6.2 Grease Interceptors, Oil Separators & Sand Traps

6.2 Grease Interceptors, Oil Separators & Sand Traps

Quick Answer: Under North Carolina Plumbing Code (NCPC) Chapter 10, interceptors and separators are mandatory whenever waste streams carry harmful or hazardous constituents including fats, oils, and grease (FOG), petroleum distillates, sand, or solids. Hydro-mechanical Grease Interceptors (HGIs) are sized based on flow rate in Gallons Per Minute (GPM) and must provide a grease retention capacity in pounds equal to at least twice the GPM rating ($lbs = 2 \times GPM$). HGIs mandate an approved flow control fitting with a vented air intake. Oil/water separators for automotive facilities require a minimum base capacity of 6 cubic feet (45 gallons) for the first 100 sq ft plus 1 cubic foot (7.48 gal) for each additional 100 sq ft of drained area, vented independently to the atmosphere with a minimum 2-inch dedicated vent.

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Interceptor Classifications: HGIs vs. Gravity Grease Interceptors (GGIs)

Commercial food service facilities produce large volumes of animal fats, vegetable oils, and grease (FOG). Left untreated, FOG solidifies in municipal sewer mains, causing severe sanitary sewer overflows (SSOs). The code categorizes grease recovery into two primary engineering technologies:

graph TD
    A["Commercial Kitchen Waste Stream"] --> B{"Select Interceptor Technology"}
    B --> C["Hydro-mechanical Grease Interceptor (HGI)<br/>• Point-of-use indoor unit (ASME A112.14.3 / PDI G101)<br/>• Sized by GPM (10 to 100 GPM)<br/>• Grease Capacity = 2 × GPM rating (lbs)<br/>• Requires Vented Flow Control Fitting"]
    B --> D["Gravity Grease Interceptor (GGI)<br/>• Outdoor underground concrete/poly vault (IAPMO Z1001)<br/>• Minimum 500 to 1,500+ liquid gallons<br/>• Sized for 30-minute retention time<br/>• Standard gravity baffle separation"]

Technical Comparison: HGI vs. GGI

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HGI Sizing Calculations & Fixture Flow Rate Math

Hydro-mechanical interceptors are sized by calculating the total peak drainage flow from all connected fixtures.

1. Fixture Volumetric Sizing Formula

To determine the required GPM rating for a commercial fixture (such as a 3-compartment sink), calculate its internal volume in cubic inches, convert to liquid gallons, apply the 75% load factor, and divide by the drain-down period (typically 1 minute or 2 minutes):

$\text{Total Compartment Volume (cu in)} = \text{Length (in)} \times \text{Width (in)} \times \text{Depth (in)} \times (\text{Number of Compartments})$

$\text{Total Liquid Capacity (Gallons)} = \frac{\text{Total Volume (cu in)}}{231\text{ cu in/gal}}$

$\text{Design Drainage Load (75\% Fill)} = \text{Total Capacity (Gallons)} \times 0.75$

$\mathbf{\text{Required Flow Rate (GPM)}} = \frac{\text{Design Drainage Load (Gallons)}}{\text{Drain-down Time (1 or 2 Minutes)}}$

$\mathbf{\text{Minimum Grease Retention Capacity (lbs)}} = \text{Required Flow Rate (GPM)} \times \mathbf{2}$

Worked Step-by-Step Problem: Sizing a 3-Compartment Pot Sink HGI

Scenario: A restaurant installs a commercial 3-compartment stainless steel pot sink. Each compartment measures 24 inches long, 24 inches wide, and 14 inches deep. Determine the required HGI flow rate (GPM) and grease retention capacity (lbs) based on a 1-minute drain-down time.

1. Calculate Total Volume in Cubic Inches: $\text{Volume per Compartment} = 24" \times 24" \times 14" = 8,064\text{ cu in}$ $\text{Total Volume (3 Compartments)} = 8,064 \times 3 = \mathbf{24,192\text{ cu in}}$
2. Convert to Liquid Gallons ($231\text{ cu in/gal}$): $\text{Total Capacity} = \frac{24,192}{231} = \mathbf{104.73\text{ gallons}}$
3. Apply the 75% Displacement / Fill Factor: $\text{Design Load} = 104.73 \times 0.75 = \mathbf{78.55\text{ gallons}}$
4. Calculate Flow Rate for 1-Minute Drainage: $\text{Flow Rate} = \frac{78.55\text{ gal}}{1\text{ min}} = \mathbf{78.55\text{ GPM}}$ (Rounding up to the next standard commercial HGI size yields a 100 GPM interceptor, or 50 GPM if evaluated under a 2-minute drain-down schedule: $78.55 / 2 = 39.28\text{ GPM} \implies 50\text{ GPM}$).
5. Calculate Minimum Grease Retention Capacity for a 50 GPM Unit: $\text{Grease Retention Capacity} = 50\text{ GPM} \times 2 = \mathbf{100\text{ lbs of grease}}$

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The Flow Control Fitting & Air Intake Mechanism

A critical requirement of ASME A112.14.3 / PDI G101 hydro-mechanical grease interceptors is the flow control fitting with an air intake:

[Fixture Drain] ──► [Flow Control Orifice] ──► [Air Intake Vent] ──► [HGI Inlet]
                           │                         │
                    Regulates GPM             Entrains Air Bubbles
                   to match rating           for Grease Flotation

• Flow Regulation: Without a flow control fitting, a surge from a filled sink would overwhelm the interceptor at 60+ GPM, causing turbulence that flushes accumulated grease out into the building sewer.
• Air Entrainment Flotation: The air intake pulls atmospheric air into the flowing stream via venturi effect. The resulting microscopic air bubbles adhere to lightweight grease droplets, rapidly floating them to the surface grease mat before wastewater escapes under the downstream baffle.
• Air Intake Venting: The air intake of the flow control device must be extended to the atmosphere or connect to the sanitary vent system at an elevation higher than the fixture flood level rim.

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Fixture Connection Rules & Prohibitions (NCPC 1003.3)

Oil/Water Separators for Automotive Facilities (NCPC 1003.4)

Automotive repair garages, gasoline service stations, car washes, and commercial parking structures generate oily runoff containing volatile hydrocarbons, lubricants, and transmission fluids that present severe fire, explosion, and environmental hazards.

1. Code Mandate & Sizing Formula (NCPC 1003.4)

Under NCPC Section 1003.4, an approved oil/water separator must be installed wherever oily and flammable liquid wastes are generated. The liquid capacity of the separator is mathematically calculated based on drained floor area:

$\mathbf{\text{Minimum Base Capacity}} = \mathbf{6.0\text{ cubic feet (45.0 gallons)}}\text{ for the first 100 sq ft of floor area}$ $\mathbf{\text{Additional Capacity}} = \mathbf{1.0\text{ cubic foot (7.48 gallons)}}\text{ for each additional 100 sq ft (or fraction thereof)}$

$\text{Total Liquid Volume (cu ft)} = 6.0 + \left(\frac{\text{Total Drained Floor Area (sq ft)} - 100}{100}\right)$ $\text{Total Liquid Volume (Gallons)} = \text{Total Volume (cu ft)} \times 7.48\text{ gal/cu ft}$

Worked Step-by-Step Problem: Sizing an Auto Shop Oil Separator

Scenario: An automotive service garage in Charlotte, NC has a repair bay floor area of 1,600 square feet draining into floor drains. What is the minimum required liquid capacity of the oil separator in cubic feet and in gallons?

1. Base Capacity for First 100 sq ft: $\text{Base Volume} = 6.0\text{ cu ft}$
2. Calculate Remaining Floor Area: $\text{Remaining Area} = 1,600\text{ sq ft} - 100\text{ sq ft} = 1,500\text{ sq ft}$
3. Calculate Incremental Volume ($1.0\text{ cu ft per 100 sq ft}$): $\text{Incremental Volume} = \frac{1,500}{100} \times 1.0 = 15.0\text{ cu ft}$
4. Calculate Total Volume in Cubic Feet: $\text{Total Volume} = 6.0\text{ cu ft} + 15.0\text{ cu ft} = \mathbf{21.0\text{ cubic feet}}$
5. Convert to Gallons ($7.48\text{ gal/cu ft}$): $\text{Total Gallons} = 21.0\text{ cu ft} \times 7.48\text{ gal/cu ft} = \mathbf{157.08\text{ gallons}}$

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Structural Construction & Dedicated Independent Venting Rules

Oil/water separators are subject to stringent construction and venting requirements due to explosive hydrocarbon vapor accumulation:

graph TD
    A["Automotive Repair Bay Drains"] --> B["Sand / Grit Catch Basin (Settles Dirt)"]
    B --> C["Vapor-Tight Oil Separator Vault"]
    C -->|Bottom-Draw Inverted Trap Leg| D["Effluent to Sanitary Sewer"]
    C -->|Dedicated ≥ 2-inch Vent Pipe| E["Terminates Independently Through Roof"]
    style E fill:#d9534f,color:#fff

1. Vapor-Tight Enclosure: The separator body, access manholes, and inspection covers must be manufactured from non-combustible, corrosion-resistant cast iron, steel, or reinforced concrete sealed gas-tight and vapor-tight.
2. Deep Water Trap Seal: The discharge outlet must be configured with a submerged inverted elbow or baffle providing a water seal of not less than 4 inches (102 mm) to prevent flammable vapors from entering the public sewer.
3. Mandatory Independent 2-Inch Vent (NCPC 1003.4.2.2):
   • Every oil separator must be provided with an independent vent pipe not less than 2 inches (51 mm) in diameter.
   • The vent must connect to the vapor space above the liquid line inside the separator.
   • The vent must extend independently through the roof to the outer air and CANNOT be tied into any sanitary DWV vent or plumbing stack vent.
   • Why? Interconnecting oil separator vents with building sanitary vents could allow explosive gasoline vapors to migrate into building plumbing fixtures.

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Sand, Grit & Solids Interceptors (NCPC 1003.5 / 1003.8)

Where wastewater contains heavy suspended mineral solids, sand, grit, glass, or lint, approved sand/solids interceptors are mandatory:

• Automotive Wash Racks & Car Washes: Must be equipped with a sediment interceptor (sand trap pit) having a minimum liquid depth of 24 inches below the invert of the outlet pipe and a cleanout access opening of at least 2 square feet before discharging into an oil separator.
• Commercial Laundries: Interceptors must be equipped with removable wire mesh lint screens or baskets with maximum 1/2-inch openings to capture fibrous lint before fouling drain mains.
• Bottling Plants & Machine Shops: Interceptors must retain glass chips, metal shavings, and heavy grit.