Dealing with 'Washout'

April 2, 2008
Studies find advanced vortex separators provide best protection against re-suspension and discharge of pollutants captured in wet-weather events

About the author: Robert Y.G. Andoh is director of innovation for Hydro Intl. Andoh can be reached at 207.756.6200.


For years, municipalities around the globe have invested in storm water treatment systems to remove runoff pollutants. Whether used for stand-alone treatment or as pretreatment devices to keep infiltration bays from clogging, these “sedimentation devices” have an important role in storm water treatment. A troubling probem, however, has emerged and become more apparent: Many proprietary systems do not hold what they catch.

When flows increase, previous captured pollutants can re-suspend and discharge out of the system. Moreover, many communities are not aware of this “washout” phenomenon, leaving them with a false sense of security about their ability to keep waterways clean.

While most treatment systems incorporate high-flow bypasses, studies have shown that these mechanisms do not eliminate washout. In a treatment device with no or poorly designed internal components, sediment can be removed efficiently at low flow rates. However, flows from moderate to heavy rains are high enough to wash previously captured pollutants right out of the device, whether operating in bypass mode or not. Washout means the catch basin itself becomes a pollution source.

Vetting Treatment Systems

Vendors frequently state the performance of their systems only in terms of pollutant-removal efficiency. However, to gain the full environmental benefit of storm water treatment, organizations should consider two key parameters: pollutant-removal efficiency (the ability to remove pollutants from influent) and pollutant-retention efficiency (the ability to retain pollutants once collected).

Most treatment system studies have focused exclusively on the efficacy of initial pollutant removal. While this is a good indication of the device’s abilities in low-flow conditions, it does not provide a complete picture, particularly as to how the system will work during moderate to heavy rain events. Also, differences between design methods, unit sizes and capacities, test protocols and site conditions have made side-by-side comparisons impossible.

Comparing Treatment Systems

The most popular chamber systems can be categorized in three general groupings. Gravity sedimentation devices rely on simple gravitational settlement to function. Simple vortex separators rely on enhanced gravitational settlement to function, through the use of a rotating-flow field. Finally, advanced vortex separators operate in a similar manner to simple vortex separators but use specially designed internal components to control and enhance performance and provide isolated storage zones for captured pollutants.

Avoiding Washout

Until recently, simple catch basins were used as entry points to storm drainage systems. However, a recent study by the Centre for Environmental Technology at Liverpool John Moore University (LCET) revealed that these catch basins represent a source of pollutants because of ineffective pollution retention. LCET researchers found that the least efficient chambers have exposed storage zones and the better performers have storage zones sheltered from main flows.

The researchers went on to examine the interrelationship between a treatment system’s chamber design and pollutant washout. To this end, they compared four chamber configurations: gravity sedimentation device with opposed intake and exit, simple vortex separator, simple vortex separator with skirt and advanced vortex separator.


For the study, a sediment stimulant was allowed to settle in the chamber base, corresponding to the region where it would be stored in practice. Researchers added dye to color the initial water contained. Flow was passed through the chamber of each device, and particles that were re-suspended and washed out were collected at the outlet.

Through studying video of the preceding activities, researchers confirmed that washout varied greatly depending on the type of chamber design, with re-suspension rates ranging from close to zero to 100 percent depending on device configuration, nature and type of flow modifying components and whether sediment was stored in isolated storage zones.


For the gravity sedimentation device, the perpendicular orientation of the inlet resulted in the formation of a vertical plane-oriented circulating flow in the chamber, sweeping horizontally from the inlet and then down the opposite wall into the sump. This created turbulence in the region, such that the solid material was swept upward from the base and to the overflow. At the end of the test, no solid material was left in the chamber, and all the dye had been washed out.

The simple vortex separator configuration also showed high levels of sump turbulence. The vortex flow in the chamber drew solid material toward the center and upward into the main flow, from which point it exited the chamber. At the end of the test, the dye had been washed out, but a portion of the solid material remained. The liquid exchange rate exceeded the re-entrainment rate, but the solid material retained was largely in suspension and vulnerable to further washout.

For the simple vortex separator with skirt configuration, the circular opening in the center of the skirt allowed the core of the vortex to penetrate into the pump, drawing solid material upward into the main flow. The dye washout rate exceeded the re-entrainment rate, but the overall retention rate was similar. The simple vortex separators, with and without skirt configurations, retained 50 to 51 percent of stored solids, respectively, indicating that the skirt component on its own did not offer any benefit.

The advanced vortex separator configuration’s internal components prevented excessive flow penetration into the sump region, minimizing disturbance of stored solid materials. Unlike with the two simple vortex separator configurations, retained solid material was not in suspension and thus extremely unlikely to be washed out. A substantial proportion of dye remained in sheltered regions of the chamber during testing. Retention efficacy was close to 100 percent.

Ultimately, those developing and testing storm water treatment systems should reach out the community, educating municipalities about the importance of not only initial pollutant capture rates but also continuous performance and solids retention under a range of conditions. This will ensure that those who invest in the technologies are well-protected, even during the rainiest seasons.