6.3 Grease Interceptors & Hydromechanical Traps

Grease Interceptors & Hydromechanical Traps

1. Fundamentals of Fats, Oils, and Grease (FOG) Management

Wastewater discharged from commercial food service establishments—such as restaurants, institutional cafeterias, bakeries, and food processing plants—contains high concentrations of Fats, Oils, and Grease (FOG). When hot, greasy wastewater enters drainage piping, it cools below its congealing temperature (typically $110^\circ\text{F}$ to $120^\circ\text{F}$ / $43^\circ\text{C}$ to $49^\circ\text{C}$). FOG solidifies on pipe walls, reacts with mineral salts to undergo saponification (forming hard soap deposits), and severely restricts sewer pipe capacity, leading to Sanitary Sewer Overflows (SSOs).

To protect municipal sewer infrastructure, model plumbing codes mandate that all fixtures receiving grease-laden waste—including 3-compartment pots-and-pans sinks, pre-rinse sink stations, wok stoves, floor drains in food prep areas, and automatic dishwashers—must discharge through an approved Grease Interceptor before joining the sanitary building drain.

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

Plumbing codes divide grease removal systems into two distinct technological categories:

┌──────────────────────────────────────────────────────────────────────────┐
│               HYDROMECHANICAL (HGI) VS. GRAVITY (GGI)                    │
├──────────────────────────┬───────────────────────────────────────────────┤
│ Feature                  │ Hydromechanical (HGI) │ Gravity (GGI)         │
├──────────────────────────┼───────────────────────┼───────────────────────┤
│ Primary Standard         │ ASME A112.14.3 / PFI  │ IAPMO/ANSI Z1001      │
│ Sizing Metric            │ Flow (GPM) & Cap (lbs)│ Volume (Gallons)      │
│ Typical Location         │ Indoors (near sink)   │ Outdoors (underground)│
│ Retention Time           │ Short (< 5 minutes)   │ Long (20 to 30 min)   │
│ Flow Control Fitting     │ Mandatory (Vented)    │ Not Required          │
│ Separation Principle     │ Air entrainment/baffle│ Gravity flotation     │
└──────────────────────────┴───────────────────────┴───────────────────────┘

1. Hydromechanical Grease Interceptors (HGIs)

HGIs are compact units installed inside the building, either above floor under sink compartments or recessed into floor slabs. HGIs are tested and certified under ASME A112.14.3 or PFI G101 standards:

• Flow Rating & Capacity: HGIs are rated by liquid flow rate in Gallons Per Minute (GPM) (standard sizes: 7, 10, 15, 20, 25, 35, 50, 75, and 100 GPM) and grease retention capacity in pounds (lbs).
• The 2-to-1 Capacity Rule: Under ASME A112.14.3 testing, an HGI must hold a minimum of 2 pounds of grease for every 1 GPM of certified flow rating. For example, a 20 GPM interceptor must have a certified grease capacity of at least 40 lbs of grease before breakdown occurs.
• Air Entrainment Mechanism: HGIs rely on a vented flow control device equipped with an air intake. As water passes through the orifice of the flow control fitting, venturi action draws air into the liquid stream. Tiny air bubbles attach to floating grease globules, increasing their buoyancy according to Stokes' Law and causing them to rise rapidly into the top grease reservoir.

2. Gravity Grease Interceptors (GGIs)

GGIs are large multi-compartment concrete, fiberglass, or polyethylene vaults installed underground outdoors. GGIs are governed by IAPMO/ANSI Z1001 standards:

• Volumetric Retention Sizing: GGIs are sized strictly by total liquid volume in gallons (typically 500 to 5,000+ gallons).
• Retention Time: GGIs rely purely on extended hydraulic retention time—typically 20 to 30 minutes of quiet flow—allowing low specific gravity FOG (0.85 to 0.92) to float naturally to the surface while heavy food solids settle into the bottom sludge chamber.
• Baffles and Dip Tubes: GGIs feature interior baffle walls and submerged outlet dip tubes extending into the clear water zone between the top floating grease layer and the bottom sludge layer.

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3. HGI Sizing Calculations by Fixture Volume

To size an indoor Hydromechanical Grease Interceptor (HGI) for a commercial kitchen sink setup, journeymen use the fixture capacity formula under IPC and UPC guidelines.

                          FIXTURE DRAINAGE VOLUME FORMULA
   ┌──────────────────────────────────────────────────────────────────┐
   │ 1. Calculate Internal Volume (cu in): V = Length x Width x Depth │
   │ 2. Convert to Gallons: V_gal = V_cu_in / 231 cu in/gal           │
   │ 3. Apply 75% Fill Factor: V_actual = V_gal x 0.75                │
   │ 4. Determine Flow Rate: Q_GPM = V_actual / Drain Time (1 or 2 min)│
   └──────────────────────────────────────────────────────────────────┘

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

A commercial kitchen features a 3-compartment pot sink. Each compartment measures 18 inches long, 18 inches wide, and 12 inches deep.

• Step 1: Calculate Volume of One Compartment: $V_{\text{comp}} = 18 \text{ in} \times 18 \text{ in} \times 12 \text{ in} = 3,888 \text{ cubic inches}$

• Step 2: Calculate Total Volume for 3 Compartments: $V_{\text{total}} = 3 \times 3,888 = 11,664 \text{ cubic inches}$

• Step 3: Convert Cubic Inches to Liquid Gallons: $V_{\text{gallons}} = \frac{11,664 \text{ cu in}}{231 \text{ cu in/gal}} = 50.49 \text{ gallons}$

• Step 4: Apply the 75% Displacement Fill Factor: Sink compartments are rarely filled to the top rim due to displacement and overflow guards. $V_{\text{actual}} = 50.49 \text{ gal} \times 0.75 = 37.87 \text{ gallons}$

• Step 5: Determine Flow Rate ($Q$) based on Drain Down Time:

  • For a 1-minute drain period (fast drainage): $Q = \frac{37.87 \text{ gal}}{1 \text{ min}} = 37.87 \text{ GPM} \rightarrow \text{Select a } \mathbf{50 \text{ GPM / 100 lb HGI}}$
  • For a 2-minute drain period (standard drainage): $Q = \frac{37.87 \text{ gal}}{2 \text{ min}} = 18.93 \text{ GPM} \rightarrow \text{Select a } \mathbf{20 \text{ GPM / 40 lb HGI}}$

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4. Flow Control Devices and Vented Air Intakes

For an HGI to perform at its certified efficiency (typically $\ge 90%$ FOG removal efficiency), it must be equipped with an external flow control fitting installed on the inlet piping:

   Sink Fixture Tailpiece
           │
           ▼
   ┌───────────────┐ Air Intake Pipe (Extends above flood rim
   │ FLOW CONTROL  │◄───────────── or connects to vent stack)
   │   FITTING     │ Venturi Air Entrainment
   └───────┬───────┘
           │ Regulated Flow Stream with Micro Air Bubbles
           ▼
   ┌───────────────┐
   │  HGI Vessel   │ (FOG Flotation Accelerated)
   └───────────────┘

1. Flow Velocity Restriction: The internal orifice plate of the flow control fitting restricts the maximum instantaneous flow rate entering the interceptor so it does not exceed the unit's rated GPM capacity.
2. Air Intake Connection: A vented air intake (minimum 2-inch line) must connect to the flow control fitting and extend either to the open atmosphere above the fixture flood level rim or tie into the building DWV vent stack. Air entrainment is essential to bubble formation for FOG separation.
3. Placement: The flow control valve must be installed between the fixture outlet and the interceptor inlet, positioned as close to the fixture as possible.

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5. Maintenance Regulations and The 25% Rule

Both IPC and UPC mandate regular maintenance schedules enforced by municipal pretreating inspectors based on the 25% Rule:

$\text{Total FOG Thickness} + \text{Bottom Sludge Thickness} \ge 0.25 \times \text{Total Liquid Depth}$

  ┌─────────────────────────────────────────────────────────┐ ◄── Top Cover Rim
  │ ▒▒▒▒▒▒▒▒▒▒▒▒ FLOATING FOG LAYER ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ │ ◄── 4 inches FOG
  ├─────────────────────────────────────────────────────────┤
  │                                                         │
  │                  Clear Water Zone                       │ Total Depth: 20 inches
  │                                                         │ 25% Limit = 5 inches
  ├─────────────────────────────────────────────────────────┤ Current Total = 6 inches
  │ ░░░░░░░░░░░░ BOTTOM SLUDGE LAYER ░░░░░░░░░░░░░░░░░░░░░░ │ ◄── 2 inches Sludge
  └─────────────────────────────────────────────────────────┘ ══ PUMPING MANDATED!

Why 25% Exceeded Triggers Failure

When combined floating grease and bottom solids exceed 25% of the total liquid height within the vault, the effective liquid volume in the middle clear water zone is severely reduced. Fluid velocity through the shortened channel increases dramatically, shortening retention time. High velocity pulls grease droplets straight out through the submerged discharge dip tube, short-circuiting the interceptor and releasing FOG into the municipal sewer line.

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6. Prohibited Connections and Code Violations

Model codes establish strict prohibitions regarding what fixtures may and may not connect to grease interceptors:

• Prohibited Discharges (Blackwater): Sanitary waste from water closets, urinals, bidets, or clinical sinks must never drain into a grease interceptor. Human waste introduces pathogenic bacteria and heavy solids that contaminate grease recovery and create severe public health hazards.
• High-Temperature Discharge Limits: Commercial dishwashers discharging water at temperatures exceeding $140^\circ\text{F}$ ($60^\circ\text{C}$) should not discharge directly into small HGIs without cooling, as high heat re-emulsifies trapped grease, causing it to flow through into the drainage system.
• Garbage Disposers / Food Waste Grinders: Food waste grinders must not discharge into an HGI unless an approved solids interceptor (lint/food scraper) is installed upstream to trap heavy ground organic matter before it enters the grease chamber.