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Downstream Defender

A high-efficiency advanced vortex separator designed for treating a large flow range while keeping headlosses low. The Downstream Defender® removes oil, fine sediment, trash and debris from stormwater in a footprint as much as half the size of conventional gravity separators. 

The Downstream Defender®  is the most advanced vortex separator available for the removal of sediment, oil and floatables from stormwater runoff. It combines specially designed polyethylene internal components in a standard concrete manhole structure. The Downstream Defender® is proven to be more efficient than other structural treatment devices in as little as 1/2 the footprint and is the only separator with internal components proven to prevent pollutant washout.


  • Removal of sediment and TSS
  • 100% capture of floatable trash
  • Removal of oils and grease from stormwater runoff (meets Class II Oil Collector requirements)
  • Stormwater treatment for new developments and redevelopment projects
  • Stormwater treatment Municipal/ DOT expansions or improvements for streets, roadways and parking lots
  • Pretreatment for infiltration, stormwater detention and stormwater filtration
  • Available in 4-ft, 6-ft, 8-ft, 10-ft and 12-ft units. 

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Download Leaflet Download Detailed Technical Brochure



  • Most efficient separator available. 
  • Smaller footprint
  • Lower capital cost than other devices
  • Low system headloss
  • Proven to prevent pollutant washout
  • Verified through nationally recognized programs
  • Easy to maintain. Direct access to pollutants stored in the sump from the surface, and no man-entry required.

The Downstream Defender® has internal components designed to advance vortex separation by minimizing turbulence and headloss, increasing efficiency, and preventing washout of stored pollutants. 

Watch the animation at right or the scale model testing below to see how it works.

Stormwater is introduced tangentially into the side of the vessel, generating a rotating flow that spirals around the outside of the dip plate.

Oils, trash and floatable debris rise to the water surface and are trapped in the oil and floatables storage volume.

As flow continues to spiral down around the dip plate cylinder, low energy vortex motion directs sediment inward along the benching skirt and into the protected sediment storage zone (brown zone).

The benching skirt and center cone redirect the rotating flow up and inward between the center shaft and dip plate cylinder away from the stored sediment. The outlet pipe discharges treated effluent from within the dip plate cylinder ensuring the longest possible residence time.


Frequently asked questions

Can it be modified from the specifications shown in the brochure?

Some dimensions of the Downstream Defender® cannot be modified as it relies on certain critical dimensions and minimum pipe sizes in order to function correctly. However, extended sump depths and other dimensional modifications may be available. Please inquire or submit a Request For Proposal to our engineering team and we will work with you to determine the best solution for your application.

Why is the inlet pipe submerged?

To minimize turbulence within the treatment chamber, thus increasing the pollutant capture and retention efficiency.

What size inlet/outlet pipe is acceptable?

Acceptable inlet/outlet pipe sizes vary by model size. Please refer to the sizing chart on the Downstream Defender flyer for acceptable pipe sizes.

Can the Downstream Defender® accept two inlet pipes?

In some cases, yes. The inlet pipes connect to the manhole on a tangent, so in some cases multiple inlet pipes cannot be accommodated.

What is maintenance procedure for cleanout?  Is confined space entry required?  How often is maintenance needed?

No man entry is required. Maintenance is safe and easy with a standard vactor truck. Maintenance consists of removing sediments from the sump and floatable oils, grease, trash and other debris from the floatables capture zone. Watch the maintenance process:

Why is there no internal bypass?

The standard Downstream Defender® models are designed to be “online” units that treat the entire flow,  therefore  there is no internal bypass.  However, the Downstream Defender® can be designed “offline” with an upstream bypass diversion structure. 

Where is the Downstream Defender® approved for use?

The Downstream Defender® is generally accepted for stormwater treatment or pretreatment in most areas of the United States. Please inquire for specific sizing criteria applicable to your area.

What materials are used to manufacture the Downstream Defender®?  Are the materials made in USA?

The Downstream Defender is 100% made in the US. The internal components are rotomolded polyethylene supported by a stainless steel frame. The chamber is a standard concrete manhole.

What sizes are available?  How do I know what size is appropriate for my application?

The Downstream Defender® is available in 4, 6, 8, 10 and 12-ft diameters. The unit size needed for your specific application depends upon many factors including the level of treatment desired, the target pollutant and peak drainage system flow. Please inquire for more site-specific technical assistance.

What does the Downstream Defender® cost? 

The price varies slightly with location based on local precast concrete pricing, as well as design criteria such as overall height of the concrete manhole. Please request a proposal on the Inquiry page.

Are Downstream Defender® units available everywhere in the US? Where can I buy one? 

The Downstream Defender® is available worldwide.  In the United States, Canada and Mexico, contact Hydro International in Portland, ME. For availability in the United Kingdom, please visit our UK website. For all other international locations, please visit our international website.

Where do I get general arrangement drawings and specifications for the Downstream Defender®?

General Arrangement drawings and Specifications are available from the Downloads tab.

Where is the pump? Doesn’t the Downstream Defender® need a pump because of the submerged inlet?

No pump is needed. Based on simple hydraulics, the water level in the Downstream Defender® will fill to the invert of the outlet during the first storm without any assistance required from a pump.

What is removal efficiency of the different size units?

Removal efficiency is dependent upon the particle size and specific gravity of the pollutant, in addition to the flow rate of the stormwater runoff. Please submit a request to our technical team for assistance selecting the appropriate model size.

Does installation of the Downstream Defender® require any special equipment or experience?

The installation process of a Downstream Defender® is similar to installing a standard manhole. The inlet pipe is grouted in to the manhole chamber while the outlet pipe is connected with a Fernco coupling or a pipe boot.  Submit a request for an installation manual from our technical team for additional details.


Does the Downstream Defender® remove oil and hydrocarbons? Is it an oil collector?  

The Downstream Defender® has been independently tested and has been shown to meet the requirements required for a Class II oil collector.

The Downstream Defender® doesn’t have a screen. Is it an effective trash capture device?

The Downstream Defender® uses the principles of flotation and sedimentation to capture all floatable and neutrally buoyant trash.

Download Case Study

In ambitious Low Impact Development retrofit project, the Downstream Defender® provides the final level of treatment in an LID treatment train before runoff is discharged into a sensitive aquatic preserve.

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Two Downstream Defender® devices were installed upstream of retention ponds to provide effective, reliable pretreatment.

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A Downstream Defender® and Up-Flo® Filter were used to treat harvested stormwater in a revitalized Monterrey neighborhood once plagued by frequent flooding. 

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Downstream Defender® devices pay for themselves by reducing expensive maintenance costs of large stormwater ponds.

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The Downstream Defender® helps greater Boston's Greenbush Rail Line manage stormwater runoff wtih a heavy sediment load.

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Downstream Defender® reduces expense of manufactured stormwater treatment devices.

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The Downstream Defender® provides state-of-the-art stormwater treatment technology for a state-of-the-art sports complex.

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York Hospital situated in historic York, Maine known for its natural beauty and scenic beaches, chose the Downstream Defender® to protect the fragile ecosystem of a nearby pond.

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Heavy duty porous asphalt and Downstream Defender® are project’s key stormwater controls.

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The Downstream Defender® meets stormwater treatment requirements while reducing cost and constrution time for massive project.