Oil/Grit Separators

From the Massachusetts Stormwater Handbook

Description

Oil/grit separators are underground storage tanks with three chambers designed to remove heavy particulates, floating debris and hydrocarbons from stormwater.

Stormwater enters the first chamber where heavy sediments and solids drop out. The flow moves into the second chamber where oils and greases are removed and further settling of suspended solids takes place. Oil and grease are stored in this second chamber for future removal. After moving into the third outlet chamber, the clarified stormwater runoff is then discharged to a pipe and another BMP. There are other separators that may be used for spill control.

Advantages/Benefits

Located underground so limited lot size not a deterrent in urban areas with small lots
Can be used for retrofits
Can be installed in any soil or terrain.
Public safety risks are low.

Ability to Meet Massachusetts Stormwater Management Standards

Standard	Description
2 - Peak Flow	Provides no peak flow attenuation
3 - Recharge	Provides no groundwater recharge
4 – TSS Removal	25% TSS removal credit when used for pretreatment and placed off-line.
5 - Higher Pollutant Loading	Provides no groundwater recharge
6 - Discharges near or to Critical Areas	Provides no groundwater recharge
7 - Redevelopment	Provides no groundwater recharge

May be a pretreatment BMP when combined with other practices. May serve as a spill control device. Highly suitable.

Disadvantages/Limitations

Limited pollutant removal; cannot effectively remove soluble pollutants, fine particles, or bacteria
Can become a source of pollutants due to resuspension of sediment unless properly maintained
Susceptible to flushing during large storms
Limited to relatively small contributing drainage areas
Requires proper disposal of trapped sediments and oils
May be expensive to construct and maintain
Entrapment hazard for amphibians and other small animals

Pollutant Removal Efficiencies

Total Suspended Solids (TSS): - 25% for oil grit separator, only when placed off-line and only when used for pretreatment
Nutrients (Nitrogen, phosphorus): - Insufficient data
Metals (copper, lead, zinc, cadmium): - Insufficient data
Pathogens (coliform, e coli): - Insufficient data

Maintenance

Activity	Frequency
Inspect units	After every major storm but at least monthly
Clean units	Twice a year

Applicability

Oil grit separators must be used to manage runoff from land uses with higher potential pollutant loads where there is a risk that the stormwater is contaminated with oil or grease. These uses include the following:

High-Intensity-Use Parking Lots
Gas Fueling Stations
Vehicles (including boats, buses, cars, and trucks) and Equipment Service and Maintenance Areas
Fleet Storage Areas

Design Considerations

Dovetail design practices, source controls and pollution prevention measures with separator design.
Place separators before all other structural stormwater treatment practices (except for structures associated with source control/ pollution prevention such as drip pans and structural treatment practices such as deep sump catch basins that double as inlets).
Limit the contributing drainage area to the oil/grit separator to one acre or less of impervious cover.
Use oil grit separators only in off-line configurations to treat the required water quality volume.
Provide pool storage in the first chamber to accommodate the required water quality volume or 400 cubic feet per acre of impervious surface. Confirm that the oil/grit separator is designed to treat the required water quality volume.
Make the permanent pool at least 4 feet deep.
Design the device to pass the 2-year 24-hour storm without interference and provide a bypass for larger storms to prevent resuspension of solids.
Make oil/grit separator units watertight to prevent possible groundwater contamination.
Use a trash rack or screen to cover the discharge outlet and orifices between chambers.
Provide each chamber with manholes and access stepladders to facilitate maintenance and allow cleaning without confined space entry.
Seal potential mosquito entry points.
Install any pump mechanism downstream of the separator to prevent oil emulsification.
Locate an inverted elbow pipe between the second and third chambers and with the bottom of the elbow pipe at least 3 feet below the second chamber’s permanent pool.
Provide appropriate removal covers that allow access for observation and maintenance.
Where the structure is located below the seasonal high groundwater table, design the structure to prevent flotation.
For gas stations, automobile maintenance and service areas, and other areas where large volumes of petroleum and oil are handled, consider adding coalescing plates to increase the effectiveness of the device and reduce the size of the units. A series of coalescing plates constructed of oil-attracting materials such as polypropylene typically spaced one inch apart attracts small droplets of oil, which begin to concentrate until they are large enough to float to the surface.

Maintenance

Sediments and associated pollutants and trash are removed only when inlets or sumps are cleaned out, so regular maintenance is essential. Most studies have linked the failure of oil grit separators to the lack of regular maintenance. The more frequent the cleaning, the less likely sediments will be resuspended and subsequently discharged. In addition, frequent cleaning also makes more volume available for future storms and enhances overall performance. Cleaning includes removal of accumulated oil and grease and sediment using a vacuum truck or other ordinary catch basin cleaning device. In areas of high sediment loading, inspect and clean inlets after every major storm. At a minimum, inspect oil grit separators monthly, and clean them out at least twice per year. Polluted water or sediments removed from an oil grit separator should be disposed of in accordance with all applicable local, state and federal laws and regulations including M.G.L.c. 21C and 310 CMR 30.00.

References

American Petroleum Institute, 2002, Management of Water Discharges: Design and Operations of Oil-Water Separators, 1st Edition, Revision 90, American Petroleum Institute.

Arizona Department of Environmental Quality, 1996, BADCT Guidance Document for Pretreatment with Oil/ Water Separators, OFR 96-15, http://www.azdeq.gov/environ/water/permits/download/owsbadct.pdf

Beychok, Milton, Wikipedia, API Oil-Water Separator, http://en.wikipedia.org/wiki/API_oil-water_separator

Center for Watershed Protection, Performance of Oil-Grit Separators in Removing Pollutants at Small Sites, Technical Note #101 from Watershed Protection Techniques. 2(3): 539-542

Houston, City of, Harris County, Harris County Flood Control District, 2001, Storm Water Quality Management Guidance Manual, Section 4.4.2, p. 4-84 to 4-89, http://www.cleanwaterclearchoice.org/downloads/professional/guidance_manual_full.pdf

Idaho Department of Environmental Quality, 2005, Storm Water Best Management Practices Catalog, Oil/ Water Separator, BMP 18, pp. 91 to 95, http://www.deq.idaho.gov/water/data_reports/storm_water/catalog/sec_4/bmps/18.pdf

Massachusetts Highway Department, 2004, Storm Water Handbook for Highways and Bridges, p. Minton, Gary. 2002, Stormwater Treatment, RPA Associates, Seattle, WA, p. 120

New Zealand Water Environment Research Foundation, 2004, On-Site Stormwater Management Guideline, Section 5.10, pp. 23 to 24, http://www.nzwwa.org.nz/Section%205.pdf

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

U.S. EPA, 1999, Storm Water Technology Fact Sheet, Water Quality Inlets, EPA 832-F-99-029, http://www.epa.gov/owm/mtb/wtrqlty.pdf