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Facilities - Oil water separator

Oil / water separator

An oil / water separator treats stormwater by trapping oil at the surface of the water and sediment at the bottom, allowing treated water to pass through. It is most commonly used as the first pre-treatment facility in a series of stormwater management facilities and is usually found in parking lots, service and fuel stations. A unit should be inspected monthly and after a major storm. Make sure to keep records of your inspections and maintenance activities.  


Maintenance is needed if you see these signs

oil water separater good
Good/after: This oil water separator has been cleaned.
oil water separater needs maintenance
Bad/before: This oil water separator needs to be cleaned.

Discharge water shows obvious signs of oil or other contaminants

Thick layer of oil (more than 1 inch) on surface of water in vault

Sediment accumulation of more than 6 inches on the bottom of the vault

Accumulation of trash or debris

Plates, baffles or inlet/outlet pipes cracked or damaged

Cracks in pipe joints wider than 1/2 inch and/or evidence of soil particles entering through cracks


Tips for fixing problems and general maintenance

Inspection or maintenance that requires entering the underground facility must be performed by a trained and certified expert. Cleaning and proper disposal of sediment and oils can be done by most environmental contractors. Be sure to keep copies of the disposal records.

Remove trash and debris regularly.

Identify sources of leaks or spills and contain them as quickly as possible.

Oils and sediment should be removed and properly dispsed of (not in the trash).

Secured, floating, oil-absorbant pads can be used to extend the maintenance period.

Additional oil water separator elements

Flow control structure / flow restrictor

Flow control structure

Underground flow control structures and flow restrictors direct or restrict flow in or out of facilities. They slowly release stormwater at a specific rate. It is important to make sure they do not become plugged or damaged because the facility could release water too quickly or too slowly and overflow. They are usually found in manholes.


Coalescing plates

oil water coalescing plates sq.jpg

Some oil / water separators include coalescing plates (wavy-edged sheets shown above) that separate oil from water, so it can then be skimmed off. Plates should be white when clean.