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Pretreatment for Bioretention: Capture of Gross Solids and Sediment
December 2020 (volume 15 - issue 3)
Contributed by Andy Erickson and Matt Hernick; St. Anthony Falls Laboratory, University of Minnesota
This article was developed from the full report by Erickson, A. and M. Hernick. (2019). “Capture of Gross Solids and Sediment by Pretreatment Practices for Bioretention.” SAFL Project Report No. 586, University of Minnesota, Minneapolis, MN. January 2019. http://hdl.handle.net/11299/201607
Bioretention practices, often called rain gardens, have become an increasingly common stormwater treatment option in Minnesota. Beyond stormwater treatment, bioretention areas have aesthetic and other benefits and may be designed in a variety of ways to fit the characteristics of a given site. A primary purpose for these practices is to capture sediment from stormwater while it infiltrates into the bioretention media. This sediment can accumulate over time and eventually clog a bioretention cell. Thus, pretreatment of incoming stormwater is an integral part of the treatment process and is required for bioretention by the Minnesota Stormwater Manual (MPCA 2017a).
Pretreatment practices are intended to reduce maintenance and prolong the lifespan of structural stormwater BMPs by removing trash, debris, organic materials, coarse sediments, and associated pollutants prior to entering structural stormwater BMPs (MPCA 2017b). The performance goal set forth by the Minnesota Pollution Control Agency (MPCA) is capture of gross solids and 25% of sediment greater than 100μm. In addition, proper pretreatment practices can provide a stable inlet into a bioretention practice that prevents erosion and minimizes disturbance of ground cover (e.g., mulch) within the bioretention.
Actual data on the effectiveness of pretreatment practices, whether from field studies or laboratory or field testing, is limited or varies widely in method and results. The purpose of this project was to measure the performance of five pretreatment practices for bioretention, both proprietary and non-proprietary, commonly used in Minnesota. The field-based performance testing protocol was developed to measure capture of sediment and gross solids when adding the design storage volume (full storage volume before bypass) of the bioretention practice.
Five pretreatment practices were tested as part of this study: grass lined inlet, Rain Guardian Bunker, Rain Guardian Turret, rock lined inlet, and in-line shallow sump grit chamber. The primary treatment mechanisms for stormwater pretreatment are screening, settling, and filtration and are described for each of the five practices tested in this project in Table 1. The full description and dimensions of the practices are in the full report (Erickson and Hernick, 2019).
Table 1. Pretreatment practices, brief description, and treatment mechanisms (Erickson & Hernick, 2019).
|Grass Lined Inlet||Non-proprietary, grassed conveyance, sloped between curb cut and bottom of bioretention.||
|Rain Guardian Bunker||Proprietary rectangular chamber with top grate, concrete bottom, screened exit wall, and skimming debris wall.||
|Rain Guardian Turret||Proprietary cylindrical chamber with top grate, concrete bottom, screened exit wall, and skimming debris wall.||
|Rock Lined Inlet||Non-proprietary, rock-covered conveyance, sloped between curb cut and bottom of bioretention.||
|Shallow Sump Grit Chamber||Non-proprietary, shallow sump below gutter and connected to bioretention by three sub-surface PVC pipes.||
A grass lined inlet (GLI) in a non-proprietary grassed conveyance that is sloped between the curb cut and the bottom of bioretention, as shown in Figure 1a. It is also sometimes called a filter strip, buffer strip, or vegetative filter. The Rain Guardian Bunker (RGB) is a proprietary, rectangular chamber with top grate, concrete bottom, screened exit wall, and skimmer beam, as shown in Figure 1b. The Rain Guardian Turret (RGT) is a proprietary, cylindrical chamber with top grate, concrete bottom, screened exit wall, and skimmer beam as shown in Figure 1c. To facilitate testing of the RGT, diversion plates were constructed from lightweight insulation panels (pink, shown in Figure 1c) to divert flow into the opening of the RGT. A rock lined inlet (RLI) in a non-proprietary rock-covered conveyance that is sloped between the curb cut and the bottom of bioretention, as shown in Figure 1d. It is also sometimes called a riprap entrance, rock channel, or rock buffer strip.
Figure 1. a) Grass Lined Inlet: Flow at 0.25 cfs (GLI-025-B). b) Rain Guardian Bunker: Flow at 0.25 cfs (RGB-025-B). c) Rain Guardian Turret: Flow at 0.25 cfs (RGT-025-A). d) Rock Lined Inlet: Flow at 0.50 cfs.
The in-line shallow sump grit chamber tested during this project comprises a rectangular catch basin, approximately 36 inches long by 24 inches wide with a 12-inch sump. There are five 4-inch holes in the bottom of the concrete chamber floor which allow for infiltration of water from the sump into the subsurface soils. The grit chamber is installed in-line with the gutter (Figure 2) and has three 4-inch outlet pipes that route water underneath the curb to the bioretention basin. When the water level in the sump and bioretention practice exceed the elevation of the grate within the gutter, water begins to “bypass” the bioretention practice as it flows through or over the in-line sump into the downstream gutter.
Figure 2. Shallow sump pretreatment with surface grate removed. This photo was taken upon arrival at the site, before cleaning the sump in preparation for testing.
For this project, a ratio of 80% sediment and 20% gross solids by mass was used to create the total solids at a concentration of 200 mg/L based on studies in the Twin Cities Metropolitan Area (Brezonik and Stadelmann, 2002; Kalinosky, 2015). Each pretreatment practice was thoroughly cleaned prior to each test replicate. Three narrow-graded classes of silica sand including a coarse sediment (D50 = 1.17mm), a medium sediment (D50 = 0.41mm), and a fine sediment (D50 = 0.12mm) and three types of gross solids (plastic forks, synthetic leaves, and wood dowels) were added to water from a fire hydrant throughout the duration of each test. After testing was complete, sediment and gross solids were collected and then analyzed at St. Anthony Falls Laboratory to determine capture performance.
The full design storage volume was added from a fire hydrant to the pretreatment and bioretention within 40 minutes (low intensity) or within 20 minutes (high intensity). Four of the pretreatment practices were tested with this process: grass lined inlet (i.e., grassed buffer strip), Rain Guardian Bunker proprietary device, Rain Guardian Turret proprietary device, and rock lined inlet (i.e., riprap). The pretreatment and bioretention practices were not allowed to overflow or bypass during the design volume tests. A timelapse video of the testing setup and operation is shown in Figure 3.
Figure 3: Timelapse video of testing setup and operation for rock lined inlet and grass lined inlet.
The in-line shallow sump grit chamber was tested with a revised protocol in which the design storage volume was added over 30 minutes (low intensity) and 15 minutes (high intensity). The shallow sump grit chamber was also tested with bypass conditions, which involved adding approximately two and a half times the design volume to the pretreatment and bioretention practice, causing the system to overflow and bypass some water and solids to the downstream conveyance system.
Summary of Results
All tests and replicates were performed from an initially clean condition, so the performance for several sequential tests and maintenance needed for long-term operation of these pretreatment practices was not measured in this project. All five pretreatment practices captured greater than 88% of the total sediment and greater than 65% of the fine sediment fraction (D50 = 0.12mm) in the low intensity tests (data not shown; see Erickson & Hernick 2019). During the high intensity tests, all practices captured greater than 70% of the total sediment mass and greater than 30% of the fine sediment fraction (see Figure 3). This level of performance exceeds the performance goal of 25% capture of sediment greater than 100μm, which is set by the Minnesota Pollution Control Agency (MPCA 2017b). Four of the five pretreatment practices captured 75% of the gross solids during low intensity tests and more than 55% of the gross solids during high intensity tests (see Figure 4). While the MPCA does not have a numeric target for gross solids, this level of performance for pretreatment practices is substantial. The grass lined inlet captured the least gross solids; 20% during low intensity and 30% during high intensity.
Figure 4: a) Sediment capture by percent for design volume high intensity tests (Q = 0.50cfs, duration = 20 minutes); and b) Gross solids capture by percent for design storage volume tests, Q = 0.50cfs, duration = 20 minutes.
Bypass tests were conducted to determine the performance of an in-line shallow sump grit chamber under bypass conditions. During these tests, overall sediment capture decreased from 95% during low intensity design volume tests down to 80% capture during high intensity bypass tests. Gross solids capture decreased from greater than 80% to below 40% (see Figure 5). Thus, bypass at these flow rates had minimal effect on the sediment capture, but measurable effect on the gross solids capture.
Figure 5: Sediment capture (top) and gross solids capture (bottom) by the shallow sump grit chamber for two design volume tests (a) Q = 0.06cfs for 30 minutes and (b) Q = 0.12cfs for 15 minutes; and two bypass tests (c) Q = 0.12cfs for 40 minutes and (d) Q = 0.25cfs for 20 minutes. BP = Bypass; TD = Total Duration.
Though at least four of the five pretreatment practices performed similarly in terms of sediment and gross solids capture, only three out of the five appear to be simple to inspect and maintain. When maintenance is required, the grass lined inlet and rock lined inlet likely require the same amount of effort and cost to maintain them as would be needed to install them. Of the pretreatment practices tested in this study, the grass lined inlet and rock lined inlet are among the most difficult and costly to maintain.
To maintain the Rain Guardian Bunker, Rain Guardian Turret, and shallow sump, one would need to remove the top grate and either shovel or hydro-vac the collected sediment and gross solids from within the collection chamber. The Bunker and Turret are both easily visible from the street so visual inspections of accumulated sediment depth are simple. The shallow sump is hidden underground, which makes assessing sediment accumulation depth more challenging. Of the pretreatment practices tested in this study, the Bunker and Turret are among the easiest to maintain, and the shallow sump is moderately easy to maintain.
Get the Full Report
The full report for this research project is available and citable as: Erickson, A. and M. Hernick. (2019). Capture of Gross Solids and Sediment by Pretreatment Practices for Bioretention. SAFL Project Report No. 586, University of Minnesota, Minneapolis, MN. January 2019. http://hdl.handle.net/11299/201607
This project was supported by the University of Minnesota Water Resources Center and by the Minnesota Stormwater Research Council with financial contributions from:
- Capitol Region Watershed District
- Mississippi Watershed Management Organization
- Ramsey-Washington Metro Watershed District
- South Washington Watershed District
- Valley Branch Watershed District, and
- City of Edina
For more information about the Center and the Council, visit https://www.wrc.umn.edu/projects/storm-waste-water
In addition, this project was supported by the Anoka Conservation District (ACD). The authors wish the thank the Water Resources Center and the Minnesota Stormwater Research Council and affiliated entities for provided funding to support this project. In addition, the authors wish to thank the ACD for provided funding and in-kind match (labor and materials) for this project.
Support and assistance from several organizations and individuals are listed below and is greatly appreciated. Support and assistance for the contracting process was provided by Jeff Peterson, John Bilotta, Ann Lewandowski, Cheryel Konate, Jenni Larson, Chris Lord, and Jared Wagner. In addition, Chris Lord, Jared Wagner, Mitch Haustein, and Jackson Miller (MN Conservation Corps Apprentice) provided in-kind support via labor and materials throughout testing conducted in Anoka. Support provided by St. Anthony Falls Laboratory (SAFL) staff and students include Rob Gabrielson, Peter Olson, Ben Erickson, Jim Tucker, Rikita Patel, Camila Merino-Franco, and Parker Brown.
The Cities of Anoka and Bloomington, MN provided staff, access to fire hydrants, and supplied water meters and hose for use in field testing. The authors wish to thank Marcus Mihelich from the City of Anoka, and Steve Gurney, Pat Conrad, and Ben Whitcomb from the City of Bloomington for their assistance. The City of Anoka donated 12,939 cubic feet of water and the City of Bloomington donated 1,560 cubic feet of water for field testing. In addition, owners of the property on which the rain gardens were located cooperated with field testing and supplied garden hose and donated water for field testing.
Brezonik, P.L., and T.H. Stadelmann. (2002). Analysis and predictive models of stormwater runoff volumes, loads, and pollutant concentration from watersheds in the Twin Cities metropolitan area, Minnesota, USA. Water Research 36:1743-1757. https://doi.org/10.1016/S0043-1354(01)00375-X
Erickson, A. and M. Hernick. (2019). Capture of Gross Solids and Sediment by Pretreatment Practices for Bioretention. SAFL Project Report No. 586, University of Minnesota, Minneapolis, MN. January 2019. http://hdl.handle.net/11299/201607
Kalinosky, P. (2015). Quantifying solids and nutrient recovered through street sweeping in a suburban watershed. Master of Science Thesis. University of Minnesota, St. Paul, MN. http://hdl.handle.net/11299/172600
Minnesota Pollution Control Agency (MPCA). (2017a). “Design Criteria for Bioretention.” Minnesota Stormwater Manual. Retrieved June 8, 2017 from https://stormwater.pca.state.mn.us/index.php/Design_criteria_for_bioretention
Minnesota Pollution Control Agency (MPCA). (2017b). “Overview and methods of Pretreatment.” (2017, January 23). Minnesota Stormwater Manual. Retrieved May 25, 2017 from https://stormwater.pca.state.mn.us/index.php?title=Overview_and_methods_of_pretreatment https://stormwater.pca.state.mn.us/index.php?title=Pretreatment&oldid=31119