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Stormwater Best Management Practices in an Ultra-Urban Setting: Selection and Monitoring

Fact Sheet - Manufactured Systems

Cylindrical access hole and box structure stormwater treatment devices have become increasingly popular for the removal of particulate matter normally found in stormwater runoff. The two main treatment mechanisms are vortex motion particle and particulate settling and oil-water separation. The devices operate by intercepting a portion of the flow traveling through the storm drain system and using a vortex motion and/or conventional settling chamber to separate out large sediments and oils. Two common types of access hole treatment devices include the Stormceptor® and the Downstream DefenderTM. An example of a box-type treatment unit is the VortechsTM Stormwater Treatment System. Figures 23, 24, and 25 show the Stormceptor®, Downstream DefenderTM, and VortechsTM treatment devices, respectively.

Figure 23. Stormceptor® operation during average flow conditions
(Stormceptor®, 1995)

Inlet is in middle of side of unit, runoff flows through to enter treatment area below perpendicular to inlet. Flow out of unit is on oposite side of chamber and is paralell to flow into chamber. Flow then makes a right degree turn to outlet. Sediment collects on bottom, oil is trapped underneath the insert.


Figure 24. Downstream DefenderTM (Hydro International, 1996)

Shows location of access ports, support frame, dip plate, tangential inlet pipe, benching skirt, floatables lid, center shaft and cone, concrete manhole, sediment storage facility, and outlet pipe.


Figure 25. VortechsTM Stormwater Treatment System
(Vortechnics, 1996)

Plan and Elevation Views: Flow swirls due to swirl concentrator into the grit and oil chambers, under oil barrier into flow control chamber, past weir plates into the outlet chamber.

The Stormceptor® and Downstream DefenderTM can be designed to retrofit an existing stormwater access hole structure or be designed as a new storm drain system. Each of the devices is designed to treat low to moderate storm flows. The incoming stormwater and pollutants enter a diversion chamber and are diverted into the lower chamber for treatment. While oils and floatable particulate matter rise to the surface, sediments settle out to the bottom. During peak or high flows, the excess stormwater bypasses the lower treatment chamber and flows directly to the downstream storm drain system.

The Stormceptor® is divided into two water quality chambers designed for removal of the oil and sediment normally found in urban stormwater runoff. Stormwater flows into the upper chamber and is diverted by a V-shaped weir down a drop pipe and into the lower chamber. The flow is then redirected horizontally around the circular walls of the lower chamber and through an outlet pipe. The inlet drop pipe and outlet riser pipe are set at the same elevation to provide storage for oil and sediment within the lower chamber.

The Downstream DefenderTM operates by introducing stormwater into its cylindrical base, where the runoff spirals down the perimeter, allowing the larger sediments to settle out. The internal components of the Downstream DefenderTM allow oils, grease, and floatables to be trapped. Unlike the conventional oil/grit separator unit, the Stormceptor® and Downstream DefenderTM are designed to prevent the resuspension of sediment, thereby providing actual removal during every storm event.

The VortechsTM system consists of four chambers. The first chamber is termed the grit or swirl chamber. Settleable particles are swept to the center of this chamber, where they are induced to settle out. The higher the flow rate through the system, the greater the strength of the vortex settling motion. Particles eventually migrate toward the center of the cylindrical chamber, where velocities are low and conditions are tranquil. The particles remain trapped until the system is cleaned. The first chamber is designed to prevent wash-outs that occur in conventional water quality inlet devices. The second chamber is the oil chamber. The oil barrier traps floatables, oils, and grease. Unlike conventional oil traps that lack flow controls and extra tank capacity, the VortechsTM system is designed to handle most flow surges. The third chamber is the flow control chamber, which is designed to reduce forces that encourage resuspension and wash-out. During conditions of intense storm surge through the unit, the low-flow control within the VortechsTM system causes the inlet pipe to become submerged. This process floats oily constituents up above the inlet pipe and out of the influent stream; thus, oils and grease are kept within the trap. The fourth chamber is the outlet chamber.


The Stormceptor® and Downstream DefenderTM treatment systems are used primarily for treatment of stormwater runoff from impervious surfaces. The devices are ideal for use in ultra-urban settings since each is composed of a precast structure that is installed beneath the ground and can either be retrofitted to an existing storm drain system or replace a proposed access hole in a storm drain system. The structures are designed to capture and treat a portion of the flow that enters into the storm drain system; however, the volume of runoff treated is limited to the available volume in the lower chamber structure. Because of this, Stormceptor® and Downstream DefenderTM might treat less than a typical water quality treatment volume and should be placed at the beginning of the storm drain line for maximum treatment efficiency. The Stormceptor® and Downstream DefenderTM treatment devices do not significantly reduce either biological or nutrient pollutants that are not sorbed to particles (Weatherbe, et al., 1995; Bryant, et al., 1995; Hydro International, 1996).

The VortechsTM system is designed to counter the resuspension problem associated with conventional oil/grit separator water quality inlets. Data for a VortechsTM system obtained through in-field monitoring of an actual installation in Freeport, Maine, showed that particulate matter within the unit increased over a 20-month period (Vortechnics, 1996).


There are only a few independently verified studies of the effectiveness of manufactured systems. Field testing at over 21 installed and operating Stormceptor® units in the Toronto, Canada, area has shown that 86 percent of the trapped sediments were in the clay and silt particle size range (Weatherbe, et al., 1995). The average annual accumulation rate was determined to be about 0.70 m3/ha (0.37 yd3/ac) of land. Unlike conventional oil/grit separators, the study showed that the accumulation was increasing over time. This was important because it showed captured sediments (both fine and coarse) were not being resuspended by subsequent storms. On average, monitoring studies have reported a 96 percent removal of oil, 83 percent removal of sand, and 72 percent removal of peat. Depending on the size of the unit, treatment rates range between 7,079 and 4,201 L/min (285 and 1,110 gal/min); all flow greater than the treatment rate is bypassed.

Preliminary results for the Downstream DefenderTM show overall removal efficiencies in excess of 90 percent of particles greater than 150 microns (sand-sized particles). The device intercepts the first flush and retains floatables, oils, and grease. Head loss across the Downstream DefenderTM is typically less than 30.5 cm (12 in); thus backwater effects are generally not a problem.

Bench-scale testing performed on the VortechsTM system showed that for silt-sized sediments, the average removal efficiency was in excess of 80 percent. The removal efficiency is greater for larger-sized particles. For example, for a single 2-month storm event in Portland, Maine, the same bench-scale test showed that the VortechsTM unit exhibited a removal efficiency of approximately 89 percent for sand-sized particles (Vortechnics, 1996).

Siting and Design Considerations

Vendors of manufactured systems are often willing to provide services to build, install, and maintain manufactured systems. These services frequently include technical support to design a system for a customer in the process of making a sale. If not carefully evaluated by the customer, however, these systems may become a problem, especially with respect to maintenance considerations (see below). The Stormceptor®, Downstream DefenderTM, and VortechsTM units are structural precast BMP water quality devices that can be installed on-line in new storm drain systems. The structures come in various sizes and are best suited for land uses with drainage areas of 4 ha (10 ac) or less. The Stormceptor®, Downstream DefenderTM, and VortechsTM systems are stand-alone BMPs and do not require any pretreatment; however, they can be used to pretreat stormwater runoff to other BMPs such as ponds, sand filters, or infiltration/exfiltration trenches. On the other hand, some BMPs, such as water quality inlets (see Section 3.6), should be used only for pretreatment and never as a stand-alone BMP.

The Stormceptor® comes in eight different precast sizes and can treat 0.018 to 0.07 m3/s (0.64 to 2.5 ft3/s, respectively) of stormwater runoff prior to bypass. The individual size of the Stormceptor® would depend on the amount of stormwater runoff expected to drain to the device. The Downstream DefenderTM comes in four different precast sizes and can treat 0.021 to 0.37 m3/s (0.75 to 13 ft3/s) of stormwater runoff prior to bypass. VortechsTM systems are sized based on required design flow rate. The precast units come in nine different sizes that handle flow rates between 0.04 and 0.7 m3/s (1.6 and 25 ft3/s).

Design specifications for these manufactured systems can be obtained from their manufacturers or distributors. Current information is readily available on the web sites for each manufacturer. Web site addresses are:

  • Stormceptor®: http://www.stormceptor.com.
  • Downstream DefenderTM: http://www.hil-tech.com.
  • VortechsTM: http://www.vortechnics.com.

Maintenance Considerations

The Stormceptor® and Downstream DefenderTM systems are access hole structures that are engineered to be installed within roadways in residential, commercial, industrial, or institutional areas. The access hole includes a built-in internal device that diverts stormwater runoff to the lower treatment chamber. Normal installations take only a few hours once the excavation is complete. The general maintenance procedure for the Stormceptor® is to clean out the unit once a year, or when 15 percent of the operating storage volume is filled with solids, or when oil levels reach 25 mm (1.0 in) or greater (Stormceptor®, 1996). The sediment holding capacity of the Stormceptor® units range from 2.12 to 20.56 m3 (2.77 to 26.87 yd3). The manufacturer of the Downstream DefenderTM recommends cleaning out the units at least twice a year using a conventional vacuum truck (Hydro International, 1996).

The VortechsTM system sediment storage capacity ranges from 0.57 to 5.4 m3 (0.75 to 7 yd3), depending on the size of the unit. Routine inspections are necessary to schedule cleaning. The VortechsTM system can be cleaned by a conventional vacuum truck (Vortechnics, 1996).

If not properly maintained, manufactured systems can become exporters of oil and grease and other constituents. Generally, however, manufactured systems are designed to counter the resuspension problem associated with conventional oil/grit separators.

Cost Considerations

Stormceptor® and Downstream DefenderTM units are precast manhole structures that contain a built-in diversion device. The structures are delivered to the site partially assembled. Contractors need only set the grade and alignment to properly install the units. The Stormceptor® comes in eight standard sizes, with the cost of the units ranging from $7,600 to $33,560. Based on the maximum impervious drainage area in hectares treated for the 60 percent TSS removal rate, the cost per impervious hectare ranges from $9,900 to $26,800. On average, the cost of maintaining the system is about $300 to $500 per cleaning (pumping, dewatering, and disposing of solids). The expected life of the Stormceptor® is 50 to 100 years (Stormceptor®, 1996).

Downstream DefenderTM devices are available in four standard sizes. An average cost at capacity is $44,100 per m3/s ($1,250 per ft3/s) (Hydro International, 1996).

The VortechsTM unit comes in nine different sizes depending on the quantity of stormwater for treatment. The average cost is $52,900 to $123,500 per m3/s of capacity ($1,500 to $3,500 per ft3/s) (Vortechnics, 1997). Installation costs for all of the structures are site-dependent but generally run about 25 to 35 percent of the unit cost of the structures.


Bryant, G., F. Misa, D.G. Weatherbe, and W. Snodgrass. 1995. Field Monitoring of Stormceptor Performance, Case Study of Stormceptors in the Greater Toronto Area.

Hydro International. 1996. Downstream Defender, Portland, ME. Product Literature.

Hydro International. 1996. Personal communication from A. Knight. Hydro International, Portland, ME.

Stormceptor® Corporation. 1996. Product Literature.

Vortechnics. 1996. Vortechs Stormwater Treatment System, Portland, ME. Product Literature.

Vortechnics. 1997. Personal communication from G. Novick. Vortechnics, Inc., Portland, ME.

Weatherbe, D.G., G. Bryant, and W. Snodgrass. 1995. Performance of the Stormceptor Water Quality Inlet. Proceedings of the Water Environment Federation Specialty Conference, Toronto, Canada.

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Questions and feedback should be directed to Susan Jones (Susan.Jones@dot.gov, 202-493-2139) and Marcel Tchaou (Marcel.Tchaou@dot.gov, 202-366-4196).

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