A grease interceptor (sometimes called a grease trap, though the two are technically different devices) separates fats, oils, and grease (FOG) from wastewater before it enters the sanitary sewer system. FOG that reaches the sewer solidifies, accumulates, and causes blockages and sanitary sewer overflows. Most jurisdictions require grease interceptors for any food service establishment, and the sizing must be approved by the local sewer district before a plumbing permit is issued.

Grease Trap vs. Grease Interceptor

These terms are often used interchangeably, but they refer to different devices:

• Grease trap (point-of-use) — a small device (typically 8 to 100 GPM capacity) installed under or near individual fixtures like kitchen sinks or dishwashers. Located inside the building. Sized per UPC Section 1014.1 or IPC Section 1003.3.
• Grease interceptor (gravity) — a large underground tank (typically 500 to 5,000 gallons) installed outside the building. All FOG-producing fixtures drain to it before connecting to the sewer. Sized per UPC Section 1014.2 or IPC Section 1003.3.4.

Most sewer districts require an exterior gravity grease interceptor for commercial food service, not just an under-sink grease trap. The interceptor provides longer retention time and higher separation efficiency.

When a Grease Interceptor Is Required

Per UPC Section 1009.1 and IPC Section 1003.1, a grease interceptor is required for any establishment that produces FOG in quantities that could cause sewer blockages. In practice, this means:

• Required: Restaurants, cafeterias, commercial kitchens, bakeries, food processing facilities, grocery store delis, school cafeterias, hospital kitchens, catering operations
• Usually required: Bars (if they prepare food), coffee shops with food preparation, food trucks with fixed sewer connections
• Typically NOT required: Office break rooms, residential kitchens, retail stores without food preparation, bars that only serve beverages

Sizing Method 1: UPC Fixture Unit Method

The Uniform Plumbing Code (UPC Section 1014.2.1) provides a sizing formula based on the number and types of fixtures draining to the interceptor:

Interceptor Size (gallons) = D x GL x HR / 2

Where:

• D = number of drainage fixture units (DFU) connected to the interceptor
• GL = grease loading factor (1 for light, 2 for moderate, 3 for heavy grease production)
• HR = hours of operation per peak meal period

However, many jurisdictions have moved away from this formula in favor of the flow-rate method or the Plumbing and Drainage Institute (PDI) method because it often undersizes the interceptor.

Sizing Method 2: Flow Rate Method (Most Common)

The flow rate method sizes the interceptor based on the peak drainage flow rate in GPM. The interceptor must provide a minimum retention time to allow grease to separate and float to the surface.

1. Calculate the total flow rate of all fixtures draining to the interceptor. Use the fixture GPM values from the plumbing code (UPC Table 702.1).
2. Apply a simultaneity factor — not all fixtures run at the same time. Most districts use 75% to 100% of the total fixture flow rate, depending on the type of establishment.
3. Determine retention time — minimum 30 minutes for most gravity interceptors per ASME A112.14.6. Many districts require longer retention times.
4. Calculate interceptor volume: Volume (gallons) = Flow Rate (GPM) x Retention Time (minutes).

Common Fixture Flow Rates

Worked Example

A restaurant with the following kitchen fixtures draining to the interceptor:

• 1 three-compartment sink: 20 GPM
• 1 pre-rinse spray: 6 GPM
• 1 commercial dishwasher: 20 GPM
• 1 prep sink: 10 GPM
• 1 mop sink: 12 GPM
• 2 floor drains: 2 x 5 = 10 GPM

Total flow rate: 78 GPM. With a 75% simultaneity factor: 78 x 0.75 = 58.5 GPM. At 30 minutes retention: 58.5 x 30 = 1,755 gallons. Round up to the nearest standard size: 2,000-gallon interceptor.

Standard Interceptor Sizes

Installation and Site Design Considerations

• Access for pump trucks. The interceptor must be accessible to a vacuum pump truck for regular cleaning. The truck needs to park within 50 to 100 feet of the interceptor access covers. Design a paved access path that can support the truck's weight (typically 50,000 to 80,000 lbs GVW).
• Grade and flow. The interceptor is a gravity device. The kitchen waste line must enter at the top and exit at a lower elevation. The outlet connects to the building sewer lateral at a lower invert than the interceptor outlet.
• Minimum setbacks. Most jurisdictions require the interceptor to be at least 5 feet from the building and 5 feet from property lines. Confirm with local code.
• No connection to garbage disposal. Per UPC Section 1014.2.2, food waste disposers (garbage disposals) should not discharge through the grease interceptor. The ground food particles interfere with grease separation.
• Traffic loading. If the interceptor is installed under a parking lot or drive aisle, it must be rated for H-20 (HS-20) traffic loading. Standard interceptors are not rated for traffic — specify a traffic-rated unit or relocate outside the traffic area.

Maintenance Requirements

Grease interceptors must be pumped and cleaned regularly to function properly. Most sewer districts require pumping when the grease and solids layer reaches 25 percent of the liquid depth. In practice, this means pumping every 1 to 3 months for a busy restaurant. The sewer district may require a maintenance log and periodic inspection, and failure to maintain the interceptor can result in fines and enforcement action.