The Downstream Defender® is an advanced hydrodynamic vortex separator that provides impressive and reliable removals of fine and coarse particles, hydrocarbons and floatable debris from stormwater runoff, delivering high levels of treatment over a wide range of flow rates. 

The Downstream Defender® is one of the Hydro StormTrain® Series of stormwater treatment systems which can be used independently or as part of a surface water treatment train.

Available in a range of sizes, it can function as either pretreatment or as a standalone device.  The Downstream Defender® has large storage volumes for pollutants captured from stormwater runoff whilst also offering protection to watercourses from accidental spills of oil/light liquids.

The Downstream Defender® has been independently tested, verified and approved.

Targeted Pollutants:

  • Fine particles
  • Floatable debris
  • Liquid and sediment bound hydrocarbons
  • Sediment bound heavy metals
  • Sediment bound nutrients


The Downstream Defender® reliably treats runoff from any impermeable surfaces including highways, car parks and industrial areas:

  • As part of Sustainable Drainage Systems (SuDS), Water Sensitive Urban Design (WSUD), LID or LEED systems.
  • Areas where high solids and trash/litter capture are a must.
  • Control of silting upstream of wetlands, ponds and basins


Designed for

Fine particles
Oils & hydrocarbons
Trash & floatables


Save space and money

The Downstream Defender® delivers high removal of pollutants across a wide range of flows in a much smaller footprint than conventional or other swirl-type devices. A smaller footprint means an easier install that's less expensive and less work.

No risk of pollutant wash out

The Downstream Defender® is proven to prevent wash out of captured pollutants. Carefully designed internal components isolate the pollution storage areas ensuring what is captured is retained, even during high flows.



The Downstream Defender® can accommodate a change in outlet pipe direction to suit site specific requirements, helping meet Building Regulations without the need to construct a second manhole.

Cut headloss

Low headloss means more site flexibility and provides the option for a shallower site. 


How it works

How it works

  1. Stormwater is introduced into the side of the vessel, generating a rotating flow that spirals around the outside of the dip plate.
  2. Oils, litter and floatable debris rise to the water surface and are trapped in the oil and floatables storage volume.
  3. 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.
  4. The benching skirt and centre cone redirect the rotating flow up and inward between the centre shaft and dip plate cylinder away from the stored sediment.
  5. The outlet pipe discharges treated effluent from within the dip plate cylinder ensuring the longest possible residence time. 
  6. Excess flows by-pass the vortex treatment chamber and also both the sediment and oil storage zones.  These excess flows pass forward untreated via the orientated outlet pipe. 


Technical Information

Technical information

Unit sizes (diameter)

  • 1.2 m
  • 1.8 m
  • 2.55 m
  • 3.0 m

Performance characteristics        

Treatment flow rate - Fine: 30-190 l/s

Treatment flow rate - Coarse: 38-237 l/s

Hydraulic capacity: 120-750 l/s

Oil storage capacity: 270-4,550 l

Sediment storage capacity: 0.38-3.81 m3

Maximum Headloss at Treatment Flow Rate - Coarse:  150 - 375 mm

Downstream Defender Flow Rate Table





Case Studies

Case Studies