How Grease Traps Work

What Is a Grease Trap?

A grease trap — also called a grease interceptor, FOG interceptor, or gravity grease interceptor (GGI) — is a plumbing device installed between a commercial kitchen's wastewater fixtures and the municipal sewer. Its single purpose is to intercept fats, oils, and grease (FOG) so they don't congeal inside sewer pipes and cause sanitary sewer overflows (SSOs).

According to the U.S. Environmental Protection Agency, FOG is the leading cause of sewer blockages nationwide. Every major American city therefore requires food service establishments to install grease traps and pump them on a regulated schedule. Browse our database of grease trap requirements for 246 US cities to find the exact rules for your location.

How a Grease Trap Works: The Physics

A grease trap exploits three properties of FOG to separate it from water:

1. Density. Most fats, oils, and grease have a specific gravity of 0.85 to 0.92, meaning they float on water (specific gravity 1.0).
2. Temperature. Warm FOG is liquid and mixes with wastewater in suspension. As the liquid cools inside the trap, FOG congeals and rises.
3. Residence time. Traps slow the flow rate so the water sits inside long enough for separation to occur — typically 30 minutes for gravity interceptors.

The trap is essentially a box with baffles that forces wastewater to take an indirect path. By the time water reaches the outlet pipe, it has spent enough time in the trap for FOG to float to the top and solids to settle at the bottom. The middle layer — relatively clean wastewater — exits to the sewer.

Components of a Grease Trap

Every gravity grease interceptor — regardless of size — contains the same basic parts. Understanding each component helps restaurant owners work with plumbers, haulers, and inspectors.

• Inlet pipe. Carries kitchen wastewater from sinks, dishwashers, pre-rinse stations, wok stoves, and floor drains into the trap. Most codes prohibit connecting toilets or urinals to a grease trap.
• Flow control device (air intake / vented fitting). Limits peak flow into the trap so wastewater doesn't bypass the FOG separation process. Required by both the IPC and UPC.
• Baffles. Internal walls that force water to travel a longer path and extend residence time.
• FOG storage zone. The upper portion where floating grease collects between pump-outs.
• Outlet pipe. Draws from the middle layer, bypassing both the FOG cap and the sludge layer.
• Access covers / manholes. Allow haulers to pump the trap and inspectors to measure the grease and sludge depths.
• Sample port (larger interceptors). Lets the city test effluent for BOD, TSS, and FOG concentration.

Types of Grease Traps

Grease traps come in two main configurations, and your building's size and layout dictate which type applies.

1. Passive Hydromechanical Grease Interceptors (HGI)

Also called "under-the-sink" or "point-of-use" grease traps. Small units rated 20 to 50 gallons per minute (GPM), installed directly beneath a 3-compartment sink or pre-rinse station. Capacity ranges from 20 to several hundred pounds of FOG. HGIs are common in coffee shops, small cafes, and food trucks where flow volumes are modest.

2. Gravity Grease Interceptors (GGI)

Large in-ground or exterior vault interceptors holding 500 to 5,000 gallons. Required for most full-service restaurants, commissaries, and institutional kitchens. GGIs are pumped by licensed haulers with vacuum trucks and typically sit in a parking lot, alley, or basement mechanical room.

How Grease Traps Are Sized

Sizing a grease trap is not optional — undersized traps overflow, back up into kitchens, and violate plumbing code. Two methods dominate in the United States, and which one applies depends on the plumbing code adopted by your state.

IPC Method (Drainage Fixture Units)

States following the International Plumbing Code (IPC) size interceptors based on drainage fixture units (DFUs). Section 1003.3.4 of the IPC sets minimum trap capacity based on fixture count, with a floor typically around 20 GPM and capacity sized to 2x peak flow. States including New York, Texas, Georgia, and Ohio use the IPC.

UPC Method (DWV Fixture Units + Factor)

States following the Uniform Plumbing Code (UPC) use Section 1014, which applies a grease interceptor sizing formula based on fixture drainage load and a retention time factor. California, Washington, Oregon, Minnesota, Idaho, and several other states use the UPC.

Most cities layer local ordinances on top of these codes that may require a minimum trap size regardless of fixture count — often 1,000 or 1,500 gallons for full-service restaurants. Our free grease trap sizing calculator handles both IPC and UPC math and outputs a recommendation in GPM.

The 25 Percent Rule and Pumping Schedules

Every grease trap must be pumped out regularly. Skip a pumping cycle and the FOG layer grows past the outlet pipe, pushing grease into the sewer. This is where the 25 percent rule comes in.

The 25 percent rule — often phrased as "the 25/50 rule" or "quarter rule" — requires that a grease trap be pumped before the combined FOG and settled solids reach 25 percent of the trap's total liquid depth. Many jurisdictions pair this with a 50 percent rule for sludge depth. If either threshold is exceeded at inspection, the establishment is out of compliance and subject to fines.

Pumping frequencies vary by city, ranging from every 30 days (most of Florida, much of Texas) to every 90 days (typical default) to annually for low-FOG establishments. Read our complete guide to the 25/50 rule and pumping schedules for jurisdiction-specific requirements.

Who Regulates Grease Traps?

Three regulatory layers apply to every grease trap in the United States:

1. Federal. The EPA sets pretreatment standards under 40 CFR Part 403, which require publicly owned treatment works (POTWs) to control non-domestic discharges, including FOG.
2. State. Each state adopts a plumbing code (IPC or UPC) that governs installation and sizing requirements.
3. Local. Cities and counties set pumping schedules, permit fees, record-keeping rules, and fines through their wastewater utility's FOG control program.

In practice, your local wastewater authority — often a city sewer department, county utility, or water reclamation district — handles day-to-day enforcement. Find your jurisdiction's specific rules in our state-by-state regulations database.

What Happens If a Grease Trap Fails?

A neglected grease trap causes cascading problems:

• Sewer backups. Grease flowing past the trap hardens inside pipes, eventually blocking flow and forcing sewage back up through floor drains.
• Sanitary sewer overflows (SSOs). Blocked mains can flood streets or overflow into waterways, triggering EPA Clean Water Act violations.
• Kitchen shutdowns. Health inspectors can order a restaurant to close until the trap is compliant.
• Fines and permit revocations. Fines range from a few hundred dollars per day in smaller cities to $25,000+ per day under federal Clean Water Act violations.

How to Stay Compliant: A 5-Step Checklist

1. Know your rule. Look up your city's pumping frequency, permit requirements, and fine schedule in our city database.
2. Schedule pumping in advance. Don't wait for inspections. Most cities require a licensed hauler with a signed manifest.
3. Keep records on-site. Store pumping logs, hauler manifests, and inspection reports for 3 years minimum (many cities require 5).
4. Train staff. Prohibit pouring grease down drains, installing disposals on FOG fixtures, or using hot water / degreasers to flush grease into the sewer.
5. Verify sizing annually. If you add seating, change the menu, or install new fixtures, your existing trap may be too small. Run the numbers again with our sizing calculator.

Frequently Asked Questions

Related Guides

Disclaimer: This guide is for general informational purposes. Grease trap regulations change frequently and vary by jurisdiction. Always verify current requirements with your local wastewater authority, plumbing inspector, or licensed plumber before making compliance decisions. Last updated: April 2026.