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4 Steps on How Grease Traps Work

Updated: May 21

Grease blockages cause 70% of sewer overflows. Blockages are very costly to locate, clean out, and clean up. Grease traps are the last line of defense. Let's talk about how grease traps work. In this blog, we are referring to gravity-based outdoor traps sized on capacity and retention. They are also known as grease interceptors.

How Grease Traps Work
How Grease Traps Work - Gainey's Wastewater Conference Product Demo

The Flow


Grease wastewater accumulates from kitchen sinks and floor drains. The grease wastewater is a mixture of soap, water, trash, and FOG (fats, oils, grease). The grease wastewater flows through the sewer drains to the outdoor grease trap. Now that the greasy water has made it to the trap, let's go into detail about how grease traps work internally.

How Grease Traps Work
How Grease Traps Work - The Flow

How Grease Traps Work


As the grease wastewater enters through the grease trap inlet pipe, the flow is buffered by a baffle. The baffle is a vertical barrier within the grease trap that helps slow down the flow of wastewater. The baffle also allows retention time for the grease to separate from the wastewater and float to the top. A crossover opening within the baffle structure allows the flow of wastewater from one chamber to another. These crossovers are near the bottom of the baffle closest to the clear water zone, where the water is the cleanest. After the water slowly passes through the crossovers, any remaining grease separates on the other side of the baffle. The sludge that sinks to the bottom of the trap contains any waste that makes it through the sink, for example, food and trash. The clean water travels through the outlet pipe to the sewage system to be treated. The FOG is efficiently captured and retained for later removal.

How Grease Traps Work
How Grease Traps Work

Design Considerations


During the design process, here are some things to check:

  • 6”-12” minimum space is needed from the bottom of the crossover to the bottom of the tank for sludge accumulation

  • The inlet pipe should be 6” underneath the water to avoid remixing the FOG back into the water

  • The inlet should be 2”-3” higher than the outlet for optimal gravity effectiveness

  • The grease trap must be vented to avoid any hazardous gases going back up the sewer line and into the restaurant

  • Make sure the retention time is a minimum of 30 minutes

    • Recommendation by Metcalf & Eddy, Inc.

    • Retention time is the amount of time it takes one particle of influent to travel through the system and discharge out

Balancing the Design


Size the grease trap large enough that when the fluid passes through the system it is slow enough to allow the grease to rise to the top and the heavier food particles to settle down to the bottom. If you have to choose between two sizes to purchase, go with the larger size. It might be a little bit more expensive now but will save you money on maintenance. The bigger, the better… that is how grease traps work!


Contact Us

If you need grease trap design assistance, please utilize our Grease Trap Sizing Calculator which is available to you 24/7! If you would like to learn more about this topic, please schedule a Lunch N Learn with us. You can also contact Cyndi if you have any questions. While you are here on our website, check out what we do!

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