5.3 Grease Interceptors & Trap Sizing (Hydromechanical & Gravity)

5.3 Grease Interceptors & Trap Sizing (Hydromechanical & Gravity)

Quick Answer: Fats, Oils, and Grease (FOG) discharged from commercial kitchens cause severe sewer blockages and Sanitary Sewer Overflows (SSOs). Plumbing codes mandate two primary interceptor designs: Hydromechanical Grease Interceptors (HGIs), which are indoor units rated by GPM flow capacity and equipped with flow control fittings, and Gravity Grease Interceptors (GGIs), which are large outdoor underground concrete tanks rated by liquid volume (500–1,500+ gallons).

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Fats, Oils & Grease (FOG) Management in Commercial Plumbing

Commercial food service establishments (FSEs)—including restaurants, cafeterias, hotel kitchens, butcher shops, and bakeries—generate large quantities of warm liquefied grease. As wastewater cools in sewer lines, FOG solidifies, adhering to pipe walls and forming massive obstruction blockages known as "fatbergs."

Plumbing codes (UPC Chapter 10 and Iowa Administrative Code 481—Chapter 425) mandate that all liquid waste containing FOG pass through an approved interceptor prior to entering the building drainage system.

Fixtures Required to Connect to Grease Interceptors

• Commercial 3-compartment pot and pan washing sinks
• Pre-rinse sink stations
• Food waste disposers (where permitted by AHJ)
• Floor drains and trench drains in food prep and cooking areas
• Commercial wok stations and tilt kettles
• Mop sinks receiving kitchen floor washings

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Hydromechanical vs. Gravity Grease Interceptors

Journeyman plumber candidates must master the structural, operational, and code testing distinctions between HGIs and GGIs.

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Step-by-Step HGI Sizing Procedure (UPC Chapter 10)

HGIs are sized based on the total volumetric capacity of all connected sink compartments, adjusted for liquid displacement and drain period speed.

HGI SIZING MATHEMATICAL WORKFLOW:
1. Compartment Volume  = Length (in) x Width (in) x Depth (in)
2. Liquid Capacity     = Volume (cu in) / 231 (cu in/gal)
3. Actual Liquid Fill  = Liquid Capacity x 0.75 (Displacement Factor)
4. Total Sink Fill     = Actual Liquid Fill x Number of Compartments
5. Required GPM Rating = Total Sink Fill / Drain Period (1 min or 2 min)

Step-by-Step Calculation Formula

1. Calculate Volume of One Compartment (Cubic Inches): $\text{Volume} = L \text{ (in)} \times W \text{ (in)} \times D \text{ (in)}$
2. Convert Cubic Inches to Gallons: $\text{Gallons} = \frac{\text{Volume (cu in)}}{231}$
3. Apply 75% Liquid Displacement Factor: (Accounts for 2 inches of freeboard and displacement by food/utensils). $\text{Actual Liquid Volume} = \text{Gallons} \times 0.75$
4. Multiply by Number of Sink Compartments: $\text{Total Liquid Fill} = \text{Actual Liquid Volume} \times N$
5. Determine Required GPM Rating Based on Drain Period: $\text{Required GPM} = \frac{\text{Total Liquid Fill}}{\text{Drain Period (1 or 2 minutes)}}$

Worked Exam Example Problem

Problem: Size a hydromechanical grease interceptor for a commercial 3-compartment sink where each compartment measures 18 inches long by 18 inches wide by 12 inches deep, using a 1-minute drain period.

• Step 1: Volume per compartment $= 18 \times 18 \times 12 = 3,888\text{ cubic inches}$.
• Step 2: Gallons per compartment $= 3,888 / 231 = 16.83\text{ gallons}$.
• Step 3: Liquid fill at 75% $= 16.83 \times 0.75 = 12.62\text{ gallons}$.
• Step 4: Total fill for 3 compartments $= 12.62 \times 3 = 37.86\text{ gallons}$.
• Step 5: Required flow rate for 1-minute drain period $= 37.86 / 1 = 37.86\text{ GPM}$.
• Selection: Select the next standard manufactured interceptor size: 40 GPM (with an 80 lb grease retention rating).

Vented Air-Entraining Flow Control Device

An HGI must be fitted with an external flow control valve on its inlet pipe. This valve restricts incoming fluid velocity to match the interceptor's rated GPM capacity while drawing in ambient air. Air bubbles attach to microscopic grease droplets, floating them rapidly to the liquid surface. The flow control fitting must be fully accessible and vented to the atmosphere above the sink flood rim.

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Gravity Grease Interceptor (GGI) Sizing & Design Rules

GGIs rely on long liquid retention times (25 to 30 minutes) to allow FOG to rise to the top while heavy food solids settle to the bottom.

The meal-capacity sizing equation

[!IMPORTANT] Appendix H is not adopted in Iowa. Rule 481—425.1 adopts UPC Sections 101 and 102, Chapters 2 through 17, and Appendices A and M only. Appendix H, Private Sewage Disposal Systems, is therefore not part of the Iowa State Plumbing Code, and neither are the other unadopted appendices. The meal-capacity formula below is the standard industry sizing method used by manufacturers and by many municipal pretreatment ordinances, and it remains the right engineering approach — but on the exam, size a gravity interceptor from UPC Chapter 10 and from what the local pretreatment authority requires, not from a citation to Appendix H.

Gravity grease interceptors are commonly sized with the meal-capacity formula:

$\text{Volume (Gallons)} = M \times F \times R \times S$

• $M$ (Meals per Peak Hour): Total seating capacity multiplied by peak occupancy turnover rate.
• $F$ (Waste Flow Rate): 4.5 gallons per meal for full-service restaurant with dishwashing; 2.5 gallons for fast-food single-service establishment.
• $R$ (Retention Time): Standard 2.5 hours.
• $S$ (Storage Factor): 1.0 for 8-hour operation; 1.5 for 16-hour operation; 2.0 for 24-hour operation.

Structural GGI Code Requirements

1. Compartmentation: Tanks must feature at least two compartments. The first compartment must comprise 2/3 of total volume, and the second compartment 1/3.
2. Baffle Tees: Inlet and outlet pipes must feature sanitary tees extending vertically. The outlet tee must draw liquid from the middle water zone, extending down to within 12 inches of the tank floor.
3. Manhole Access: Tanks must feature minimum 20-inch diameter manhole access covers over each compartment, brought flush to finished grade with liquid-tight gaskets.

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Maintenance Rules, Temperature Limits & Prohibitions

Proper operation of grease interceptors requires strict adherence to maintenance limits and chemical prohibitions.

The 25% Maintenance Rule

Under plumbing codes and municipal pretreatment regulations, grease interceptors must be fully pumped out and cleaned whenever the combined depth of the top floating FOG layer and bottom settled solids reaches 25% of the total liquid depth of the tank. Operating beyond 25% capacity causes fluid velocity channeling, sweeping captured grease directly into public sewers.

THE 25% LIQUID DEPTH RULE:

 +-----------------------------------+ --- Top Liquid Surface
 | ~ ~ Floating FOG Layer ~ ~ ~ ~ ~  |  |  (Grease + Solids
 +-----------------------------------+  |   must NOT exceed
 |                                   |  |   25% of total depth)
 |        Clear Water Zone           |  | 
 |                                   |  |  Total Liquid Depth
 +-----------------------------------+  | 
 | . . Bottom Settled Sludge . . . . |  | 
 +-----------------------------------+ --- Tank Floor

Temperature Discharge Limits

Wastewater entering grease interceptors must not exceed 140°F (60°C). Temperatures above 140°F liquefy captured grease, allowing FOG to escape the interceptor in liquid form. Commercial high-temperature sanitizing dishwashers ($180^\circ\text{F}$) must not discharge into hydromechanical grease interceptors unless routed through an approved cooling method or separate gravity interceptor system.

Prohibited Additives

Chemical emulsifiers, bio-additives, enzymes, bacteria cultures, and heavy solvents are STRICTLY PROHIBITED from being introduced into grease interceptors. These chemicals break grease into micro-emulsions that pass through the interceptor. As the chemicals dilute downstream in city sewer mains, the grease re-solidifies, causing catastrophic municipal pipe blockages.