v>EPA
United States
Environmental Protection
Agency
Office of Water
Washington, D.C.
EPA832-F-99-017
September 1999
Storm Water
Technology Fact Sheet
Hydrodynamic Separators
DESCRIPTION
Hydrodynamic separators are flow-through
structures with a settling or separation unit to
remove sediments and other pollutants that are
widely used in storm water treatment. No outside
power source is required, because the energy of the
flowing water allows the sediments to efficiently
separate. Depending on the type of unit, this
separation may be by means of swirl action or
indirect filtration. A generalized schematic of a unit
is shown in Figure 1. Variations of this unit have
been designed to meet specific needs.
CENTER CONE
Source: Fenner and Tyack, 1997.
FIGURE 1 GENERALIZED HYDRODYNAMIC
SEPARATOR
Hydrodynamic separators are most effective where
the materials to be removed from runoff are heavy
particulates - which can be settled - or floatables -
which can be captured, rather than solids with poor
settleability or dissolved pollutants.
In addition to the standard units, some vendors offer
supplemental features to reduce the velocity of the
flow entering the system. This increases the
efficiency of the unit by allowing more sediments to
settle out.
APPLICABILITY
This technology may be used by itself or in
conjunction with other storm water BMPs as part of
an overall storm water control strategy.
Hydrodynamic separators come in a wide size range
and some are small enough to fit in conventional
manholes. This makes hydrodynamic separators
ideal for areas where land availability is limited.
Also, because they can be placed in almost any
specific location in a system, hydrodynamic
separators are ideal for use in potential storm water
"hotspots"~areas such as near gas stations, where
higher concentrations of pollutants are more likely
to occur.
The need for hydrodynamic separators is growing as
a result of decreasing land availability for the
installation of storm water BMPs. This fact sheet
discusses hydrodynamic separator systems from four
vendors. Although there are many hydrodynamic
separation systems available, these four address the
major types.
They are the following:
• Continuous Deflective Separation (CDS).
-------
• Downstream Defender™.
• Storm cep tor®.
• Vortechs™.
Continuous Deflective Separation (CDS)
CDS' hydrodynamic separator technology is suitable
for gross pollutant removal. The system utilizes the
natural motion of water to separate and trap
sediments by indirect filtration. As the storm water
flows through the system, a very fine screen deflects
the pollutants, which are captured in a litter sump in
the center of the system. Floatables are retained
separately. This non-blocking separation technique
is the only technology covered in this fact sheet that
does not rely on secondary flow currents induced by
vortex action.
The processing capacities of CDS units vary from 3
to 300 cubic feet per second (cfs), depending on the
application. Precast modules are available for flows
up to 62 cfs, while higher flow processing requires
cast-in-place construction. Every unit requires a
detailed hydraulic analysis before it is installed to
ensure that it achieves optimum solids separation.
The cost per unit (including installation) ranges from
$2,300 to $7,200 per cfs capacity, depending on
site-specific conditions and does not include any
required maintenance.
Maintenance of the CDS technology is site-specific
but the manufacturer recommends that the unit be
checked after every runoff event for the first 30 days
after installation. During this initial installation
period the unit should be visually inspected and the
amount of deposition should be measured, to give
the operator an idea of the expected rate of
sediment deposition. Deposition can be measured
with a calibrated "dip stick". After this initial
operation period, CDS Technologies recommends
that the unit should be inspected at least once every
thirty days during the wet season. During these
inspections, the floatables should be removed and
the sump cleaned out (if it is more than 85 percent
full). It is also recommended that the unit be
pumped out and the screen inspected for damage at
least once per year.
A recently completed study by UCLA for CDS
Technologies evaluating the effectiveness of four
different sorbent materials in removing used motor
oil at concentrations typically found in storm water
runoff. They applied the sorbents in a CDS unit
separation chamber and reported captures of 80-90
percent. The test found that polypropylene or co-
polymer sorbents to be the most effective in the
capture of the used motor oil.
Downstream Defender
The Downstream Defender, manufactured by H.I.L.
Technology, Inc., regulates both the quality and
quantity of storm water runoff. The Downstream
Defender is designed to capture settleable solids,
floatables, and oil and grease. It utilizes a sloping
base, a dip plate and internal components to aid in
pollutant removal. As water flows through the unit,
hydrodynamic forces cause solids to begin settling
out. A unique feature of this unit is its sloping base
(see Figure 1), which is joined to a benching skirt at
a 30-degree angle. This feature helps solids to settle
out of the water column. The unit's dip plate
encourages solids separation and aids in the capture
of floatables and oil and grease. All settled solids
are stored in a collection facility, while flow is
discharged through an outlet pipe. H.I.L.
Technology reports that this resulting discharge is
90 percent free of the particles greater than 150
microns that originally entered the system.
The Downstream Defender comes in predesigned
standard manhole size, typically ranging from 4 to
10 feet in diameter. These units have achieved 90
percent removal for flows from 0.75 cfs to 13 cfs.
To meet specific performance criteria, or for larger
flow applications, units may be custom designed up
to 40 feet in diameter. (These are not able to fit in
conventional manholes.) The approximate capital
and installation costs, range from $10,000 to
$35,000 per pre-cast unit.
Inspecting the Downstream Defender periodically
(once a month) over the first year of operation will
aid in determining the rate of sediment and
floatables accumulation. A probe (or dipstick) may
be used to help determine the sediment depth in the
collection facility. (With this inspection information
a maintenance schedule may be established.) A
-------
sump vac (commercial or municipally-owned) may
be used to remove captured floatables and solids.
With proper upkeep, H.I.L. Technology reports the
Downstream Defender will treat storm water for
more than 30 years.
Stormceptor
Stormceptor Corporation is based in Canada and
has licensed manufacturers throughout Canada and
the United States. Stormceptor is designed to trap
and retain a variety of non-point source pollutants,
using a by-pass chamber and treatment chamber.
Stormceptor reports that it is capable of removing
50 to 80 percent of the total sediment load when
used properly.
Stormceptor units are available in prefabricated
sizes up to 12 feet in diameter by 6 to 8 feet deep.
Customized units are also available for limited
spaces. Stormceptor recommends its units for the
following areas:
• Redevelopment projects of more than 2,500
square feet where there was no previous
storm water management (even if the
existing impervious area is merely being
replaced).
• Projects that result in doubling the
impervious area.
• Projects that disturb at least half of the
existing site.
The cost of the Stormceptor unit is based on the
costs of two important system elements:
• A treatment chamber and by-pass insert.
• Access way and fittings.
Typically, the cost for installation of a unit for a one
acre drainage area is $9,000. This cost will vary
depending on site-specific conditions. Stormceptor
units range from 900 to 7,200 gallons and cost
between $7,600 and $33,560. Cleaning costs
depend on several factors, including the size of the
installed unit and travel costs for the cleaning crew.
Cleaning usually takes place once per year and costs
approximately $1,000 per structure.
Vacuum trucks are used to clean out the
Stormceptor unit. Although annual maintenance is
recommended, maintenance frequency will be based
on site-specific conditions. The need for
maintenance is indicated by sediment depth;
typically, when the unit is filled to within one foot of
capacity, it should be cleaned. Visual inspections
may also be performed and are especially
recommended for units that may capture petroleum-
based pollutants. The visual inspection is
accomplished by removing the manhole cover and
using a dipstick to determine the petroleum or oil
accumulation in the unit.
If the Stormceptor unit is not maintained properly,
approximately 15 percent of its total sediment
capacity will be reduced each year.
Vortechs
The Vortechs™ storm water treatment system,
manufactured by Vortechnics™ of Portland, Maine,
has been available since 1988. Like the other
hydrodynamic separators, Vortechs removes
floating pollutants and settleable solids from surface
runoff. This system combines swirl-concentrator
and flow-control technologies to separate solids
from the flow. Constructed of precast concrete,
Vortechs uses four structures to optimize storm
water treatment through its system. These are:
• Baffle wall: Situated permanently below the
water line, this structure helps to contain
floating pollutants during high flows and
during clean outs.
• Circular grit chamber. This structure aids
in directing the influent into a vortex path.
The vortex action encourages sediment to
be caught in the swirling flow path and to
settle out later, when the storm event is
complete.
• Flow control chamber: This device helps
keep pollutants trapped by reducing the
forces that encourage resuspension and
washout. This chamber also helps to
-------
eliminate turbulence within the
system.
• Oil chamber: This structure helps to
contain floatables.
Vortechnics manufactures nine standard-sized units.
These range from 9 feet by 3 feet to 18 feet by 12
feet. The unit sizes depend on the estimated runoff
volume to be treated. For specific applications,
dimensions of the runoff area are used to customize
the unit. Vortechnics reports that Vortechs systems
are able to treat runoff flows ranging from 1.6 cfs to
25 cfs. The cost for these units ranges from
$10,000 to $40,000, not including shipment or
installation.
As with other hydrodynamic separator systems,
maintenance of the Vortechs system is site-specific.
Frequent inspections (once a month) are
recommended during the first year and whenever
there may be heavy contaminant loadings: after
winter sandings, soil disturbances, fuel spills, or
sometimes, intense rain or wind.
The Vortechs unit requires cleaning only when the
system has nearly reached capacity. This occurs
when the sediment reaches within one foot of the
inlet pipe. The depth may be gauged by measuring
the sediment in the grit chamber with a rod or
dipstick. To clean out the system, the manhole
cover above the grit chamber is lifted and the
sediment is removed using as vacuum truck.
Following sediment removal, the manhole cover is
replaced securely to ensure that runoff does not leak
into the unit.
Hydrodynamic separators are most effective where
the separation of heavy particulate or floatable from
wet weather runoff is required. (The typical
concentrations of heavy particulate and floatable
pollutants found in storm water are shown in Table
1.) They are designed to remove settleable solids
and capture floatables; however, suspended solids
are not effectively removed. Most units are small
(depending on the flow entering needing to be
treated) and may be able to fit into pre-existing
manholes. For this reason, this technology is
particularly well suited to locations where there is
limited land available.
TABLE 1 CONCENTRATION OF
POLLUTANTS IN STORM WATER
Pollutant
TSS
Total P
TKN
Total Cu
Total Pb
Total Zn
Concentration
100mg/L
0.33 mg/L
1.50 mg/L
34 |jg/L
144 |jg/L
160 |jg L
Source: U.S. EPA, 1995.
The units designed for hydrodynamic separators are
generally prefabricated in set sizes up to twelve feet
in diameter, but they may be customized for a
specific site if needed. Some structures are available
in concrete or fiberglass. (Fiberglass is
recommended for areas of potential hazardous
material spills.) These materials are both suitable
for retrofit applications.
Hydrodynamic separators are also good for
potential storm water "hotspots" or sites that fall
under industrial NPDES storm water requirements.
"Hotspots" are areas such as gas stations, where a
higher concentration of pollutants is more likely to
be found.
ADVANTAGES AND DISADVANTAGES
The use of hydrodynamic separators as wet weather
treatment options may be limited by the variability
of net solids removal. While some data suggest
excellent removal rates, these rates often depend on
site-specific conditions, as well as other contributing
factors. Pollutants such as nutrients, which adhere
to fine particulates or are dissolved, will not be
significantly removed by the unit.
Site constraints, including the availability of suitable
land, appropriate soil depth, and stable soil to
support the unit structurally, may also limit the
applicability of the hydrodynamic separator. The
slope of the site or collection system may
-------
necessitate the use of an underground unit, which
can result in an extensive excavation.
Observable improvements in waterways are often
attributable to the use of hydrodynamic separators.
This is due to the reduction of sediments,
floatables, and oil and grease in the flow out of the
unit. These positive impacts are only achievable
when proper design and O&M of the unit are
implemented.
PERFORMANCE
Hydrodynamic separators are designed primarily for
removing floatable and gritty materials; they may
have difficulty removing the less-settleable solids
generally found in storm water. The reported
removal rates of sediments, floatables, and oil and
grease differ depending on the vendor. Proper
design and maintenance also affect the unit's
performance.
OPERATION AND MAINTENANCE
Hydrodynamic separators do not have any moving
parts, and are consequently not maintenance
intensive. However, maintaining the system
properly is very important in ensuring that it is
operating as efficiently as possible. Proper
maintenance involves frequent inspections
throughout the first year of installation. The unit is
full when the sediment level comes within one foot
of the unit's top. This is recognized through
experience or the use of a "dip stick" or rod for
measuring the sediment depth. When the unit has
reached capacity, it must be cleaned out. This may
be performed with a sump vac or vacuum truck,
depending on which unit is used. In general,
hydrodynamic separators require a minimal amount
of maintenance, but lack of attention will lower their
overall efficiency.
COSTS
The capital costs for hydrodynamic separators
depend on site-specific conditions. These costs are
based on several factors including the amount of
runoff (in cfs) required to be treated, the amount of
land available, and any other treatment technologies
that are presently being used. Capitol costs can
range from $2,300 to $40,000 per pre-cast unit.
Units which are site-specifically designed, typically
cost more and the price is based on the individual
site.
Total costs for hydrodynamic separators often
include predesign costs, capital costs, and operation
and maintenance (O&M) costs. Again, these costs
are site-specific. The predesign costs depend upon
the complexity of the intended site. O&M costs
vary based on the company contracted to clean out
the unit, and may depend on travel distances and
cleaning frequency. These costs generally are low
(maximum of $1,000 a year) and vary from year to
year.
The individual unit prices are discussed in the
current status section previously mentioned. This
covers a more in depth price range of the various
systems.
REFERENCES
1. City of Alexandria, Virginia, 1998. Warren
Bell, City of Alexandria Department of
Transportation and Environmental Services,
personal communication with Parsons
Engineering Science, Inc.
2. Allison, R.A., T.H.F. Wong, and T.A.
McMahon, 1996. "Field Trials of the
Pollutec Stormwater Pollution Trap."
Water, Vol. 23, No. 5, pp. 29-33.
3. CDS Technologies, Inc., 1998. Literature
provided by manufacturer.
4. Downstream Defender, 1998. Literature
provided by manufacturer.
5. England, Gordon, 1998. "Baffle Boxes and
Inlet Devices for Storm Water BMPs."
Internet site at [http://www.
stormwater-resources.com/], accessed July
1998.
6. The Massachusetts Strategic
Envirotechnology Partnership (STEP)
Technology Assessment, Stormceptor,
January 1998. Internet site at
-------
[http://www.state.ma.us/step/strmc
ptr.htm], accessed July 1998.
7. Stenstrom, M. K. and Sim-Lin Lau. July,
1998. Oil and Grease Removal by Floating
Sorbent in a CDS Device. Los Angeles,
CA. Prepared for CDS Technologies.
8. Stormceptor, 1998. Literature provided by
manufacturer.
9. Tyack, J.N., and R.A. Fenner, 1997. "The
Use of Scaling Laws in Characterising
Residence Time in Hydrodynamic
Separators." Presented at the 1997 IAWQ
Conference, Aalborg, Denmark.
10. U.S. EPA, July 5, 1995. EPA Clean Water
Act Section 403 Report to Congress,
NPDES Permitting Program. EPA 842-R-
94-001.
H.I.L. Technology, Inc.
Pam Deahl
94 Hutchins Drive
Portland, ME 04102
Stormceptor
Vincent H. Berg, PE
600 Jefferson Plaza, Suite 304
Rockville, MD 20852
Vortechnics
Greg Norvick
41 Evergreen Drive
Portland, ME 04103
The mention of trade names or commercial products
does not constitute endorsement or recommendation
for the use by the U.S. Environmental Protection
Agency.
11. Virginia Department of Environmental
Quality, 1998. Joe Battiata, Virginia
Department of Environmental Quality,
personal communication with Parsons
Engineering Science, Inc.
12. Vortechs. July, 1998. Literature provided
by manufacturer.
13. Wong, Tony H.F., DjulaFabian andRichard
M. Wootton, 1996. "Hydraulic
Performance and Sediment Trapping
Efficiencies of a Dual Outlet CDS Device."
Provided by CDS Technologies, Inc.,
submitted for publication in the ASCE
Journal of Hydraulic Engineering.
ADDITIONAL INFORMATION
CDS Technologies Inc.
Ernest Mathia
1005 Wetherby Way
Alpharetta, GA 30022
Center for Watershed Protection
Tom Schueler
8391 Main Street
Ellicott City, MD 21043
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
401 M St., S.W.
MTB
I
Exceience fh compliance through optftnal technical safaris:
MUNICIPAL TECHNOLOGY
------- |