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Oil and Grease Separator


Oil and grease separators are devices that are designed to remove oil, grease, sediments, trash, and other debris from stormwater. These three chambered, underground devices use the difference between the specific gravities of water and petroleum products to separate the two. However, other types of chemical pollutants, such as solvents and detergents, are not removed by this practice. As a result, they are best used as pre-treatment devices in conjunction with other management practices.

In low flow conditions, water enters the separator via a storm drain inlet and passes through the chambers of the device. As it passes, oil and grease are either separated to the surface and are skimmed off or settle to the bottom with sediments and held in the basin. The effluent is then discharged to additional management practices. However, in high flow situations, the volume of water is often greater than the capacity of the separator and stormwater may bypass the device without treatment.


  • Widely applicable
  • Requires minimal land area
  • Compatible with most storm drain systems
  • Can be used in retrofit applications


  • Does not remove emulsified oils or other pollutants
  • Ineffective for removing nutrients and heavy metals
  • Limited effectiveness with large storm events, providing little detention time
  • Resuspension of trapped sediments is probable if not maintained properly
  • Does not reduce peak flows
  • Requires frequent maintenance
  • Only used as pre-treatment devices in conjunction with other management practices

Oil and grease separators are applicable on a wide variety of sites with drainage areas of less than 5 acres. To prevent the volume of stormwater from exceeding the capacity of the separator, the contributing drainage area should have an impervious area of less than 1 acre. These devices are most often used at gas stations, industrial sites, parking lots, loading areas, or any other area hydrocarbons are likely to be present in large quantities. Because they operate underground, they are often used in retrofit applications where other management practices are not practical.

Oil and grease separators are available commercially in a variety of configurations. This section is intended to provide general design criteria only, as the device selected should be installed, operated, and maintained following all manufacturers’ specifications. In addition, Dane County does not endorse any particular type or brand of proprietary product. These devices provide only limited pollutant removal and do not reduce peak flows and, as a result, they are best used as pre-treatment devices in conjunction with other management practices.


Oil and grease separators should provide at least 400 cubic feet of wet storage for each acre of impervious area they drain. This volume should be sufficient to provide enough detention time so that oil, grease, and sediment may be separated from the stormwater. In addition, separators should provide separate storage volumes for separated petroleum products at the top of the chamber and for accumulated solids at the bottom.

The permanent pool should have a depth of at least four feet and flow velocities in the separator should not exceed three feet per minute to allow for proper treatment.


  • Installation should follow all manufacturers’ specifications
  • Separators should be constructed with reinforced concrete and must be water tight to prevent groundwater pollution
  • Installation should provide access for maintenance activities
  • Separators may be installed off-line to prevent the capacity of the separator from being exceeded during large storm events


All maintenance activities should follow the manufacturers’ recommendations and specifications. However, additional maintenance requirements may be required and are listed below.
  • Accumulated sediment must be removed when it reaches 15% of the designed storage capacity of the practice- failure to do so may result in the resuspension of accumulated particles
  • Separators should be inspected regularly to ensure that the practice is functioning properly
  • All accumulated oil, grease, sediment and other debris should be disposed of properly

Method to Determine Practice Efficiency

The manufacturer of the device determines the efficiency of this practice. It is dependent upon several factors, including the volume of water that enters the device and that proper maintenance activities are performed in a timely manner. Due to these factors and because oil and grease separators are rate based practices and are not designed to handle a specific storm event, efficiency is dependent upon external factors that are beyond the control of the designer. As a result, the efficiency for this practice is at the discretion of the permitting agency.

For standards that this device may be used to meet, please reference Table 3 on the Stormwater page.