Pre-treatment considerations for stormwater infiltration

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Infiltration Best Management Practices (BMPs) are susceptible to clogging from the trash, debris, and suspended sediments present in runoff. Pretreatment can remove debris and coarser sediments in an easier-to-maintain pretreatment device that will extend the life and reduce maintenance for the infiltration BMP. If work is being done under an MPCA Permit, then it is REQUIRED that some form of pretreatment be installed upstream of an infiltration BMP. In all other cases pretreatment is highly recommended.

Warning: To prevent clogging of the infiltration or filtration system, the Permittee(s) must use a pretreatment device such as a vegetated filter strip, small sedimentation basin, or water quality inlet (e.g., grit chamber) to settle particulates before the stormwater discharges into the infiltration or filtration system.

Situations where pretreatment is particularly important

Conditions when pretreatment is of particular importance (Sources: 1-Environmental Health and Safety, Western Michigan University, with permission) Storm Water; 2-Winegrad, Gerald. 2015. Chesapeake Bay Action Plan; 3-WI DNR. 2012. Soil Erosion from New Construction.4-Betts, Lynn. 1999. Runoff of Soil & Fertilizer. 5-Hoogestraat, Galen. 2013. Runoff Flowing through Arrowhead Golf Course. 6-Sepp, Siim. 2011. Rounded Fine-Grained Eolian Sand Sample from the Gobi Desert.

Pretreatment is of particular importance in the following situations.

• High density urban areas. High density urban areas are more likely to contain high concentrations of trash, sediments, and pollutants which can be carried by the stormwater runoff. High density urban areas contain a higher concentration of roadways; of particular concern with roadways are trash, debris and sediments (making pretreatment an important part of the BMP) as well as metals and chlorides.
• Areas with high potential for erosion. Areas that are susceptible to erosion are of concern because of high sediment loads that reduce the BMP’s infiltration capacity. This will clog the infiltration BMP.
• Areas where stormwater has a high pollutant load. Areas with a high pollutant load, or the presence of certain pollutants that are not easily removed from runoff are a concern because they have the potential to contaminate the groundwater.
• Storm sewers that convey runoff at a high velocity. A high velocity will keep sediment in suspension. Pretreatment should be installed to facilitate the proper settling of the sediment, which will prevent clogging. In addition, high velocities can reduce the volume of runoff that can be infiltrated.

Common pretreatment methods

Forebays (small sediment basins) are the most common pretreatment method, though there are many others, including cisterns, drain inlet inserts, green roofs, oil/water separators, proprietary settling/swirl chambers, vegetated filter strips, and vegetated swales. It is important to note that many of these pretreatment techniques will require routine maintenance.

Forebays. Forebays, also known as plunge pools, are small basins upstream of other BMPs that dissipate the velocity of the incoming water and provide stilling, sedimentation, and trapping of gross pollutants.

Cisterns. Cisterns are tanks located above or below ground that are used for storing a specific amount of runoff for the purpose of non-potable reuse (e.g. irrigation or vehicle washing). Small cisterns are also known as rain barrels. These completely remove the runoff from the treatment train and, therefore, provide 100 percent pollution reduction from the volume of water retained. Cisterns are often used to collect stormwater runoff from rooftops (termed rainwater harvesting), but they can also be used to intercept runoff from other impervious areas.

Drain Inlet Inserts. Drain inlet inserts are devices placed into stormwater drains or catch basins to remove pollutants from stormwater prior to entering the storm sewer system. These inserts utilize an inert filter material, such as polypropylene, to enhance pollutant removal (WEF, 2012). Drain inlet inserts have the ability to remove debris, trash, large sediments and, if a filter material is present, can also remove oils/greases and other pollutant types.

Green roofs. Green roofs capture rainfall on a roof top vegetated surface before the rainfall is allowed to become runoff. These are located at the beginning of the stormwater treatment train to provide volume reduction and to prevent runoff from collecting pollutants associated with rooftops. As green roofs employ a media for the vegetation, they are often times not effective at removing phosphorous and do not get credit for phosphorous removal in this Manual.

Oil/water separators. Oil/water separators are structures designed specifically to remove petroleum hydrocarbons, grease, sand, and grit. These separators can be split into two categories, American Petroleum Institute (API) separators and coalescing plate separators (WEF, 2012). API separators are a larger vault with baffles which enhance hydraulic efficiency. Coalescing plate separators use sloped plates or extruded tubes to achieve sediment and oil removal, and are smaller than the API structures.

Proprietary settling/swirl chambers. Proprietary settling/swirl chambers or concentrators, also known as hydrodynamic devices, cause the stormwater to move in a circular motion which enhances the settling out of sediments. These devices often remove solids, oils/grease, floatable sand, and other larger debris from stormwater runoff.

Vegetated filter strips. Vegetative filter strips reduce the velocity of stormwater runoff, allowing the sediments to settle out. Filter strips work best when receiving runoff as sheet flow, making them suitable alongside roads, parking lots, and other paved surfaces.

Vegetated swale. Vegetated swales are similar to vegetated filter strips. These devices are better suited for concentrated flow in addition to sheet flow of runoff. The flow path through the swale allows for the settling and filtration of coarse particles, and limited infiltration.

Unit process for pretreatment BMPs

The following table provides a summary of unit processes for pretreatment BMP.

Unit processes of stormwater pretreatment techniques

Additional studies

Mohamed, Lucke, and Boogaard, 2013. The authors looked at the potential to increase the effective life of permeable pavement systems by first routing the runoff through a swale. The study took place in Australia with the objective of determining the variation in pollutant removal performance along the length of the swale. The experiment showed that the grassed swales studied were effective at removing the sediment from the runoff, and would thus slow down the rate at which the permeable pavement would become clogged. The authors concluded that excessively long swales are not a cost effective solution because most of the removal happens in the first 10 meters. They also concluded that removal of 50 percent of the TSS would significantly increase the life span of the permeable pavements.

Browne, Deletic, Fletcher, and Mudd, 2011. The authors developed a dynamic two dimensional variably saturated flow model that allows a user to represent the storage and clogging of an infiltration trench. The authors modeled the hydrologic effectiveness of infiltration trenches and infiltration basins with no clogging, clogging for 10 years, and clogging for 50 years. The BMPs were modeled in sandy loam and sandy clay. The results showed that there was a significant decrease in the hydrologic effectiveness of the BMPs in sandy loam after 10 years of clogging, and another decrease after 50 years of clogging. With the BMPs in the sandy clay, there was no noticeable decrease after 10 years of clogging, but there was a decrease after 50 years. The results of this experiment show that pretreatment can increase the lifespan of an infiltration BMP. http://stormwater.pca.state.mn.us/index.php?title=Pre-treatment_considerations_for_stormwater_infiltration&action=edit

Related pages

• Overview
• BMPs
• Pollutant fate and transport in stormwater infiltration systems
• Surface water and groundwater quality impacts from stormwater infiltration
• Groundwater mounding
• Setback distances
• Karst
• Shallow soils and shallow depth to bedrock
• Shallow groundwater
• Soils with low infiltration capacity
• Potential stormwater hotspots
• Wellhead protection
• Contaminated soils and groundwater
• Decision tools
• Research needs
• References for stormwater infiltration