How Grease Traps Work

How Does a Grease Trap Work?

Every commercial kitchen sends fats, oil, and grease (FOG) down the drain. A grease trap catches it before it reaches the sewer. The trap slows the wastewater down so the grease has time to float to the top and food solids have time to settle on the bottom. Only the clean water in the middle passes through the outlet pipe and into the sewer line.

That step matters because FOG does not stay liquid once it leaves a warm kitchen. It cools, hardens, and builds up along the inside of sewer pipes until the line closes off. FOG clogs are the leading cause of sanitary sewer overflows, which is why local governments require every food service establishment to run a working trap and keep it maintained.

What Is a Grease Trap?

A grease trap is a plumbing device that captures FOG and food solids from kitchen wastewater before it reaches the municipal sewer system. Lines from the three-compartment sink, the dishwasher, mop sinks, and the floor drain all run into it, so everything from food preparation gets filtered on the way out. A trap is not a septic tank. A septic tank treats all of a building’s wastewater on site, while a trap handles only the kitchen drainage system.

Small indoor units are usually stainless steel or coated steel and hold 20 to a few hundred gallons. Larger units sit underground outside and hold a thousand gallons or more.

Parts of a Grease Trap

• Inlet pipe: brings wastewater from the kitchen into the trap.
• Inlet T fitting: slows the flow of water and drops its temperature. There should be no cap on the inlet side.
• Baffle walls: metal barriers that hold the flow back on the inlet side so grease has time to rise. Their job is to keep FOG away from the outlet.
• Outlet fitting and cap: sits about 18 inches off the base of the trap so only water leaves for the sanitary sewer system.
• Outlet pipe: carries the filtered water out to the sewer line.
• Manhole covers: cap each side of the trap. They should be solid steel, never cement.

How a Grease Trap Separates FOG From Wastewater

1. Wastewater from sinks, dishwashers, and floor drains enters through the inlet pipe.
2. The inlet T fitting slows and cools the water, which lets the grease firm up and break away.
3. FOG rises to the surface, and food particles settle on the bottom. The water between the two layers moves through a transition pipe.
4. Baffle walls hold the floating grease back so it cannot follow the water to the outlet side.
5. Clean water passes the outlet baffle, exits through the outlet pipe, and enters the sewer system.

The trap does nothing to the grease after that. It only holds it, which is why grease trap maintenance belongs on a schedule.

Grease Trap or Grease Interceptor: What Is the Difference?

The terms get used interchangeably, and the equipment does the same job. Your local regulations decide which one your kitchen needs and how big it has to be.

• Hydromechanical grease interceptors are the compact indoor units rated by flow. They fill fast and need frequent cleaning, often every week or two in a busy restaurant.
• Gravity grease interceptors are the large tanks buried outside. Capacity buys time between services, but pumping takes a truck and a licensed hauler.
• Automatic grease removal units skim the top layer into a separate bin on their own. Solids still settle, so the tank still needs regular cleaning.

How Often Should You Clean a Grease Trap?

Most commercial kitchens land on a monthly or quarterly schedule, depending on trap size and how much grease the menu produces. The one-fourths rule is the standard to work from. Once FOG and solids fill 25 percent of the trap, it stops separating properly and needs service.

Skip it, and grease buildup shows up as foul odors and a slow drain, then a backup that shuts down service mid-shift. You end up paying for emergency work, costly repairs to the trap or sewer line, and whatever penalty comes with a discharge violation.

What Should Never Go Down the Drain

Proper waste disposal starts at the sink.

• Fryer oil, vegetable oils, and animal fats. Proper disposal means a collection container and a used cooking oil recycling program.
• Food scraps. A garbage disposal grinds food into pieces that still settle as solid waste in the trap.
• Drain additives and degreasers. They break grease into droplets that slip past the baffles and reform downstream.
• Sauces, dairy, and dressings. Anything that congeals as it cools behaves like grease.

Grease Trap Cleaning From Mahoney Environmental

Mahoney Environmental has cleaned restaurant grease traps since 1953. We are a licensed EPA recycler serving more than 90,000 foodservice customers nationwide, and we can handle grease trap cleaning and used cooking oil (UCO) collection on the same visit.

Our maintenance services cover scheduled cleaning built around your hours, rodding and jetting for blocked lines, repairs found during service, and a full inspection report on every visit for your compliance records.

A restaurant owner should not have to think about the trap between services. That is our job. Call Mahoney Environmental at (800) 892-9392 or contact us to set up a cleaning schedule.