What Is the Purpose of a Grease Trap?

The purpose of a grease trap is to capture fats, oils, and grease before they enter the public sewer. That is the whole job. Everything else a grease trap does comes back to that one function.

It sounds minor until you know what grease does once it gets past the trap.

What happens without one

Every sink, dishwasher, and floor drain in a commercial kitchen sends water down the same lines. In a foodservice kitchen, that water carries a lot of fats, oils, and grease, collectively abbreviated as FOG.

FOG is liquid when it is hot, but it doesn’t stay that way. As it cools inside your pipes, it hardens and clings to the pipe walls. Layers collect on top of one another, narrowing the pipe walls until water can no longer pass through.

The result? A backup. Sometimes that means a slow drain; sometimes it means wastewater coming up through the floor drains in the middle of a dinner rush. Either way, the kitchen stops, and that lost time coupled with a hefty repair bill means a lot of money out of your pocket.

A grease trap sits between your drains and the sewer, stopping FOG before the clogs can happen.

How a grease trap does its job

The mechanism is simple. Grease is lighter than water, so it floats. Food solids are heavier, so they sink.

When wastewater enters the trap, it slows down and cools. The FOG rises to the top and collects there. The solids drop to the bottom. The cleaner water in the middle passes out through the outlet and continues to the sewer the way it should.

The grease and solids stay behind until someone removes them. That last part is where a lot of kitchens get into trouble, so it helps to understand how often a grease trap needs to be cleaned before you commit to a unit.

Why every commercial kitchen needs one

There are three reasons, and they stack.

The first is legal. Nearly every municipality in the country requires foodservice establishments to install and maintain a grease interceptor. Skip it, or let it fail an inspection, and the fines follow.

The second is your own plumbing. A working trap is the difference between a routine cleaning schedule and an emergency plumber on a Saturday night.

The third is the sewer system everyone shares. Grease is the leading cause of sewer blockages, and those blockages push untreated sewage into streets and waterways. The trap in your kitchen is part of keeping that from happening.

Where automatic units fit in

Three types of grease interceptor are common in the United States. Small passive units sit under the sink and fill up fast. Larger interceptors made of concrete, steel, fiberglass, or plastic handle high volume and get pumped on a schedule. Automatic grease removal devices do the separating and move the grease into a separate container for you.

The Big Dipper is a point-source automatic unit. It handles the FOG right where it is generated instead of sending it down the line to a larger tank. For kitchens that produce more than a compact unit can keep up with, the Trapzilla interceptor holds a much higher volume in a smaller footprint than a traditional concrete tank.

Which one fits depends on your menu, your flow rate, and your local code. Our guide to what a grease trap is walks through the differences if you are still deciding.

Common questions

Does a grease trap remove the grease, or just hold it? A passive trap holds it until you clean it out by hand or with a pump truck. An automatic unit moves the grease into a separate collection container as it works. Either way, the grease has to leave the building eventually.

Is a grease trap the same as a grease interceptor? The terms get used interchangeably. Some codes call small indoor units traps and large outdoor units interceptors, but they serve the same purpose.

Do I still need one if I do not have a fryer? Yes. Grease comes off dishes, pots, and pans at the wash sink long before anything reaches a fryer. Most kitchens produce more FOG than they expect. There is more on this in our FAQ.

The purpose of a grease trap never changes. What changes is how much work it takes off your plate, and that is where the type you choose starts to matter.