Source: Fenner and Tyack, 1997.

FIGURE 1 GENERALIZED HYDRODYNAMIC

SEPARATOR

United States

Environmental Protection

Agency

Office of Water

Washington, D.C.

EPA 832-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.

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:

C Continuous Deflective Separation (CDS).
C Downstream DefenderTM.

• Stormceptor®.

C VortechsTM.

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 copolymer 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:

C 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).

C Projects that result in doubling the

impervious area.

C 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:

C A treatment chamber and by-pass insert.

C 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 petroleumbased 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 VortechsTM storm water treatment system,

manufactured by VortechnicsTM 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
TABLE 1 CONCENTRATION OF

POLLUTANTS IN STORM WATER

Pollutant Concentration

TSS 100 mg/L

Total P 0.33 mg/L

TKN 1.50 mg/L

Total Cu 34 μg/L

Total Pb 144 μg/L

Total Zn 160 μg L

Source: U.S. EPA, 1995.

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.

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 Strategi c

Envirotechnology Partnership (STEP)

Technology Assessment, Stormceptor,

January 1998. Internet site at
For more information contact:

Municipal Technology Branch

U.S. EPA

Mail Code 4204

401 M St., S.W.

[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-R94-001.

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., Djula Fabian and Richard

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

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.