Is Your Stormwater System a Washout?
A guide to ensuring your pollutant removal system doesn't unintentionally release contaminants
Throughout the world, thousands
of stormwater pollutant
removal systems are
being installed in an effort to
prevent watercourses from being polluted.
As supply has risen to meet
demand, a variety of proprietary,
chamber-based systems have
emerged, including hydrodynamic
separators, which are designed to settle
out and store sediments and associated
pollutants, preventing them
from being discharged to the natural
environment.
However, recent research suggests
that some systems may not be as
effective as claimed. Some systems
are subject to “washout” or “scour,”
whereby captured and stored pollutants
are flushed from a system during
extreme wet weather.
Conventional Sizing Strategies
A common measure of stormwater
separator performance is “pollutant
removal efficiency” – the ability to
remove pollutants from a contaminated
flow. Net performance will depend on
actual flow conditions faced, and the
nature and characteristics of the pollutant
load.
For a typical hydrodynamic treatment
device, instantaneous efficiencies will
tend to be highest at low flows and lowest
at high flows. This means that over
the duration of a storm, the net efficiency
will fall somewhere in between.
This conceptual approach is useful,
allowing designs to be assessed in line
with defined efficiency targets. However, it assumes that pollutants, once captured,
will remain captured, even during
the most extreme storm conditions.
Retention Efficiency and Washout
In practice, poorly configured
stormwater separators can be prone to
“washout,” which occurs when captured
and stored pollutants get resuspended
and flushed out at high flows.
This can be included in the performance
calculation through “retention efficiency”
– the ability to retain previously captured
and stored pollutants.
Most real stormwater treatment systems
are protected with high-flow
bypasses. This prevents hydraulic overloading
and also helps to prevent
washout. However, it must be recognized
that any bypassed flow will not be
treated. A preferred approach is to
select stormwater separators that have
good retention performance (i.e., that do not washout) and then to maximize the
treatment threshold.
Chamber Assessment
The “retention” efficiency of a
stormwater chamber is, in practice, difficult
to measure and express quantitatively,
which may explain why it has not
often been considered in practical
assessments. However, using the outputs
of visual experimentation and flow simulation,
combined with a little intuition,
it is actually relatively straightforward to
predict which devices might washout
and which might not.
Extensive research has been carried
out at the Centre for Environmental
Technology at Liverpool John Moores
University in the UK (LCET), examining
the interrelation between chamber
design and pollutants washout. This
work finds that the least-efficient chambers
have exposed storage zones. The
better performers have storage zones
that are sheltered from the main flows. LCET has looked at a range of “generic”
designs of chamber, configured to
mimic the primary features of proprietary
systems. These have included symmetrical
flow chambers and vortex flow
chambers, both with and without internal
components.
Symmetrical Flow Chamber
Symmetrical flow chambers typically
have perpendicular inlet and outlet
points. They may contain internal baffles,
though many do not. Catch basins
and some proprietary systems fall into
this category.
LCET looked at the retention of unexpanded
polystyrene beads (equivalent in
settling velocity to 75-micron sediment
particles) during a 5-minute period of
moderate-high flow to the chamber.
Video clips were obtained, and washout
quantities measured.
This system was found to exhibit high
levels of washout, in fact approaching
100 percent -- i.e., none of the material
stored in the base of the chamber
remained at the end of the test.
Vortex Flow Chamber
Three different configurations of vortex
chamber were looked at by LCET, including
one with no internal components,
one with a ring positioned part way up
from the base, and a further one with a
ring and base shield/outlet assembly.
The vortex flow chambers generally
exhibited less washout than the symmetrical
chamber, though this was
found to depend on the actual configuration.
With no internals, around half of
the stored sediment was lost. Adding the
ring component, while sheltering parts
of the base, the vortex tended to penetrate
and accelerate into this region, and
again, around half of the stored sediment
was lost. The chamber with the
ring and base shield/outlet assembly,
however, retained 99.7 percent of the
stored material, along with dye added to
the water at the start of the test.
Combined results from the work are
presented on page 20, confirming the
inter-relation between chamber configuration
and performance.
Computer Simulations
The work carried out at LCET has
been backed up by fluid flow simulations,
carried out in collaboration with
Fluent, a world leading provider of computational
fluid dynamics software. The
predictions confirm what is observed in
practice. In the symmetrical flow chamber,
the inlet flow creates significant
mixing, pulling particles up from the
base, directly into the path of the inlet
flow. This corresponds to substantial
washout in practice.
The vortex chamber was found to be
more effective, having a more structured
flow, and fewer particles passing to the
outlet, though particles are nevertheless
accelerated and stirred up into the main
treatment area.
Adding a ring above the sediment
storage region has some impact on the
flow structure, but the vortex is still able
to penetrate into the base, pulling material
up.
The vortex chamber with ring and
base shield/outlet assembly shows very
little susceptibility to washout. While
particles stored in the sump are lifted
and rotated, their velocities are low, and
they are contained in this area. This cor-
responds with the experimental outputs,
demonstrating this type of configuration,
with a sheltered sump, as being
best at retaining stored material.
The Liverpool study revealed the
following:
• The phenomenon of poor retention
leading to the washout of previously
captured solids from stormwater
treatment chambers must be taken
into account in system selection for
practical application.
• In the worst cases, washout begins
almost immediately at flows likely to
be encountered even in the early
stages of a storm event.
• The rate of stored pollutants washout
is extremely sensitive to chamber
design. In the current studies, reentrainment
rates ranged from zero to
100 percent.
• The best-performing chambers were
found to be those that had induced
rotary flows and internal arrangements
and flow modifying components
that resulted in the sediment
storage region being hydraulically
separated from the main treatment
region.
Does Your Stormwater System Have
Washout Woes?
Washout is an important factor to
consider in the selection of stormwater
separators, yet it is clearly not taken into
account in the design or verification of
many proprietary systems. Those
responsible for selecting the best solutions
to meet particular site needs are
faced with an array of technological
options and an array of performance
claims. However, the process of identifying
the “performers” from the “failers” is
not as complicated is it might appear. The key is in asking the right questions,
and also applying a little intuition.
• Are stored sediments located in a
region that is suitably sheltered from
the main treatment region? Simple
test – if you can draw a vertical line /
vertical lines from the treatment
region to the storage region without
hitting a physical boundary then the
answer is probably “NO” – the system
may be liable to washout.
• Does the system have a “high-flows”
bypass? If the answer is “YES,”
washout may be avoided. However,
beware that any bypassed flow will
not be treated. Also, beware that systems
that bypass directly through the
main treatment region may still be
prone to washout – again, the vertical
line test can be applied.
Washout is a particularly important
phenomenon, as
stormwater treatment
devices are
typically maintained
on an infrequent
basis. In
addition to removing
pollutants,
these systems must
also retain and
store them for later
removal during
maintenance visits.
A device that is
effective at removing
pollutants
under low-flow conditions
but is prone
to resuspending
and washing out
previously captured
materials when
flows increase does
not serve the functions
it claims. It
provides very little
overall environmental
benefit. It
also provides a false
sense of security to
those relying on it
to keep their water clean.
This article originally appeared in the 09/01/2007 issue of Environmental Protection.
About the Author
Richard E. Ayres, JD is also a partner in the law firm.