Information: Much of the information on this page has been updated. Please see Pre-treatment

Flow-through water quality treatment structures are commonly used for pretreatment to remove coarse sediment (i.e., coarse silts, fine sands and larger particles) and floatables (i.e., debris and oil). These structures vary greatly in size and can include the following categories that are discussed in greater detail below:

  • Underground storage structures
  • Hydrodynamic separators
  • Sump catch basins or manholes

Pretreatment considerations

Pretreatment is a required stormwater management practice for infiltration practices intended to extend the life of downstream BMPs. Flow-through structures are intended to remove a significant amount of coarse silts and sands during low flows; their removal efficiency often is significantly reduced at high flows. Frequent maintenance and cleaning is required as these structures are prone to exhibit washout during high flows, where the previously-accumulated sediment is resuspended and carried out of the structure. Washout concerns can be mitigated through

  • frequent maintenance;
  • appropriate sizing;
  • high-flow bypass; and
  • scour-reducing screens, baffles or protective measures.

Permit applicability

Many communities have water quality goals that target a specific reduction in total suspended solids (TSS). Because flow-through structures cannot effectively remove clays and most silts, these BMPs cannot meet water quality goals without additional downstream primary treatment. If sized appropriately, flow-through structures can effectively extend the life of downstream primary treatment systems by removing coarser sediments.

Similarly, flow-through structures cannot remove dissolved phosphorus. Many communities have specific phosphorus reduction goals that are not possible to meet with use of these structures alone. While these structures alone do not meet the stormwater runoff volume performance goals or the stormwater runoff phosphorus removal performance goal developed as part of Minimal Impact Design Standards (MIDS), flow-through structures do qualify as pretreatment under the Minnesota Pollution Control agency (MPCA) Stormwater Construction Permit.

Retrofit applicability

Flow-through structures are often used in retrofit situations or in denser development (or redevelopment) where surface treatment is not feasible or cost-effective due to competing land demands. Flow-through structures generally have a smaller footprint than other treatment approaches and due to their placement underground, their use can allow the surface to be used for other purposes, such as parking or greenspace.

Sump catch basins and manholes are a relatively common BMP; however, testing has shown that these BMPs are prone to washout, limiting their effectiveness. Sump catch basins and manholes that have already been constructed present a unique opportunity for retrofit with porous baffles that can prevent or limit washout (e.g., SAFL Baffle). If the characteristics of the existing sump catch basin or manhole are consistent with the design considerations for the installation of porous baffle retrofit, a retrofit may effectively, and inexpensively, improve the removal efficiency of the BMP.

Cold climate suitability

Flow-through structures are appropriate for usage in Minnesota and other cold climates. They are placed belowground, often below the frostline. Flow-through structures are particularly effective at capturing sediment from late-winter or early-spring snowmelt events that are often sediment-laden.

Water quality benefits

Flow-through structures can remove nutrients that are attached to larger particles (coarse silts and sands) or nutrients that are attached to floating trash or debris. These BMPs are not effective at removing dissolved nutrients or nutrients attached to silts or clays.

Water quantity benefits

Flow-through structures do not promote volume reduction; however, stormwater reuse could be incorporated into the design, allowing reuse of the permanent pool in the BMP.

General descriptions

Underground storage structures

Underground storage structures (also called underground settling devices) typically contain multiple chambers to remove sediment and floatables. The first chamber serves as a forebay, promoting settlement of suspended sediment, generally coarse silts and sands in a permanent pool with a minimum depth of 4 feet. The second chamber serves a skimming function, removing oils, grease, and floating debris. A baffle or submerged inverted pipe separates the second and third chambers, trapping floatables. The third chamber can be used for additional settlement of large particles, before the cleaner water is discharged from the structure.

A permanent pool totaling 400 cubic feet per acre of tributary impervious surface should be provided throughout the entire structure.The footprint of these systems is generally much larger and they are typically more expensive than hydrodynamic separators.

Hydrodynamic separators

Due to the large size required to remove suspended solids in the underground storage structures, smaller-footprint proprietary systems have become more common. The internal components of these devices create flow patterns and flow conditions that help remove suspended sediments during low flow conditions. Several of these hydrodynamic separators have been full-scale tested by the St. Anthony Falls Laboratory (SAFL) at the University of Minnesota, including BaySaver, CDS, Downstream Defender, ecoStorm, Environment 21, Stormceptor, and the Vortechs System (Wilson et al., 2009).

Similar to underground storage structures, hydrodynamic separators include a permanent pool for sedimentation and a solid baffle or inverted outlet pipe to provide skimming to trap floatables. These systems vary in size and may contain several chambers.

The manufacturers of the hydrodynamic separators provide product-specific design guidance, but rarely do the guidelines include considerations for high flows and associated washout.

Sump catch basins and manholes

A sump is a standard catch basin or manhole constructed with part of the structure located below the outlet, creating a permanent pool. Catch basins and manholes are not made for pretreatment of storm water; however, they can potentially function as pretreatment structures when the bottom of the structure is significantly below the invert of the outlet pipe. Similar to the proprietary hydrodynamic devices, standard catch basins and manholes with deep sumps have a smaller footprint than an underground storage structure. Sumps are generally less expensive are more readily available than the hydrodynamic separators.

Standard sumps have one chamber and sedimentation occurs due to flow patterns resulting from water plunging into the sump. Sedimentation in standard sumps occurs in the permanent pool. Sumps should be designed with a permanent pool of at least 3 feet. The outlet from the sump sometimes incorporates an inverted pipe to allow skimming of floatables such as debris and oil.

As with proprietary systems, washout is a concern with sump catch basins and manholes (Saddoris et al., 2011; Howard et al., 2012). To mitigate the effects of washout, the University of Minnesota developed a porous baffle, called the SAFL Baffle, to modify the flow characteristics through sump manholes and reduce resuspension of previously-accumulated sediment. Full-scale testing determined that the SAFL Baffle proved quite effective at reducing resuspension and washout while adding very little head loss (Howard et al., 2012).

The SAFL Baffle was tested with sump manholes and sump catch basins. Sump catch basins (receiving flow through a grate above and from an inlet pipe) present a different flow pattern than a sump manhole which does not receive flow from above. Based on research performed at SAFL, washout and resuspension is mitigated when the drainage area of the inflow pipe is at least three times greater than the drainage area of the grate above the catch basin. High flows from an inlet grate into a catch basin can plunge deeper into the sump and cause washout when horizontal flows from an inlet pipe are too low (McIntire et al., 2012). Therefore, porous screen/baffle retrofits, such as SAFL Baffle, are not recommended for headwater catch basins or when the depth of the sump is less than three feet.

Design considerations

General guidance

Flow-through structures should be selected and designed to maximize suspended sediment removal given the treatment drainage area and its land use characteristics, available space for the structure, and presence of other stormwater BMPs in series. Design assistance of proprietary systems is generally provided by the manufacturer of the flow-through system.

All flow-through structures have some propensity to washout during high flow conditions. Installation of a high-flow bypass can mitigate washout, essentially designing the systems to function as offline treatment devices, primarily treating low flows.

Design assistance using SHSAM

The Saint Anthony Falls Laboratory (SAFL) of University of Minnesota has performed extensive full-scale testing of many different types of flow-through structures to determine their removal efficiency functions that can assist the designer in selecting the appropriate type of flow-through structure and size the selected structure to meet the sediment removal goals. In 2012, ASTM International developed standards in testing removal efficiency of hydrodynamic separators (ASTM C1746/C1746M–12). The standard is primarily based on the testing conducted at the University of Minnesota. In addition to sediment removal, SAFL performed testing to determine the propensity for sediment washout under high flows for different structures.

The results of SAFL’s full-scale testing were used to build a computer program to predict the removal efficiency and washout for various flow-through devices under different conditions. The program is called SHSAM (Sizing Hydrodynamic Separators And Manholes).

SHSAM is a computer program for predicting the amount of suspended sediments removed from stormwater runoff by a given hydrodynamic separator/standard sump over a given period of time, e.g., 15 years. SHSAM is comprised of a simple continuous runoff model, a generic sediment removal response function, and a generic sediment washout function. The table below summarizes the flow-through structures available for water quality modeling by SHSAM and whether the model is able to determine sediment removal efficiency and/or washout. SHSAM can be downloaded at [1].

Device Removal efficiency considered Washout considered
BaySaver X
Downstream Defender X X
ecoStorm X X
Environment 21 X X
Stormceptor X X
Vortechs System X
Standard Sumps X X
Standard Sumps with SAFL Baffle X X

In order to assess the performance of hydrodynamic separators and sump manholes, the user of SHSAM should select local weather data and silica sand OK110 particle size distribution (PSD). The silica sand OK110 is a commercial particle size distribution used in testing these devices. This gradation has a median size of110 microns with 90 percent of particles between 100 and 240 microns, which represents fine sands in stormwater runoff. OK110 does not represent a typical particle size distribution of suspended sediments; rather, it provides an appropriate range of particle sizes that are expected to be removed by flow-through structures. The removal efficiencies achieved by flow-through structures using OK110 are not the removal efficiencies that these devices will achieve for a typical range of suspended sediment particle sizes. OK110 allows the designer to determine how well the pretreatment structure can remove coarse silt and sand which would in turn decrease the maintenance frequency of the downstream primary BMP.

Note that an ASTM standard is currently being developed for a consistent type of particle-size-distribution to be used for testing hydrodynamic separators.

Performance Assessment

The SHSAM software provides a useful tool for assessing the performance of a limited number of hydrodynamic devices and standard sumps. All these devices have been tested based on a mass-balance approach with repeatable results. To assess the performance of hydrodynamic separators which are not included in the SHSAM software, the devices should be evaluated using the testing methods described in ASTM Standards C1746 and C1745. Subsequently, the device performance can be assessed based on the site specific conditions.

Flow-through structures cannot remove fine silts and clays, and thus generally can only remove a small fraction of presumed suspended solids particle size distribution. As such, their primary purpose is pretreatment and removal of coarse silts and sands. If properly designed and maintained, flow-through structures can remove up to 70 to 80 percent of coarse silts and sands, which can be represented by OK110 particle size distribution. This removal efficiency for coarse sediment equates to approximately a 20 percent removal of TSS using a particle size distribution with a broader range of sizes (e.g., EPA NURP particle size distribution). Removal of 20 percent TSS results in approximately a 10 percent reduction in total phosphorus. The nutrient loading removal of the downstream primary treatment BMP typically assumes pretreatment; therefore, the nutrient removal from the pretreatment device (such as a flow-through structure) cannot typically be added to the nutrient removal of the downstream primary treatment BMP. Attaining typical TSS and nutrient removal goals will require treatment by a downstream primary BMP. Removing a greater fraction of coarse sediments with pretreatment structures will allow the primary treatment structure to meet or exceed its anticipated design life.


Frequent maintenance of flow-through structures is essential to prevent resuspension of settled particles in subsequent storms. Ideally, structures would be cleaned out after each water quality rain event (described as 0.5 inches of precipitation by the Minnesota Stormwater Manual), although that may not be practical or even necessary, given the drainage area characteristics and resulting sediment load. Manufacturers of hydrodynamic separators recommend a frequency of once a year, which is most effective if it is done before the beginning of winter. However, through frequent inspection of accumulated sediment and screen/baffle clogging over the first two years following installation, a structure-specific maintenance program can be developed. The SHSAM software provides site-specific guidance on frequency of cleaning to mitigate resuspension, which can be implemented during the inspection/monitoring period and modified after obtaining more precise and distinct results on the performance of the device during the inspection/monitoring period. Note that most devices have a maximum capacity of 1 foot in the sump, i.e., the pretreatment device shall be cleaned before 1 foot of sediment accumulates in the sump, however, if the sump is shallow (less than 3 feet), the sump may reach maximum capacity before 1 foot of sediment accumulation and require more frequent maintenance.

All flow-through structures should be constructed to facilitate access of maintenance vehicles and equipment. Proprietary devices may have additional maintenance requirements that should be followed. At a minimum, structures and screens/baffles should be fully cleaned once a year. Vacuum trucks can be used to remove accumulated sediment and oils/grease, although compacted sediments may require full dewatering and manual removal of sediments. Currently, sediment removed from flow-through structures is not generally considered to be potentially hazardous and is not subject to testing for proper disposal. All applicable local, state and federal laws should be followed in the disposal of accumulated sediments and floatables, including hydrocarbons.


  • Howard, A.K., O. Mohseni, J.S. Gulliver, and H.G. Stefan (2012). Hydraulic Analysis of Suspended Sediment Removal from Storm Water in a Standard Sump. Journal of Hydraulic Engineering. ASCE. June 2012. 138(6): 491-502.
  • McIntire, K. D., Howard, A. K., Mohseni, O., and Gulliver, J.S. (2012). Assessment and Recommendations for the Operation of Standard Sumps as Best Management Practices for Stormwater Treatment, Volume 2. Final Report. Minnesota Department of Transportation. May 2012.
  • MPCA (2005). The Minnesota Stormwater Manual. Minnesota Pollution Control Agency. November 2005.
  • Saddoris, D. A., McIntire, K. D., Mohseni, O., and Gulliver, J. S. (2010). Hydrodynamic Separator Sediment Retention Testing. Final Report. Minnesota Department of Transportation. March 2010.
  • Wilson, M.A., and O. Mohseni, J. S. Gulliver, R.M. Hozalski, and H.G. Stefan (2009). Assessment of Hydrodynamic Separators for Storm-Water Treatment. Journal of Hydraulic Engineering. ASCE, May 2009. 135(5): 383-392.

This page was last modified on 21 July 2016, at 14:22.

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