draft 4.10 gravity (oil-grit) separators

Volume 2 (Technical Handbook) Georgia Stormwater Management Manual 3.3-294.10-1

4.10 Gravity (Oil-Grit) Separator Limited Application

Structural Stormwater Control

Description: Hydrodynamic

separationAn oil-grit separator is a device designed to remove settleable solids, oil and grease, debris and floatables from stormwater runoff through gravitational settling, hydrodynamic separation, and trapping of pollutants. Oil-grit separators are also called gravity separators or oil-water separators. LID/GI Considerations: Gravity oil-grit

separators are not considered low impact development or green infrastructure. However, for ultra-urban development projects or stormwater retrofit designs, oil-grit separators may be one of few design options for removal of total suspended solids and/or pollutants from stormwater runoff.

3.3-36 Georgia Stormwater Management Manual Volume 2 (Technical Handbook)

REASONS FOR LIMITED USE

KEY CONSIDERATIONS

Design CRITERIA

Gravity separators are typically used for areas less than 5 acres

Recommended contributing area to any individual gravity separator should be limited to 1 acre or less of impervious cover

The total wet storage of the gravity separator unit should be at least 400 cubic feet per contributing impervious acre

The minimum depth of the permanent pools should be 4 feet

Horizontal velocity through the separation chamber should be 1 to 3 ft/min or less

No velocities in the device should exceed the entrance velocity

A trash rack should be included in the design to capture floating debris, preferably near the inlet chamber

KEY CONSIDERATIONSADVANTAGES / BENEFITS:

Well suited for use on urban development sites, where larger or above-ground BMPs are not an option, or for stormwater retrofit projects

Can be used as pretreatment for other BMPs.

Can replace a conventional junction or inlet structure

Multiple inlets can connect to a single unit

Some designs require minimal drop between inlet and outlet

DISADVANTAGES / LIMITATIONS:

Dissolved pollutants are not effectively removed by oil-grit separators

Oil-grit separators cannot alone achieve the 80% TSS removal target

Frequent maintenance required

Performance dependent on design and frequency of inspection and cleanout of unit

Some designs may require a confined space entry for maintenance and repairs

ROUTINE MAINTENANCE REQUIREMENTS:

Maintenance requirements for a proprietary system should be obtained from the manufacturer

Frequency of inspection and maintenance is dependent on land use, climatological conditions, and the design of gravity separator

Failure to provide adequate inspection and maintenance can result in the resuspension of accumulated solids

Proper disposal of oil, solids and floatables removed from the gravity separator must be ensured

Intended for the removal of settleable solids (grit and

sediment) and floatable matter, including oil and grease

Dissolved pollutants are not effectively removed Frequent maintenance required Performance dependent on design and frequency of

inspection and cleanout of unit

STORMWATER MANAGEMENT

SUITABILITY

Runoff Reduction

Water Quality

Channel Protection

Overbank Flood

Protection

Extreme Flood

Protection = suitable for this practice

= may provide partial benefits

IMPLEMENTATION

CONSIDERATIONS

L Land Requirement

M Capital Cost

H Maintenance Burden

Residential Subdivision Use: Yes

High Density/Ultra-Urban: Yes

Roadway Projects: Not Recommended

SPECIAL APPLICATIONS

Pretreatment

High Density/Ultra-Urban

Other: Hotspot areas

Residential Subdivision Use: No

Soils: Gravity oil-grit separator systems can be installed in almost any soil or terrain.

Other Considerations: Install as

an off-line device unless the separator can be sized to handle a small drainage area. Install manhole on downstream side to provide easy access for sampling of effluent.

L=Low M=Moderate H=High

Runoff Reduction Credit: Minimal Runoff Reduction is provided by gravity oil-grit separators. A pretreatment BMP or downstream regional facility should be used if runoff reduction is desired.

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4.103.3.6.1 General Description

Gravity separators (also known as oil-grit separators) are hydrodynamic separation devices that are designed to remove grit and heavy sediments, oil and grease, debris and floatable matter from stormwater runoff through gravitational settling and trapping. . Gravity separator units contain a permanent pool of water and typically consist of an inlet chamber, separation/storage chamber, a bypass chamber, and an access port for maintenance purposes (see Figure 4.10-1). . Runoff enters the inlet chamber where heavy sediments and solids drop out. . The flow moves into the main gravity separation chamber, where further settling of suspended solids takes place. . Oil and grease are skimmed and stored in a waste oil storage compartment for future removal. . After moving into the outlet chamber, the clarified runoff is then discharged. The performance of these systems is based primarily on the relatively low solubility of petroleum products in water and the difference between the specific gravity of water and the specific gravities of petroleum compounds. . Gravity separators are not designed to separate other products such as solvents, detergents, or dissolved pollutants. . The typical gravity separator unit may be enhanced with a pretreatment swirl concentrator chamber, oil draw-off devices that continuously remove the accumulated light liquids, and flow control valves regulating the flow rate into the unit. Gravity separators are best used in commercial, industrial and transportation land uses and are intended primarily as a pretreatment measure for high-density or ultra urban sites, or for use in hydrocarbon hotspots, such as gas stations and areas with high vehicular traffic. . However, gravity separators cannot be used for the removal of dissolved or emulsified oils and pollutnantspollutants such as coolants, soluble lubricants, glycols and alcohols. Since resuspension of accumulated sediments is possible during heavy storm events, gravity separator units are typically installed off-line. . Gravity separators are available as prefabricated proprietary systems from a number of different commercial vendors.


Figure 4.10-1 Schematic of an Example Gravity (Oil-Grit) Separator

(Source: NVRC, 1992[1])

4.10.2 Stormwater Management Suitability

Runoff Reduction

Gravity oil-grit separators provide minimal stormwater volume runoff reduction. Another BMP should be used in a treatment train with gravity oil-grit separators to provide runoff reduction. See section 4.1.6 for more information about using BMPs in series. Water Quality

If installed as per the recommended design criteria and properly maintained, 40% total suspended solids removal will be applied to the water quality volume (WQv) flowing to the gravity oil-grit separator. Another BMP should be used in a treatment train with gravity oil-grit separators to provide the additional required water quality treatment. (See Section 4.1.6.)

Channel Protection

Gravity oil-grit separators do not provide channel protection. Another BMP should be used in a treatment train with gravity oil-grit separators to provide runoff reduction. (See Section 4.1.6.) Additionally, the gravity oil-grit separator should be designed off-line or a bypass used for higher flows.

Overbank Flood Protection

Gravity oil-grit separators do not provide overbank flood protection. Another BMP should be used in a treatment train with gravity oil-grit separators to provide runoff reduction. Additionally, the gravity oil-grit separator should be designed off-line or a bypass used for higher flows. (See Section 4.1.6.)

Extreme Flood Protection

Gravity oil-grit separators do not provide extreme flood protection. Another BMP should be used in a treatment train with gravity oil-grit separators to provide runoff reduction. Additionally, the gravity oil-grit separator should be designed off-line or a bypass used for higher flows. (See Section 4.1.6.)

3.3.6.24.10.3 Pollutant Removal Capabilities

Testing of gravity separators has shown that they can remove between 40 and 50% of the TSS loading when used in an off-line configuration (Curran, 1996 and Henry, 1999). . Gravity

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separators also provide removal of debris, hydrocarbons, trash and other floatables. . They provide only minimal removal of nutrients and organic matter. The following design pollutant removal rates are conservative average pollutant reduction percentages for design purposes derived from sampling data, modeling and professional judgment.

Total Suspended Solids – 40%

Total Phosphorus – 5%

Total Nitrogen – 5%

Fecal Coliform – insufficient data

Heavy Metals – insufficient data Actual field testing data and pollutant removal rates from an independent source should be obtained before using a proprietary gravity separator system.

4.10.4 Application and Site Feasibility Criteria3.3.6.3 Design Criteria and

Specifications

Conventional oil-grit separator units contain a permanent pool of water and typically consist of an inlet chamber, separation/storage chamber, a bypass chamber, and an access port for maintenance purposes. Runoff enters the inlet chamber where heavy sediments and solids drop out. Then the flow moves into the main separation chamber, where further settling of suspended solids takes place. Oil and grease are skimmed and stored in a waste oil storage compartment for future removal. After moving into the outlet chamber, the clarified runoff is then discharged to the site’s stormwater conveyance system. A wide variety of separator systems are commercially-available in a variety of layouts, for which vendors have design data and procedures. Oil-grit separators are sized based on a design flow rate, the Water Quality Peak Flow Rate (Qwq). This contrasts with most other stormwater structural controls, which are sized based on capturing and treating a specific volume. General Feasibility

Suitable for Residential Subdivision Usage – YES Suitable for High Density/Ultra Urban Areas – YES Regional Stormwater Control – NO

Physical Feasibility – Physical Constraints at Project Site

Drainage Area – Gravity separators are typically used for areas less than 5 acres. It is recommended that the contributing area to any individual gravity separator be limited to 1 acre or less of impervious cover.

The total wet storage of the gravity separator unit should be at least 400 cubic feet per contributing impervious acre.

Space Required – Gravity oil-grit separators are installed underground; therefore, minimal surface area is required for the device.

Adequate maintenance access to each chamber must be provided for inspection and cleanout of a gravity separator unit.

Site Slope – Gravity oil-grit separators may be installed on sites with slopes up to 6 %. A minimum 20 foot wide maintenance right-of-way or drainage easement shall be provided

for the oil-grit separator from a driveway, public or private road. The maintenance access easement shall have a maximum slope of no more than 15% and shall have a minimum unobstructed drive path width of 12 feet, appropriately stabilized to withstand maintenance equipment and vehicles. The right-of-way shall be located such that maintenance vehicles and equipment can access the oil-grit separator.

Minimum Depth to Water Table – 2 feet Minimum Head – 4 feet (The minimum depth of the permanent pools should be 4 feet.) Soils – Gravity separator systems can be installed in almost any soil or terrain. Check with manufacturer recommendations for additional site design constraints.

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Other Constraints / Considerations

Hot spots – Gravity oil-grit separators are well-suited for hot spot runoff Damage to existing structures and facilities –

Gravity oil-grit separators should not be used in areas where their operation may create a risk for basement flooding, interfere with subsurface sewage disposal systems, or affect other underground structures.

Gravity oil-grit separators should be designed so that overflow drains away from buildings to prevent damage to building foundations.

Trout Stream - Gravity oil-grit separators are will not reduce thermal impacts of stormwater runoff, nor are the y effective at removing soluble pollutants impacts. Therefore, they are not considered an effective means of protecting trout streams. However, in urban and highly developed areas or with a stormwater retrofit, gravity oil-grit separators may be an effective BMP for total suspended solids removal and hydrocarbons.

Coastal Areas Poorly Drained Soils - Poorly draining soils do not inhibit a gravity oil-grit separator’s ability

to temporarily store and treat stormwater runoff. Flat Terrain - Flat terrain and low site slopes do not interfere with the operation of a gravity

oil-grit separator. Shallow Water Table - Review manufacturer’s instructions regarding groundwater

elevation. Anti-flotation calculations may be required when large open chambers are installed at or below the water table.

4.10.5 Planning and Design Criteria Before designing the gravity oil-grit separator, the following data is necessary: Existing and proposed site, topographic and location maps, and field reviews. Impervious and pervious areas. Other means may be used to determine the land use data. Roadway and drainage profiles, cross sections, utility plans, and soil report for the site. Design data from nearby storm sewer structure. Water surface elevation of nearby water systems as well as the depth to seasonally high

groundwater. The following criteria are to be considered minimum standards for the design of a gravity oil-grit separator. Consult with the local review authority to determine if there are any variations to these criteria or additional standards that must be followed.

4.10.5.1 Location and Layout Gravity oil-grit separators should be located upstream or downstream of other BMPs providing runoff reduction, additional treatment of the water quality volume (WQv), channel protection volume (CPv), overbank flood protection (QP25) and extreme flood protection (Qf). See Section 4.1.6 for more information on the use of multiple BMPs in a treatment train.

4.10.5.2 General Design The use of gravity (oil-grit) separators should be limited to the following applications:

Pretreatment for other structural stormwater controls

High-density, ultra urban or other space-limited development sites

Hotspot areas where the control of grit, floatables, and/or oil and grease are required Gravity separators are rate-based devices. This contrasts with most other stormwater

BMPs, which are sized based on capturing and treating a specific volume. Horizontal velocity through the separation chamber should be 1 to 3 ft/min or less. No

velocities in the device should exceed the entrance velocity.

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Gravity separators are typically used for areas less than 5 acres. It is recommended that

the contributing area to any individual gravity separator be limited to 1 acre or less of impervious cover.

4.10.5.3 Physical Specifications / Geometry

The design criteria and specifications of a proprietary gravity separator unit should be obtained from the manufacturer.

The separation chamber should provide for three separate storage volumes:

A volume for separated oil storage at the top of the chamber

A volume for settleable solids accumulation at the bottom of the chamber

A volume required to give adequate flow-through detention time for separation of oil and sediment from the stormwater flow

Gravity separator units are typically designed to bypass runoff flows in excess of the design flow rate. Some designs have built-in high flow bypass mechanisms. Other designs require a diversion structure or flow splitter ahead of the device in the drainage system. An adequate outfall must be provided.

A trash rack should be included in the design to capture floating debris, preferably near the inlet chamber to prevent debris from becoming oil impregnated.

Ideally, a gravity separator design will provide an oil draw-off mechanism to a separate chamber or storage area.

Gravity separator units should be watertight to prevent possible groundwater contamination.

4.10.5.4 Pretreatment \ Inlets Gravity oil-grit separators are typically used as a pretreatment device in a hotspot area or

where floatable debris and pollutants should be removed prior to additional treatment by another BMP,

Inlets size, slope and invert should be sized based on manufacturer’s recommendations for flow rate, volume and structure size.

4.10.5.5 Outlet Structures An important consideration when designing an oil-grit separator system for a site is how to bypass large storm events that exceed the design flow capacity around the separator without damaging the unit, exceeding the design flow capacity, or resuspending collected pollutants. Since resuspension of accumulated sediments and oil droplets is possible during heavy storm events, oil-grit separator units are typically installed off-line with a bypass installed to minimize pollutant wash-out or resuspension. An adequate outfall/outlet must be provided for both the discharge from the separator itself, and the bypassed discharge. Runoff shall be discharged in a non-erosive manner. Typically the high flow outlet will discharge at a higher elevation than the low flow outlet.

4.10.5.6 Safety Features The deep inverts, open void sections and sometimes larger pipe diameters into and out of

oil-grit separators may present a fall or entrapment hazard. It is recommended that gravity oil-grit separators be constructed with manhole covers and/or grate lids with locking mechanisms.

Structural loading calculations, such as H-20 loading for traffic areas, should be performed when sizing and installing gravity oil-grit separators.

Some oil-grit separators are considered confined spaces. Additional training may be required to perform work inside the units.

Gravity separator systems can be installed in almost any soil or terrain. Since these devices are underground, appearance is not an issue and public safety risks are low.


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Gravity separators are rate-based devices. This contrasts with most other stormwater

structural controls, which are sized based on capturing and treating a specific volume. Gravity separator units are typically designed to bypass runoff flows in excess of the design

flow rate. Some designs have built-in high flow bypass mechanisms. Other designs require a diversion structure or flow splitter ahead of the device in the drainage system. An adequate outfall must be provided.

The separation chamber should provide for three separate storage volumes: (1) A volume for separated oil storage at the top of the chamber (2) A volume for settleable solids accumulation at the bottom of the chamber (3) A volume required to give adequate flow-through detention time for separation of oil

and sediment from the stormwater flow The total wet storage of the gravity separator unit should be at least 400 cubic feet per

contributing impervious acre. The minimum depth of the permanent pools should be 4 feet. Horizontal velocity through the separation chamber should be 1 to 3 ft/min or less. No

velocities in the device should exceed the entrance velocity. A trash rack should be included in the design to capture floating debris, preferably near the

inlet chamber to prevent debris from becoming oil impregnated. Ideally, a gravity separator design will provide an oil draw-off mechanism to a separate

chamber or storage area. Adequate maintenance access to each chamber must be provided for inspection and

cleanout of a gravity separator unit. Gravity separator units should be watertight to prevent possible groundwater contamination. The design criteria and specifications of a proprietary gravity separator unit should be

obtained from the manufacturer.

4.10.5.7 Construction Considerations

Contributing drainage areas to the gravity oil-grit separator should be stabilized with appropriate erosion and sediment control devices or with temporary and/or permanent seeding before runoff can enter a newly installed-device.

Newly installed gravity oil-grit separators should be inspected prior to being placed in service. Remove sediment and debris that may have been collected during delivery and installation.

A minimum 20-foot wide maintenance right-of-way or drainage easement shall be provided for the oil-grit separator from a driveway, public or private road.

4.10.5.8 Construction and Maintenance Costs

Material and installation costs for gravity oil-grit separators can vary based on the size, location, treatment requirements, and manufacturer,

Typically, gravity oil-grit separator systems can range from approximately $5-$6,000 for a small catch basin or manhole insert type design to approximately $40,000 for a multiple-chamber, high volume, high flow device.

4.10.6 Design Procedures

In general, site designers should perform the following design procedures when designing a gravity oil-grit separator.



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Step 1. Determine the goals and primary functions of the gravity oil-grit separator

A gravity oil-grit separator can be designed to provide pre-treatment of settleable solids, oil and grease, debris and floatables

An oil-grit separator may also be used to provide some treatment of the water quality volume (WQv)

Check with local officials and other agencies to determine if there are any additional restrictions and/or surface water or watershed requirements that may apply. In addition, consider if the oil-grit separator has any site-specific design conditions or criteria. List any restrictions or other requirements that may apply or affect the design.

Step 2. Determine if the development site and conditions are appropriate for the use of a gravity oil-grit separator

Consider the application and site feasibility criteria in this chapter. In addition, determine if site conditions are suitable for a gravity oil-grit separator. Create a rough layout of the device dimensions taking into consideration existing trees, utility lines, power and telephone poles, roadways, sidewalks, curbs and other obstructions. Step 3. Compute runoff control volumes and rates Oil-grit separators are typically sized based on the Water Quality peak flow rate (Qwq). This contrasts with most other stormwater BMPs, which are sized based on capturing and treating a specific volume. Refer to manufacturer instructions and design tools to design the gravity oil-grit separator based on the appropriate design flow rate and volumes. Step 4. Compute outlet release rate and size outlet Use manufacturer-recommend design methods to determine the size, slope and invert of

the device outlet. Ensure downstream receiving BMP and/or storm drain system can receive the volume and

rate of stormwater flow from the gravity oil-grit separator. A hydraulic grade analysis should be performed to determine if the receiving stormdrain

and pipe system can accept the flow, and that the oil-grit separator does not create a hydraulic head jump that exceeds the elevation of the upstream system.

Step 5. Calculate the Target Water Quality Volume

If the gravity oil-grit separator will be used to provide water quality, calculate the Water Quality Volume using the following formula: WQv = (1.2) (RV) (A) / 12 Where: WQv = Water Quality Volume (cubic-feet) 1.2 = Target rainfall amount to be treated (inches) Rv = Volumetric runoff coefficient which can be found by: RV = 0.05+0.009(I) Where: I = new impervious area of the contributing drainage area (%) A = Site area (square feet) 12 = Unit conversion factor (in/ft)

Step 6. Calculate the Water Quality Volume treated by the gravity oil-grit separator:

Using Table 4.1.3-2 - BMP Runoff Reduction Credits, lookup the appropriate runoff WQv/TSS removal provided by the practice: Calculate the Water Quality Volume/TSS Removal provided by the selected practice WQv (provided) = (WQv/TSS%) (WQv) Where: WQv (provided) = Water Quality Volume/Total Suspended Solids Removal provided (cubic feet) by a specific BMP




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WQv/TSS% = Water Quality Volume/Total Suspended Solids Removal percentage, or credit, assigned to the specific practice WQv = As calculated in Step 5 When WQv (provided) = Target WQv, Water Quality requirements are met. If WQv (provided) is less than the Target WQv, then additional water quality treatment/TSS removal or runoff reduction volume must be provided by an upstream or downstream BMP

3.3.6.4 Inspection4.10.7 Inspection and Maintenance Requirements

For many of the current gravity oil-grit separator designs, inspection and maintenance is conducted from the surface access cover, eliminating the need for confined space entry into the unit. Often an inspection orifice is provided. A “sludge judge” and oil dip-stick can be used at the access cover to determine sediment and oil depth measurements. Proper disposal of oil, solids and floatables removed from the gravity separator must be ensured. Please refer to Table 4.10-1 for typical maintenance requirements.

Table 3.3.6-14.10-1 Typical Maintenance Activities for Gravity Oil-Grit Separators

Activity

Schedule

Prior to Placing in Service

Inspect the gravity separator unit.

Regularly (quarterlyEvery 6

months)

Clean out sediment, oil and grease, and floatables, using catch

basin cleaning equipment (vacuum pumps). . Manual removal of

pollutants may be necessary. In subsequent years, inspections can

be based on first-year observations or local requirements

Inspect the unit immediately after an oil, fuel or chemical spill. A

licensed waste management company should remove oil and

sediment and dispose responsibly

Cleaning is recommended once the sediment depth reaches 15% of

storage capacity

As Needed

Additional Maintenance Considerations and Requirements

Additional maintenance requirements for a proprietary system should be obtained from the manufacturer.

Failure to provide adequate inspection and maintenance can result in the resuspension of accumulated solids. Frequency of inspection and maintenance is dependent on land use, climatological conditions, and the design of gravity separator.

Proper disposal of oil, solids and floatables removed from the gravity separator must be ensured.

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3.3.6.5 Example Schematic

Figure 3.3.6-1 Schematic of an Example Gravity (Oil-Grit) Separator

(Source: NVRC, 1992[1])

draft 4.10 gravity (oil-grit) separators

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