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 {{alertThis page is an edit and testing page use by the wiki authors. It is not a content page for the Manual. Information on this page may not be accurate and should not be used as guidance in managing stormwater.alertdanger}}   {{alertThis page is an edit and testing page use by the wiki authors. It is not a content page for the Manual. Information on this page may not be accurate and should not be used as guidance in managing stormwater.alertdanger}} 
   
−  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.
 +  Stormwater pollutant concentrations and event mean concentrations 
   
−  To perform efficiently, pretreatment practices must be properly sized. Given the lack of a consistent approach in stormwater pretreatment sizing in the literature, we developed a sizing approach focused on removing a specific fraction of sediment. This approach is dependent on the settling velocity of different sized particles.
 +  *[https://stormwater.pca.state.mn.us/index.php?title=Municipal_Separate_Storm_Sewer_System_(MS4)_toolkit {{pop'''Municipal Separate Storm Sewer System (MS4) toolkit'''What is the MS4 ToolkitThe municipal separate storm sewer system (MS4) toolkit is a resource for stormwater program staff. It can help entities regulated under the MS4 program meet the requirements of the general permit, specifically in meeting the 6 Minimum Control Measures. However, since it offers a wide range of stormwater education information including sample fact sheets, newsletters, videos, and other resources, it can be used by all stormwater practitioners and educators.}}] 
−   +  
−  This page provides an approach to pretreatment sizing. For the technical development of this methodology, [https://stormwater.pca.state.mn.us/index.php?title=Technical_basis_for_pretreatment_sizing_for_basins_and_filter_strips link here].
 +  
−   +  
−  ==Pretreatment basins==
 +  
−  [[File: Preserver Photo 1.JPGthumb300 pxleftalt=image of Preserver pretreatment device<font size=3>Heavy Leaf and Debris Loading in Pretreatment Manhole Sump [Ramsey Conservation District, 2017]. </font size>]]
 +  
−   +  
−  Basins are stormwater best management practices (BMPs) having an engineered pool that promotes settling of solids (e.g., pond <span title="an artificial pool of water in front of a larger body of water. The larger body of water may be natural or manmade. 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."> '''forebay'''</span>, <span title="a pit or hollow in which liquid collects"> '''sump'''</span>, <span title="stormwater management devices that use cyclonic separation to control water pollution. They are designed as flowthrough structures with a settling or separation unit to remove sediment and other pollutants."> '''hydrodynamic separator'''</span>, and <span title="A catch basin is an engineered drainage structure with the sole function of collecting rainwater and snowmelt from streets and parking lots and transporting it to local waterways through a system of underground piping, culverts, and / or drainage ditches"> '''catch basin'''</span>). These pretreatment facilities are typically small (relative to the primary treatment practice) but can be sized to settle gravel and sands, and possibly silts, at low flow conditions. For preliminary sizing purposes, the small size results in a basin that is close to wellmixed, where the concentration of suspended solids is constant with depth to the bottom of the basin and the particles are settling out because of zero velocity at the wall.
 +  
−   +  
−  ===Methodology for pretreatment basins===
 +  
−  For the following calculation, ensure your units are consistent
 +  
−  *Determine, ''v''<sub>''S''</sub>, the settling velocity for the particle size targeted (recommend 80 microns for particle size. Determine v<sub>s</sub> from table on this page)
 +  
−  *Determine ''FR'', the target fraction removal (recommend 0.500.70)
 +  
−  *Determine ''A'', the area of directly connected impervious draining to the pretreatment practice
 +  
−  *Determine ''I'', the peak rain intensity (0.5 in/hr for a 1inch event, Type 2 distribution)
 +  
−  *Calculate the area of the pretreatment basin from A<sub>S</sub> = (FR * I * A * FR) / (v<sub>S</sub> * (1  FR))
 +  
−  *Determine the basin depth as the lesser of sqrt(A<sub>S</sub>) or 6 feet
 +  
−   +  
−  ===Example for pretreatment basin===
 +  
−  { class="wikitable" style="float:right; marginleft: 10px; width:500px;"
 +  
−  
 +  
−   colspan="3" style="textalign: center;" '''Settling velocity according to Equation 8 of various equivalent spherical diameter sand and silt particles at 20°C.'''
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−  
 +  
−  ! Silt and sand Diameter (um) !! v<sub>s</sub> (m/s) !! Header text (ft/s)
 +  
−  
 +  
−   10  0.000089  0.00029
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−  
 +  
−   30  0.00078  0.0026
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−  
 +  
−   80  0.0051  0.017
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−  
 +  
−   100  0.0075  0.025
 +  
−  
 +  
−   150  0.015  0.049
 +  
−  
 +  
−   200  0.023  0.075
 +  
−  
 +  
−   500  0.071  0.23
 +  
−  
 +  
−   1000  0.13  0.43
 +  
−  }
 +  
−   +  
−  One of the inlets into a 2acre pond is draining a directly connected impervious area of 30 acres. The pretreatment basin for the 2acre pond will be designed to retain 70 percent of the 80 μm finesand particles. Using adjacent table, the settling velocity (v<sub>S</sub>) for 80 μm finesand particles is 0.017 feet per second (fps). For this case, the values to be input into the calculation are
 +  
−   +  
−  *FR = 0.7
 +  
−  *A = 30 acres
 +  
−  *I = 0.5 in/hr
 +  
−  *v<sub>s</sub> = 0.017 ft/s
 +  
−   +  
−  Plugging the these into the equation A<sub>S</sub> = (FR * I * A * FR) / (v<sub>S</sub> * (1  FR)) yields a value of 1453 ft<sup>2</sup> for the area of the forebay. The depth is the lesser of 6 feet or the square root of the forebay area (square root of 1453 = 38.1). The depth is therefore 6 feet.
 +  
−   +  
−  ==Pretreatment filter strips==
 +  
−  [[File:Burnsville figure 2.png300pxthumbleftalt=figure of filter strip<font size=3>Vegetated Filter Strip at Curb Cutout (Barr Engineering Company, 2006).</font size>]]
 +  
−   +  
−  Filter strips are primarily designed to enhance settling; physical filtering of particles in a filter strip is minimal. The filter strip is located at the bottom of a shallow, widely spread, open channel flow to allow for particle settling through the media, which is often grass or rocks. The grass or rocks allow space for the settled particles to collect until cleanout is required.
 +  
−   +  
−  ===Methodology for pretreatment filter strips===
 +  
−  For the following calculation, ensure your units are consistent
 +  
−  *Determine, ''v''<sub>''S''</sub>, the settling velocity for the particle size targeted (recommend 80 microns for particle size. Determine v<sub>s</sub> from table on this page)
 +  
−  *Determine ''FR'', the target fraction removal (recommend 0.500.70)
 +  
−  *Determine ''A'', the area of directly connected impervious draining to the pretreatment practice
 +  
−  *Determine ''I'', the peak rain intensity (0.5 in/hr for a 1inch event, Type 2 distribution)
 +  
−  *Calculate the area of the filter strip from LW =  ((c * I * A) / (v<sub>S</sub>) * ln(1  FR)) where c = 0.7 for small storms
 +  
−  *Determine the length (L) and the width (W) from the above computation
 +  
−   +  
−  ===Example for pretreatment filter strip===
 +  
−  A directly connected impervious area of 10,000 ft<sup>2</sup> drains into a bioretention practice. The area is a sufficient size to install a wellsized filter strip, so the designer chose to settle 80 percent of the 80 ''u''m finesand particles in the filter strip during the design storm. For this case, the values to be used are
 +  
−  *FR = 0.8
 +  
−  *A = 10,000 ft<sup>2</sup>
 +  
−  *I = 0.5 in/hr
 +  
−  *v<sub>S</sub> = 0.017 ft/s
 +  
−  *c = 0.7
 +  
−   +  
−  Plugging these values into the equation LW =  ((0.7 * 10000 ft<sup>2</sup>) * 0.5 in/hr) / 0.017 ft/s) * ln(10.8) = 7.6 ft<sup>2</sup>
 +  
−   +  
−  Assuming a 3ft wide inlet, the filter strip would need to be a minimum of 2.5 ft (L = LW/W = 7.6 ft<sup>2</sup>/3 ft = 2.5 ft) in length by 2.5ft. To settle 80 percent of the fine sand particles entering this bioretention practice, the filter strip is recommended to be 3 ft wide by 2.5 ft long.
 +  
   
   