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Underground Stormwater Facilities: Hydrodynamic Separators

Hydrodynamic separators remove oil, grease, trash, and sediment from stormwater runoff. These underground structures include oil and grit separators and proprietary hydrodynamic separators, such as BaysaverAqua-Swirl,® and Stormceptor.® Please visit the manufacturers’ websites for more information about these devices.

Underground hydrodynamic separators are commonly located under parking lots at commercial sites or multi-family residential sites (condominium, apartments, etc.).


How Do They Work?

During a storm, rainwater collects pollutants as it flows across impervious surfaces, such as rooftops, sidewalks, and roads. Flow splitters are often used to send a certain quantity of untreated water, known as the “first flush,” to a hydrodynamic separator. The oil and grit separator captures and treats stormwater by separating oil, grease, trash, and sediment from the captured stormwater through three chambers. The clean water is then returned to the local stream or to the storm drain system.

Proprietary systems such as Baysaver,® Aqua-Swirl,® and Stormceptor,® follow similar processes to remove oil, grease, trash, and sediment from stormwater. The designs of these systems vary.


Why are Hydrodynamic Separators Important?

Hydrodynamic separators:

  • Remove pollutants

  • Improve the health of streams and rivers

  • Help to make our waters fishable and swimmable

  • Improve the quality of the Chesapeake Bay

Image of a hydrodynamic separator manhole cover.
Hydrodynamic separator manhole cover.



Maintaining Hydrodynamic Separator

As the property owner, you are responsible for all maintenance unless your hydrodynamic separator has been transferred to the Stormwater Facilities Maintenance Program. Learn more about how you can transfer your hydrodynamic separator into this program.


Image of oil and grit separator.
Cleaning an Oil and Grit Separator

Why Maintain My Hydrodynamic Separator?

An unmaintained hydrodynamic separator may:

  • Not remove pollutants as intended, sending polluted water to streams and rivers

  • Become filled with sediment and debris so water cannot be treated or stored, which may lead to downstream flooding and erosion

  • Cost more to fix if problems are left unchecked

Performing preventive maintenance regularly will prevent long-term damage and help avoid potential violations. These actions will keep your facility looking good and working correctly, which will save you money in the long term. The preventive maintenance actions shown below can be thought of as similar to preventive car maintenance (like changing the oil in your car every 3 months).

Structural maintenance is work done on the components of the filter that allow it to control rainwater and remove pollutants. Structural maintenance problems in the table below are things you should be looking out for. When they happen, call DEP for help (like calling your mechanic when the engine light comes on).


Preventive Maintenance Actions For Your Hydrodynamic Separator


As Needed

✔ Pick up trash, debris, and leaves around your property and in front of the inlets to your hydrodynamic separator

✔ Know the pollutant sources on your property and try to eliminate the pollutants at the source

✔ Have a contractor annually clean your Hydrodynamic Separator

✔ Sweep paved areas on your property to remove pollutants, such as sediment and sand


✔ Store chemicals, used oil, and pesticides in covered areas so these potential pollutants are not exposed to rainfall


✔ Do not stockpile sand or salt on your property unless they are in covered containment areas


✔ Do not wash your vehicles or mechanical equipment on paved areas and prevent wash water from entering the storm drain system


✔ Mark your hydrodynamic separator inlets to ensure snow plow operators do not damage the facility


Structural Maintenance Actions for a Hydrodynamic Separator
(Only by a Contractor)


Possible Fix

Missing or damaged manhole cover

Replace manhole cover

Trash, debris, oil and grease, or sediment accumulation

Pump-out, power wash, clean, and properly dispose of any debris

Missing steps/ladders

Repair or replace steps/ladders

Clogged pipes and trash racks

Remove and properly dispose of debris causing clogging

Corrosion of metal pipes

Repair or replace corroded metal parts

Damaged structural components (i.e., pipes, concrete, trash racks, pipe joints, proprietary unit, etc.)

Repair and/or replacement for these components to be determined by DEP



How Will I Know What Maintenance is Required? 

DEP recommends owners use a certified contractor to pump-out, power wash, and clean their stormwater facility annually to ensure the facility is properly functioning and passes inspection.  If DEP inspects the facility and finds maintenance issues, you will receive a notice of violation with a work order showing the list of required maintenance and repairs. You will have 45 days from the date of the notice to hire a certified contractor to complete the maintenance and repairs.  A civil citation may be issued if you have not complied with the maintenance and repair requirements. 

View the list of DEP-approved contractors who can perform the structural maintenance on stormwater facilities.

DEP can answer your questions and provide additional guidance about maintaining your stormwater facility. Please email or call the Montgomery County Customer Service Center at 3-1-1.


Image of the front of the Hydrodynamic Separator Maintenance Fact Sheet.
Download the Information on this Page  
(PDF, 332KB)

Hydrodynamic Separator in the Stormwater Facilities Maintenance Program

The Stormwater Facilities Maintenance Program is responsible for maintaining all public stormwater facilities as well as all private facilities that transferred their maintenance to the County.  

Not sure if your hydrodynamic separator is part of the Stormwater Facilities Maintenance Program?  Check out online Facilities Map or email


Can I remove my hydrodynamic separator after installation?

No, you cannot remove any facilities that were part of your building installation - these are permitted structures and DEP maintains a database of these facility locations as part of the Stormwater Facility Maintenance Program. DEP may perform a maintenance inspection of your practice if it is a permitted structure.  Property owners must contact DEP before any major changes can be performed to the structure. 





Resources and Financial Incentives

Have a hydrodynamic separator on your property? Financial incentives are available for you.

Image of a green roof.


The Water Quality Protection Charge Credit Program

If your property has a hydrodynamic separator that is maintained in accordance with the Department of Environmental Protection maintenance requirements, then you are now eligible to receive a credit off your annual Water Quality Protection Charge (found on your yearly property tax bill).  The credit provides an incentive for maintaining your flow splitter and other stormwater management practices.

You must apply for the WQPC credit separately – the credit will not be provided to you automatically.

Residential property owners can receive up to 50% off their WQPC depending on the type and size of stormwater management practices on the property.

Non-residential and multi-family properties can receive up to 50-60% off their WQPC depending on the type and size of the stormwater management practices on the property.

Learn more on the WQPC Credit webpage.