How a Grease Trap (Grease Interceptor) Works

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A grease trap is a chamber that sits in a kitchen’s drain line and slows the flow of wastewater so that fats, oils, and grease have time to separate out before the water reaches the sewer. It does not filter, heat, or chemically treat anything. It simply gives gravity a quiet place to do its job. Hot, greasy dishwater enters fast and turbulent, then slows down inside the chamber. As the water sits and cools, the grease rises, the food solids settle, and only the cleaner middle layer is allowed to leave.

That single idea, slowing the water down so it separates, is the whole device. Understanding it makes everything else about these units make sense: why they have baffles, why size and location vary so much, and why a unit that is run too fast stops working even when nothing is broken. This guide explains the mechanics. It does not cover how often a unit needs cleaning (see our guide on how often a grease trap needs cleaning (221)), why food businesses are required to install one (see our guide on why restaurants are required to have grease interceptors (220)), or how units are sized for a given kitchen (see our guide on how grease interceptors are sized for a commercial kitchen (223)).

The Separation Principle: Why Grease Floats and Solids Sink

Grease floats because it is lighter than water, and food scraps sink because they are heavier. That difference in density is the entire engine of the device. Fats, oils, and grease have a specific gravity of roughly 0.9, meaning they weigh about 90 percent of what an equal volume of water weighs, so they rise to the top. Most food solids are denser than water, so they drop to the bottom. The water itself ends up in the middle.

The catch is that this separation only happens when the water is calm and given time. In a drain line, greasy water moves too fast and stays too warm and churned-up for the layers to form. Warm grease stays dispersed through the water as tiny droplets, almost like a salad dressing that has just been shaken. The grease trap fixes both problems at once. By widening the path into a chamber, it drops the water’s speed, and by holding the water for a stretch of time, it lets the water cool. As the temperature falls, the grease thickens and the droplets coalesce and float. That is why a unit run too hard, or fed water that is too hot, passes grease straight through: the layers never get the still, cool conditions they need to form.

Inside the Chamber: Inlet Baffle, Retention Zone, and Outlet

Inside the box, three features do the work: an inlet baffle that calms incoming flow, a retention zone where separation happens, and an outlet that draws only from the clean middle layer. Each one exists to protect the calm.

Water arrives through the inlet, where a baffle or tee directs the incoming stream downward and slows its energy so it does not blast across the chamber and stir up what has already settled. From there the water enters the retention zone, the main body of the chamber, where it slows nearly to a standstill. This is where the layering takes place. Grease accumulates at the top as a thickening cap, solids build up as sludge on the floor, and a band of relatively clean water occupies the space between them.

The outlet is the clever part. Instead of skimming off the top, the outlet pipe draws from below the grease layer and above the settled solids, pulling only from that clean middle band. An outlet baffle or a submerged outlet tee blocks the floating grease from following the water out. Because the grease and solids have nowhere to exit, they stay behind and build up over time, which is exactly why the unit eventually needs to be emptied. The chamber is designed to keep what it captures, not to pass it along.

Hydromechanical Traps vs. In-Ground Gravity Interceptors

The terms grease trap and grease interceptor describe the same physics at two very different scales: small indoor hydromechanical units, and large outdoor in-ground tanks. The words are often used loosely, but the distinction is real and comes down to size, location, and flow.

A hydromechanical grease interceptor is the smaller, indoor type, often called a grease trap. It is compact, typically installed under or near a sink, and uses internal baffling, and sometimes air entrainment, to drive separation in a small volume. These units are tested and rated by the Plumbing and Drainage Institute under standard PDI-G101, which assigns each one a flow rating in gallons per minute at which it still removes the great majority of incoming grease. Common ratings run from about 10 to 100 gallons per minute, and units in this class are measured in tens of gallons of volume and tens of pounds of grease capacity.

A gravity grease interceptor is the large outdoor type. It is a buried tank, often 500 to 2,000 gallons or more, set in the ground outside the building. With far more volume, it holds the wastewater much longer, relies on plain gravity and dwell time rather than compact baffling tricks, and serves a whole kitchen or building rather than a single sink. The plumbing code generally points larger food-service operations toward this gravity type because the bigger the grease load, the more retention volume the separation needs. Where the line between the two falls in your building, and which one your code requires, is determined by the adopted plumbing code and your local authority, not by a rule of thumb. Sizing and installation of either type are code-required design work for a licensed plumber or engineer.

Where the Interceptor Sits in a Kitchen’s Drain Line

A grease trap is plumbed between the greasy fixtures and the building’s main sewer connection, so that grease-laden water passes through it but other wastewater does not. Position is part of how the device works.

Only the drains that carry fats, oils, and grease are routed through it: pot and pan sinks, prep sinks, prerinse stations, woks, and floor drains near cooking equipment. Wastewater from restrooms and other non-greasy sources bypasses the interceptor and goes straight to the sewer. There are two reasons for keeping that separation. First, sending clean water through the unit would waste its capacity and disturb the calm pool that separation depends on. Second, sanitary waste from toilets is kept out of a grease interceptor by design.

An indoor hydromechanical unit usually sits close to the fixtures it serves, under the sink or nearby, where the water is still hot and greasy. A large gravity interceptor sits outside and downstream, taking the combined kitchen waste before it leaves the property for the public sewer. One more code detail matters here: a food waste disposer should not discharge into a grease interceptor without a separate solids interceptor ahead of it, because ground food would overwhelm the chamber with sludge. Exactly which fixtures must be routed through the unit, and how it ties into the building drain, is set by the plumbing code and belongs to a licensed plumber.

Flow Rate and Retention Time in Plain Terms

Flow rate is how fast water moves through the unit, and retention time is how long the water stays inside, and the second one is what actually makes the device work. A grease trap separates grease only if the water lingers long enough to cool and settle. Push water through too quickly and the retention time collapses, the layers never form, and grease rides straight out to the sewer.

This is why both rating systems are built around the speed-versus-dwell trade-off. A hydromechanical unit is given a gallons-per-minute rating, the flow at which it can still capture the bulk of the grease; feed it faster than that rating and its performance drops. A gravity interceptor is sized the opposite way, around a target holding time. A common code basis sizes the tank by multiplying the peak drain flow in gallons per minute by a retention time of around 30 minutes, which is part of why these tanks are so large. The exact sizing method and required retention vary by the plumbing code your area has adopted, so the binding number comes from your local code and authority. Either way, slowing the water and holding it is the entire point, and a bigger tank buys more holding time for the same flow.

Put simply, the device is a deliberate traffic jam for your dishwater. Grease and food solids are dropped off along the way, and only the cleaner water in the middle is let back onto the road to the sewer. The exact flow rating or retention volume your kitchen needs depends on its fixtures and use, which is sizing work for a licensed plumber or engineer, not a number to guess at.

This guide is general information, not professional or compliance advice. Have a licensed plumber or engineer handle the selection, sizing, and installation of any grease interceptor, and confirm requirements with your local plumbing authority and sewer district.

Frequently Asked Questions

What actually happens to the wastewater inside a grease trap?
The incoming greasy water slows down and cools inside the chamber. As it sits, grease rises and forms a cap at the top, food solids settle to the bottom as sludge, and a band of cleaner water collects in the middle. The outlet draws only from that middle band, so the grease and solids stay behind in the chamber until it is emptied.

Does a grease trap remove grease with chemicals or heat?
No. A grease trap is purely mechanical. It slows the wastewater and lets it cool so that grease floats to the top and solids settle to the bottom by gravity. There is no chemical treatment or heating involved, and the unit simply holds back what separates out.

Why does grease float to the top inside the chamber?
Because fats, oils, and grease are less dense than water, with a specific gravity around 0.9, they weigh less than the same volume of water and rise. Food solids are denser than water, so they sink. The water settles in between, and the outlet draws from that middle layer.

Does a home kitchen need a grease trap?
Under the International Plumbing Code, a grease interceptor is generally required for fixtures with grease-laden waste in commercial food preparation, such as restaurants and cafeterias, and is not required for individual dwelling units or private living quarters. Requirements vary by jurisdiction, so confirm with your local plumbing authority.

Why does a grease trap stop working if water moves through too fast?
Separation needs time. If water flows through faster than the unit is rated for, it does not stay long enough to cool and let the grease and solids separate, so grease passes straight through to the sewer. Matching the unit’s flow rating to the kitchen’s actual flow is what keeps it working.

Sources

  • International Code Council, 2021 International Plumbing Code, Section 1003.3 (Grease interceptors): https://codes.iccsafe.org/s/IPC2021P1/chapter-10-traps-interceptors-and-separators/IPC2021P1-Ch10-Sec1003.3
  • International Code Council, 2021 International Plumbing Code, Section 1003.3.1 (Grease interceptors and automatic grease removal devices, sizing and standards): https://codes.iccsafe.org/s/IPC2021P1/chapter-10-traps-interceptors-and-separators/IPC2021P1-Ch10-Sec1003.3.1
  • Plumbing and Drainage Institute, Standard PDI-G101, Testing and Rating Procedure for Hydromechanical Grease Interceptors: http://www.pdionline.org/certified-grease-interceptors/

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