Many factors influence the choice of pretreatment practices, including but not limited to: (1) contributing area characteristics (<span title="The total drainage area, including pervious and impervious surfaces, contributing to a BMP"> '''[https://stormwater.pca.state.mn.us/index.php?title=Contributing_drainage_area_to_stormwater_BMPs contributing drainage area]'''</span>, <span title="A subset of impervious cover, which is directly connected to a drainage system or a water body via continuous impervious surfaces."> '''connected imperviousness'''</span>, land uses, soils, slopes, dominant vegetation, source controls in place, and availability of public lands ); (2) existing infrastructure (above and below ground); and (3) type of structural stormwater BMP with <span title="Infiltration Best Management Practices (BMPs) treat urban stormwater runoff as it flows through a filtering medium and into underlying soil, where it may eventually percolate into groundwater. The filtering media is typically coarse-textured and may contain organic material, as in the case of bioinfiltration BMPs."> [https://stormwater.pca.state.mn.us/index.php?title=Stormwater_infiltration_Best_Management_Practices '''infiltration''']</span>/<span title="Filtration Best Management Practices (BMPs) treat urban stormwater runoff as it flows through a filtering medium, such as sand or an organic material. They are generally used on small drainage areas (5 acres or less) and are primarily designed for pollutant removal. They are effective at removing total suspended solids (TSS), particulate phosphorus, metals, and most organics. They are less effective for soluble pollutants such as dissolved phosphorus, chloride, and nitrate."> [https://stormwater.pca.state.mn.us/index.php?title=Filtration '''filtration''']</span> BMPs having that have the greatest need for removal of solids prior to treatment areas. The following section describes the specific strengths, weaknesses, and applicability of pretreatment screens.

Revision as of 22:28, 23 June 2022

image
Information: Reference to any specific commercial product, process, or service by trade name, trademark, service mark, manufacturer, or otherwise does not constitute or imply endorsement, recommendation, or favoring by the Minnesota Pollution Control Agency.
image of screen
Screened inlets to a tree trench box, Maplewood mall, Minnesota. Photos courtesy of Alisha Goldstein, U.S. EPA.
pretreatment screen
Pretreatment screen for a residential raingarden. Photo courtesy of Katherine Sullivan.

NOTE: It is strongly recommended that pretreatment devices be certified through Washington State's TAPE or New Jersey's NJCAT programs, be modeled using SHSAM, or follow guidance in this manual for vegetated filter strips and forebays.

It is important to tailor a pretreatment practice to the specific site and type of receiving structural stormwater BMP. Many factors influence the choice of pretreatment practices, including but not limited to: (1) contributing area characteristics ( contributing drainage area, connected imperviousness, land uses, soils, slopes, dominant vegetation, source controls in place, and availability of public lands ); (2) existing infrastructure (above and below ground); and (3) type of structural stormwater BMP with infiltration/ filtration BMPs having that have the greatest need for removal of solids prior to treatment areas. The following section describes the specific strengths, weaknesses, and applicability of pretreatment screens.

Processes

Screens provide a very specific and limited function of collecting large pollutants from stormwater runoff at defined sites along the stormwater flow network. Screens do not provide volume reduction, peak flow reduction, sedimentation, infiltration, filtration, sorption and biological processes.

Applicability and suitability

Pretreatment practices are NOT stand-alone treatment practices and should only be installed in conjunction with a treatment practice immediately downstream. The applicability of screens with regard to the cold climate, retrofits, and highly urban and ultra-urban environments is presented below.

Cold Climate suitability

Screens are not suitable for use in cold climates when snow and ice is able to accumulate on the screen, because screens that become frozen often result in short-circuiting (bypass) or backflows. Ice accumulation also often prevents screens from adequately pretreating high runoff volumes and pollutant loads during major thaw and spring snowmelt events in cold climates. Screens also do not provide snow storage capacity.

Retrofit suitability

Screens are often suitable to retrofit with a variety of structural stormwater BMPs due to their small footprint.

Ultra-Urban suitability

Screens are often effectively used in ultra-urban settings due to their small footprint but above ground screens are often constrained by aesthetics, traffic/safety and cold weather factors.

Applicability to MPCA stormwater permits

If designed properly, this practice meets the intent of the Construction stormwater permit as pretreatment for an infiltration or filtration system.

Advantages and limitations

The advantages and limitations of screens with regard to the pollutant removal capabilities, cost, ease of construction and maintenance, space and other design considerations and compatibility are presented below.

Advantages

Screens are an effective means of pretreatment in certain instances. They screen out sediment and debris efficiently in a small amount of space. The initial costs are often relatively inexpensive because screen systems are not complex. This also allows for simple maintenance which typically requires access to the screen and manual removal of sediment and debris, sometimes including washing with clean water to dislodge collected sediment and debris. Construction consists of installing a screen device in place, with relatively minimal labor costs and often without heavy equipment. Because screens typically have a small footprint, they are suitable for many BMPs and retrofit with many preexisting structures with minimal construction. Screens do provide discrete locations for the interception of large debris thus potentially saving expenses associated with collecting trash from structural stormwater BMPS.

Limitations

Screens should be inspected after each storm, until a site-specific, seasonal maintenance routine is developed. For most screen materials and design, pollutant removal is limited to trash, debris and coarse sediments while all other pollutants will pass through to the structural stormwater BMP. If the screen diameters are too small, collected material will clog the screen and increase cleaning frequency. Some seasons (e.g., spring snowmelt, spring leaf-out debris, autumn deciduous leaf-fall) or contributing areas (e.g., heavily wooded residential areas without street sweeping, commercial businesses with plastic bags, etc.) often produce a substantial amount of large sediment and debris, resulting in the need for inspections to determine an effective maintenance protocol. Screen systems often do not have much collection capacity and thus reduce in effectiveness or clog after a series of storm events. Without frequent maintenance, this ultimately diminishes the overall effectiveness of the system and thus careful consideration of screen design (size, diameter of openings, contributing area) is required to treat the range of expected seasonal flows and solids load.

Water quantity treatment

Stormwater volume reduction is not provided by pretreatment screens because water flowing into the device exits after the screen removes the sediment and large debris.

Water quality treatment

image of pretreatment screen
A pretreatment screen removes debris, such as leaves, and pollutants associated with the debris. Pollutants may leach out of the debris, however, such as nitrogen and phosphorus from leaves, making it important to properly maintain the screens. Image is of a RainGuardian Bunker. Source: RainGuardian

Screens used for stormwater pretreatment provide screening, or physical straining, of sediment, trash and debris from stormwater. Screens are not to be confused with filters, which refer to the passing of stormwater through a media to remove pollutants. Screens used for stormwater pretreatment are typically manufactured devices that are placed at stormwater inlets.

Points to consider when choosing a screen include the size of the screen openings, the anticipated flow rate through the screen, and the debris holding capacity of the device.

Screens may provide a reduction in nutrients, bacteria, or hydrocarbons if those pollutants are attached to trash or debris captured by the screen. For example, leaf litter can be a large source of nutrients in stormwater runoff, and effectively managing leaf litter has been used by communities to control phosphorus loads in stormwater (City of Madison, 2015). Another reason it is important to clean screens out regularly is to avoid the possibility of nutrients being leached out over time, which reduces the overall effectiveness of nutrient removal by screens. Capture of trash such as food containers, diapers and pet waste may also reduce bacteria (USEPA, 2012). Screens that capture trash and other debris should be cleaned and maintained regularly to prevent accumulation of trash, which may become a pollution source if not properly removed (USEPA, 2014).

Performance of pretreatment screens

Caution: Available information on performance of settling devices is limited and should be viewed with caution

The following discussion provides a summary of information found in the literature. The information provides a general summary and indication of the range of pollutant removal and should not be used for design purposes.

A pilot study in the state of California found that catch basin inserts with five-millimeter (mm) openings captured 100 percent of trash in runoff generated by a one-year, one-hour storm (Burns, 2014). Larger openings in the screen may allow small debris such as wrappers or cigarette butts to pass through the screen. The District Department of Environment captured 17,000 pounds of trash in 2012 using trash traps as a part of their TMDL implementation strategy (DDOE, 2013).


Related pages

To see the above pages as a single page, link here

Pretreatment sizing for basins and filters strips

Guidance for managing sediment and wastes collected by pretreatment practices

Tables

Other information and links