10.2 Grease Interceptors: Sizing, Types & Plan Review

Grease Interceptors: Sizing, Types & Plan Review

Commercial food service operations generate immense quantities of waste fats, oils, and grease (FOG). When hot FOG is rinsed down commercial drains, it enters the municipal sanitary collection network as a liquid emulsion. As this wastewater cools to ambient ground temperatures below $110^{\circ}\text{F}$ ($43.3^{\circ}\text{C}$), the lipids solidify. In the presence of calcium and alkaline minerals prevalent in domestic sewage, these free fatty acids undergo saponification—forming hard, insoluble calcium metallic soaps. Over time, these accretions form monstrous subterranean obstructions commonly known as "fatbergs." Fatbergs severely restrict sewer hydraulic capacity, trigger catastrophic sanitary sewer overflows (SSOs) into streets and waterways, and generate millions of dollars in municipal remediation costs.

IPC 1003.1 requires interceptors and separators wherever oil, grease, sand or other harmful substances would otherwise reach the public sewer, and IPC 1003.3 applies that to grease. A grease interceptor or automatic grease removal device is required for fixtures and equipment with grease-laden waste in food preparation areas, such as in restaurants, hotel kitchens, hospitals, school kitchens, bars, factory cafeterias and clubs (1003.3.1). The plans examiner evaluates interceptor type, sizing, flow control, venting and which fixtures connect.

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Hydromechanical (HGI) vs. Gravity Grease Interceptors (GGI)

The plumbing industry and the IPC divide grease interceptors into two distinct engineering classes, each governed by different product standards, separation mechanics, and spatial requirements:

   HYDROMECHANICAL GREASE INTERCEPTOR (HGI)       GRAVITY GREASE INTERCEPTOR (GGI)
   (ASME A112.14.3 / PDI-G101)                   (IAPMO/ANSI Z1001)

         INLET VENTED FLOW CONTROL                       MANHOLES TO FINISHED GRADE
               │                                              ┌──────┐    ┌──────┐
               ▼                                              │      │    │      │
        ┌─────────────┐                                  ═════╪══════╪════╪══════╪═════ GRADE
   IN ──┤ AIR INTAKE  ├──► [ INTERCEPTOR ] ──► OUT            │      │    │      │
        └─────────────┘     Compact chamber;             IN ──┤      │    │      ├──► OUT
                            Air entrainment;                  │ 2/3  │BAF │ 1/3  │
                            Baffles accelerate                │ VOL  │FLE │ VOL  │
                            flotation;                        │      │    │      │
                            Grease Cap (lbs) = 2x GPM         └──────┴────┴──────┘
                                                              Underground Vault: 750 - 1500+ gal
                                                              30-Minute Quiescent Retention

Comprehensive Engineering Comparison Matrix

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HGI Sizing Protocol: Fixture Capacity Calculation Method

Under IPC Section 1003.3.5, hydromechanical grease interceptors are sized in accordance with ASME A112.14.3, ASME A112.14.4, ASME A112.14.6, CSA B481.3 or PDI G101. The common PDI G101 fixture-capacity method uses the volume of the connected fixtures.

The Standard Five-Step Fixture Capacity Formula

1. Calculate Basin Volume in Cubic Inches: For each sink compartment connected to the interceptor, multiply length by width by depth: $V_{\text{basin}} = L \times W \times D$

2. Sum All Compartments and Convert to Liquid Gallons: Divide total cubic inches by the standard volumetric conversion constant ($231\text{ cubic inches per gallon}$): $V_{\text{total (gal)}} = \frac{\sum (L \times W \times D)}{231}$

3. Apply the 75% Operational Fill Factor: Sinks are never filled to the absolute flood rim due to displacement by washed utensils and the practical waterline established by the overflow or faucet. The industry standard fill factor is 75% ($0.75$): $V_{\text{effective (gal)}} = V_{\text{total (gal)}} \times 0.75$

4. Calculate Required Flow Rate (gpm): Divide the effective liquid volume by the design drain-out period. PDI-G101 specifies a 1-minute drain period for maximum peak flow calculations, while some municipal jurisdictions permit a 2-minute drain period: $Q_{\text{required (gpm)}} = \frac{V_{\text{effective (gal)}}}{t_{\text{drain (minutes)}}}$

5. Establish Minimum Grease Retention Capacity (IPC 1003.3.5.1): IPC Table 1003.3.5.1 lists the grease retention capacity for each flow-through rating (4 gpm = 8 lb, 20 gpm = 40 lb, 50 gpm = 100 lb, 100 gpm = 200 lb), and for ratings over 100 gpm the capacity is double the rating: $\text{Grease Retention Capacity (lbs)} = 2 \times Q_{\text{rated (gpm)}}$

Step-by-Step Worked Review Example: Commercial 3-Compartment Pot Sink

Review Scenario: A plumbing submittal indicates a commercial restaurant three-compartment scullery pot washing sink. Each compartment measures 24 inches long ($L$), 24 inches wide ($W$), and 14 inches deep ($D$). The designer proposes a 50 gpm HGI based on a 1-minute drain-out period.

• Step 1: Compute Volume of One Basin: $V_{\text{basin}} = 24\text{ in.} \times 24\text{ in.} \times 14\text{ in.} = 8,064\text{ cu in.}$

• Step 2: Aggregate All Three Basins: $V_{\text{total}} = 3 \times 8,064\text{ cu in.} = 24,192\text{ cu in.}$ $V_{\text{total (gal)}} = \frac{24,192}{231} \approx 104.73\text{ gallons}$

• Step 3: Apply the 75% Operational Fill Factor: $V_{\text{effective}} = 104.73\text{ gallons} \times 0.75 = 78.55\text{ gallons}$

• Step 4: Determine Required Flow Rate ($Q$) for a 1-Minute Drain Period: $Q = \frac{78.55\text{ gallons}}{1\text{ minute}} = 78.55\text{ gpm}$

• Step 5: Compare Against Proposed 50 gpm Unit & Determine Correct Size: Proposed Unit: 50 gpm rating is significantly lower than the required 78.55 gpm flow rate! The proposed 50 gpm unit is DEFICIENT. Required Unit Selection: The plans examiner must require the designer to select the next standard commercial HGI size: 100 gpm flow rating. Required Grease Capacity: Under IPC Table 1003.3.5.1, a 100 gpm unit must have a grease retention capacity of not less than 200 pounds ($90.7\text{ kg}$).

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Flow Control Devices and Venting (IPC 1003.3.5.2 and 1003.9)

Under IPC 1003.3.5.2, grease interceptors must be equipped with devices to control the rate of water flow so it does not exceed the rated flow. The flow-control device must be vented and terminate not less than 6 inches above the flood rim level, or be installed per the manufacturer's instructions.

                    FIXTURE DRAIN
                          │
                          ▼
            ┌───────────────────────────┐
            │    FLOW CONTROL FITTING   │
            │                           │
            │  ┌───┐             ┌───┐  │
            │  │   │ RESTRICTOR  │   ├──┼── AIR INTAKE VENT
            │  │   │  ORIFICE    │   │  │   (Extends above flood rim
            │  └───┘             └───┘  │    or connects to vent header)
            └─────────────┬─────────────┘
                          │
                          ▼ METERED FLOW + AIR BUBBLES
                 ┌─────────────────┐
                 │   HGI CHAMBER   │
                 │                 │       OUTLET TRAP & VENT
                 │  GREASE  GREASE │              ▲
                 │  ░░░░░░  ░░░░░░ │              │
                 │                 │              │
                 └────────┬────────┘              │
                          │                       │
                          └───────────────────────┴──► TO SEWER

The Role of Vented Flow Controls (ASME A112.14.3 Type A)

1. Flow Velocity Restriction: The internal orifice plate chokes high-volume surges down to the certified gpm rating, preventing the interceptor chamber from being hydraulically blown out.
2. Air Entrainment (Aeration): The flow control fitting includes an integral air intake port connected to a vent pipe extending above the fixture flood rim. As wastewater rushes through the restrictor orifice, it generates a Venturi vacuum that draws atmospheric air into the stream. Microscopic air bubbles attach themselves to suspended fat droplets, dramatically increasing their buoyancy and accelerating separation inside the interceptor.
3. Venting the Interceptor (IPC 1003.9): Interceptors and separators must be designed so they do not become air bound, and must be vented by one of the Chapter 9 methods. Follow the manufacturer's instructions for whether the unit's own seal serves as the trap.
4. Direct Connection (IPC 1003.3.8): The discharge piping from a grease interceptor is directly connected to the sanitary drainage system.
5. Slope Upstream (IPC 704.1): Drainage piping upstream of a grease interceptor slopes at least 1/4 inch per foot under the 2024 IPC.

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Gravity Grease Interceptors (GGI) Design & Construction

Where high-volume food operations (such as casino buffets, banquet halls, or multi-restaurant shopping centers) generate thousands of gallons of oily effluent daily, small indoor HGIs are impractical due to frequent cleaning requirements. Designers specify outdoor subterranean Gravity Grease Interceptors (GGIs) per IAPMO/ANSI Z1001.

Core Requirements for GGIs

1. Capacity (IPC 1003.3.7): The required capacity is the peak drain flow into the interceptor in gpm multiplied by a retention time of 30 minutes. GGIs are designed and tested per IAPMO/ANSI Z1001, and installed per the manufacturer's instructions or, where none are provided, per ASME A112.14.6 and Z1001: $\text{Tank Volume (gal)} \ge Q_{\text{peak (gpm)}} \times 30\text{ minutes}$
2. Local Minimums: Many sewer authorities set a minimum GGI size (often 750 to 1,000 gallons) in their FOG ordinances. That is a local rule, not an IPC number.
3. Compartments and Baffles: GGIs built to Z1001 are compartmented and baffled so grease floats and solids settle before the outlet. Product data shows the arrangement.
4. Invert Drop & Baffle Elevations:
   • Inlet Baffle: Must extend downward into the lower third of the liquid depth to inject influent beneath the floating grease cap without disturbing the surface mat.
   • Outlet Baffle: Must draw water from the center third of the liquid depth—safely below the top grease cap and safely above the bottom sludge layer.
5. Access (IPC 1003.10): Access must be provided to each interceptor for service and maintenance, and interceptors are maintained by periodic removal of grease, scum, oil and solids. Outdoor GGIs therefore have manholes over each compartment, with covers rated for traffic where vehicles can reach them.
6. Sampling: Many sewer authorities also require a downstream sampling box for FOG testing. Check the local pretreatment ordinance.

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Fixture Connection Rules (IPC 1003.3.1 through 1003.3.4)

   MUST DISCHARGE TO THE GREASE INTERCEPTOR       MUST NOT DISCHARGE TO IT
   (IPC 1003.3.1)                                 (IPC 1003.2, 1003.3.1, 1003.3.2)

   ✓ Pot sinks                                    ✗ Food waste disposers (1003.3.2)
   ✓ Prerinse sinks                               ✗ Water closets, urinals and other
   ✓ Soup kettles or similar devices                sanitary fixtures
   ✓ Wok stations                                 ✗ Any waste that does not need
   ✓ Floor drains or sinks into which               treatment or separation (1003.2)
     kettles are drained
   ✓ Automatic hood wash units
   ✓ Dishwashers without prerinse sinks

Key Rules

1. What connects (1003.3.1): Fixtures and equipment with grease-laden waste in food preparation areas, including pot sinks, prerinse sinks, soup kettles, wok stations, floor drains or sinks into which kettles are drained, automatic hood wash units and dishwashers without prerinse sinks. Interceptors and removal devices receive waste only from fixtures and equipment that allow fats, oils or grease to be discharged.
2. Food waste disposers (1003.3.2): A food waste disposer shall not discharge to a grease interceptor. Ground food solids fill the interceptor and displace grease capacity.
3. Additives (1003.3.3): Dispensers of performance additives are prohibited, except microbial systems for aerobic bioremediation or for inhibiting pathogens by anaerobic methods. These are allowed only where the interceptor manufacturer permits them and they conform to ASME A112.14.6. Systems that discharge emulsifiers, chemicals or enzymes are prohibited.
4. Dwelling units (1003.3.4): Not required for individual dwelling units or private living quarters.
5. Retrofits (1003.3.1): Where space prevents installing or replacing a grease interceptor, one or more may be installed on or above the floor, upstream of an existing grease interceptor.
6. Automatic grease removal devices (1003.3.6): Located downstream of each fixture or group of fixtures per the manufacturer, sized for all connected flows, with ready access.
7. Pot sinks without a separate trap (1002.1 exception): A hydromechanical interceptor may serve a combination food utensil sink of not more than three compartments without separate traps. The outlet must be within 30 inches vertically and 60 inches of developed length of the interceptor inlet, and the branch must have an emergency floor drain downstream that serves only the floor drain and the interceptor.

Dishwashers and Hot, Detergent-Laden Water

The 2024 IPC lists dishwashers without prerinse sinks among the equipment that discharges through a grease interceptor or automatic grease removal device. Where a prerinse sink removes the grease before washing, the dishwasher is not on that list. Hot water and detergents can emulsify grease, so designers place interceptors to handle that flow. Some sewer authorities set their own rules for dishwasher routing and water temperature, and the interceptor manufacturer may limit inlet temperature. Plans examiners follow the IPC unless a local amendment or pretreatment ordinance says otherwise.

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Plans Examiner Verification Checklist: Grease Interceptor Review

When evaluating food service tenant improvements or commercial kitchen plans:

• Required Fixtures (1003.3.1): Grease-laden fixtures and equipment in food preparation areas discharge to a grease interceptor or automatic grease removal device.
• HGI Sizing Calculations: Confirm fixture capacity calculations using 75% fill factor and appropriate 1-minute or 2-minute drain period.
• Grease Capacity (Table 1003.3.5.1): Grease retention capacity in pounds is at least twice the gpm rating.
• Flow Control (1003.3.5.2): A flow-control device, vented to at least 6 inches above the flood rim (or per the manufacturer).
• Venting (1003.9): The interceptor is vented by a Chapter 9 method and cannot become air bound.
• GGI Capacity (1003.3.7): Peak gpm × 30 minutes, built to IAPMO/ANSI Z1001.
• Prohibited Connections: No food waste disposers (1003.3.2), no sanitary fixtures, and no waste that needs no separation (1003.2).
• Upstream Slope (704.1): Piping to the interceptor slopes at least 1/4 inch per foot.
• Access (1003.10): Access for service and maintenance of every interceptor.