Pre-treatment considerations for stormwater infiltration

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: The Construction Stormwater General Permit states: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.

Information: Reference therein 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 MPCA.

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 have increased impervious surface and are more likely to contain high concentrations and annual loads of trash, sediments, and pollutants in stormwater runoff. Of particular concern are trash, debris, sediments, metals, organics, 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 as a result of clogging.
• 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. see the section on potential stormwater hotspots.
• 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, oil/water separators, proprietary settling/swirl chambers, and vegetated filter strips. It is important to note that many of these pretreatment techniques will require routine maintenance. Other practices typically considered to be primary BMPs, such as swales and green roofs, are often used for pretreatment.

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.

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.

Unit process for pretreatment BMPs

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

Unit processes of stormwater pretreatment techniques (Adapted from WEF, 2008)
Link to this table

Control	Vegetated filter strips	Vegetated swale	Forebays	Street/parking lot sweeping	Proprietary settling/swirl chambers	Oil/water separators	Green roofs	Cisterns	Drain inlet inserts
Peak flow attenuation					X		X
Infiltration	X	X
Dispersion	X	X					X
Evapotranspiration							X
Runoff collection and usage								X
Sedimentation	X		X	X	X	X			X
Flotation			X		X	X
Laminar separation						X
Swirl concentration					X
Sorption							X
Filtration	X	X							X
Plant metabolism	X	X					X
Temperature reduction	X	X					X

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.

Maniquiz et al., 2010 conducted research on the effectiveness of presettling basins as a component of stormwater best management practice (BMP) technologies. The authors state "optimizing the design of the presettling basin means that the storage volume ratio should be determined based on the desired captured amount of runoff and sediment from runoff to limit the frequency of maintenance caused by the accumulation of sediment. It was recommended that pretreatment of runoff should be employed when the site in which the BMP is to be sited has high TSS loading and runoff rate, and is subjected to high intensity rainfall."

Related pages

• Overview of stormwater infiltration
• Pre-treatment considerations for stormwater infiltration
• BMPs for stormwater infiltration
• Pollutant fate and transport in stormwater infiltration systems
• Surface water and groundwater quality impacts from stormwater infiltration
• Stormwater infiltration and groundwater mounding
• Stormwater infiltration and setback (separation) distances
• Karst
• Shallow soils and shallow depth to bedrock
• Shallow groundwater
• Soils with low infiltration capacity
• Potential stormwater hotspots
• Stormwater and wellhead protection
• Stormwater infiltrations and contaminated soils and groundwater
• Decision tools for stormwater infiltration
• Stormwater infiltration research needs
• References for stormwater infiltration