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Stormwater Best Management Practices in an Ultra-Urban Setting: Selection and Monitoring

Fact Sheet - Oil/Grit Separator Units

The typical oil/grit separator (OGS) unit operates by settling sediment and particulate matter, screening debris, and separating free surface oils from stormwater runoff. The unit typically consists of three or four chambers. Figure 21 is a schematic of a typical water quality oil/grit separator unit. In the case of a conventional OGS unit, the first chamber, termed the grit chamber, is designed to settle sediment and large particulate matter; the access from the first chamber to the second chamber is covered with a trash rack, which operates as a screen to prevent debris from passing through to the second chamber. The second chamber, termed the oil chamber, is designed to trap and separate free surface oils and grease from the stormwater runoff. The third chamber houses the stormwater outlet pipe that discharges the overflow to the storm drain system.

Figure 21. Schematic of an oil/grit separator (OGS)
(adapted from Schueler, 1987)

(See also above description) Side View: 3 access manholes. permanent pool 113 m<sup>3</sup> of storage per contributing acre. 1.2 m deep. Trash rack protects two 15 cm orifices between first and second chambers. Inverted elbow pipe in second chamber regulates water levels.

Most OGS units are designed to be placed in highly impervious parking areas that drain about 0.4 ha (1 ac). Results from one OGS study conducted in the State of Maryland showed that the treatment capacity of most conventional OGS units inventoried was less than 5.1 mm (0.2 in) of runoff for the service area (Schueler and Shepp, 1993). Because of the limited retention capacity, conventional OGSs are not capable of removing large quantities of stormwater constituents. Instead, they are designed and implemented to control hydrocarbons, debris, large organic matter, and coarse sediments that are commonly associated with heavily traveled parking areas.


The OGS unit is designed to trap and settle large sediments and particulate matter, debris, and hydrocarbons from highly impervious areas such as parking lots, gas stations, loading docks, and roadside rest areas. The OGS unit is constructed beneath the surface of the impervious area, and as such does not require additional space. Because of this, it can be easily retrofitted into existing impervious land use conditions, which makes it suitable for ultra-urban environments. Results from an OGS study in the State of Maryland have shown that detention times for conventional OGS units are generally less than 30 minutes during storm events (Schueler and Shepp, 1993). Trapped sediments and particles tend to resuspend during subsequent storms and exit the chambers. Because settling and trapping are temporary, actual pollutant removal occurs only when the units are cleaned out. Therefore, these devices are best suited for an off-line configuration where only a portion of the first flush is treated by the unit and clean out occurs after every major storm event. A study produced by the Metropolitan Washington Council of Governments showed that particulate matter within conventional OGS units remained the same or decreased over a 20-month period (Shepp et al., 1992).


Conventional OGS units have demonstrated poor pollutant removal capabilities. The primary removal mechanism of the OGS is settling; with short detention times, and resuspension occurring after every storm event, removal effectiveness is limited to what is physically cleaned out after every storm. If the unit is not cleaned after each storm, resuspended trace metals, nutrients, organic matter, and sediments will eventually pass through each chamber and into the storm drain system.

A study performed on OGS units in the State of Maryland showed that negative sediment deposition from storm to storm indicated that re-suspension and washout were a common problem (Schueler and Shepp, 1993). The only constituent that was trapped with some efficiency in the second chamber was total hydrocarbons. This was probably due to the inverted siphon, which is designed to retain free surface oils and grease (Schueler et al., 1992; Schueler and Shepp, 1993).

Siting and Design Considerations

The OGS unit is a structural BMP that is easily installed in areas of high imperviousness such as parking lots, gas stations, commercial and industrial sites, and shopping centers, and even along roadways. The OGS unit would be well suited for ultra-urban environments where available land area is a major constraint. OGS units typically are sized for highly impervious drainage areas of less than 0.4 ha (1 ac), though up to 0.61 ha (1.5 ac) is feasible. Locating the units off-line would alleviate some of the problems associated with the retention and resuspension of pollutants.

The OGS units are designed using a three- or four-chamber configuration. Settling of larger sediments, trash, and debris takes place in the first chamber. The primary function of the second chamber is to separate oils and grease from the stormwater runoff; some absorption of oils and grease to smaller sediments, and settling will also occur in the first chamber. The third chamber houses the overflow pipe. The OGS unit typically is sized based on the drainage area, which often includes rooftops, and the percent imperviousness of the basin. One common practice is to size the unit based on a design storm to provide some amount of storage. In general, OGS units are rectangular in shape, with the largest chamber being the initial settling chamber. Approximate dimensions for an OGS unit located in a parking area that drains 0.4 ha (1 ac) would be 1.82 m deep by 1.22 m wide by 4.23 long (6 ft deep by 4 ft wide by 14 ft long) (inside dimensions). The length of the first chamber would be 1.82 m (6 ft) with 1.22 m (4 ft) for each of the other two chambers.

Specific dimensions for each OGS design are dependent on site characteristics and local design storm requirements. Improvement in OGS performance can be achieved by extending the interior chamber walls to the top of the chamber, thereby eliminating recirculation and overflow from one chamber to another. In addition, placing the OGS off-line from the main stormwater system helps to reduce resuspension of oil and grit.

Additional design examples and information can be found in Controlling Urban Runoff: A Practical Manual for Planning and Designing Urban BMPs (Schueler, 1987), and Northern Virginia BMP Handbook: A Guide to Planning and Designing Best Management Practices in Northern Virginia (NVPDC, 1992). Because studies have shown that water quality inlets are a marginal method for removing particulate matter (Schueler and Shepp, 1993), other design references (Claytor and Schueler, 1996) do not recommend them for sand filter pretreatment.

Maintenance Considerations

Very few structural or clogging problems have been reported during the first five years of OGS operation (Schueler and Shepp, 1993). The OGS unit should be inspected after each major storm event. Clean-out would require the removal of sediments, trash, and debris. In reality, OGSs are rarely cleaned out after every storm because such intensive maintenance is beyond most budgets.

The removal of oily debris, sediments, and trash might require disposal as a hazardous waste. However, some local landfills may accept the sediment and trash if it is properly dewatered.

Cost Considerations

OGS units can be either cast-in-place or precast. Precast concrete chambers are usually delivered to the site partially assembled and tend to cost slightly less than the cast-in-place option. The cost associated with a cast-in-place concrete OGS unit is a function of several parameters. Excavation, gravel bedding, amount and size of rebar, amount of concrete and form work, and grate and clean-out access holes all contribute to the total cost of the OGS unit. In 1992, OGS units were reported to cost between $5,000 and $15,000 fully installed. On average, costs per inlet ranged from $7,000 to $8,000 (Schueler et al., 1992).


Botts, J., L. Allard, and J. Wheeler. 1996. Structural Best Management Practices for Storm Water Pollution Control at Industrial Facilities. Watershed '96, pp. 216-219.

Claytor, R.A., and T.R. Schueler. 1996. Design of Stormwater Filtering Systems. The Center for Watershed Protection, Silver Spring, MD.

NVPDC. 1992. Northern Virginia BMP Handbook: A Guide to Planning and Designing Best Management Practices in Northern Virginia. Prepared by Northern Virginia Planning District Commission (NVPDC) and Engineers and Surveyors Institute.

Schueler, T.R. 1987. Controlling Urban Runoff: A Practical Manual for Planning and Designing Urban BMPs. Metropolitan Washington Council of Governments, Washington, DC.

Schueler, T.R., P.A. Kumble, and M.A. Heraty. 1992. A Current Assessment of Urban Best Management Practices - Techniques for Reducing Non-Point Source Pollution in the Coastal Zone. Metropolitan Washington Council of Governments, Department of Environmental Programs, Anacostia Restoration Team, Washington, DC.

Schueler, T.R., and D. Shepp. 1993. The Quality of Trapped Sediments and Pool Water Within Oil-Grit Separators in Suburban Maryland. Chapter 6. Interim Report for the Maryland Department of the Environment Hydrocarbon Study, 81-115.

Shepp, D., D. Cole, and F.J. Galli. 1992. A Field Survey of the Performance of Oil/Grit Separators. Prepared for the Maryland Department of the Environment by the Metropolitan Washington Council of Governments, Department of Environmental Programs, Washington, DC.

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Questions and feedback should be directed to Susan Jones (Susan.Jones@dot.gov, 202-493-2139) and Marcel Tchaou (Marcel.Tchaou@dot.gov, 202-366-4196).

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