Grease Trap — How It Works, Cleaning, and Code Requirements

A grease trap is a plumbing interceptor device installed in a drain line to capture fats, oils, and grease before they enter the sanitary sewer system and cause blockages or damage.

Grease Trap diagram — labeled parts and installation context

What It Is

A grease trap works by slowing the flow of wastewater so that fats, oils, and grease — which are lighter than water — can float to the surface while heavier food solids settle to the bottom. The separated water in the middle layer then exits through an outlet baffle and continues to the sewer. Grease accumulates in the trap until it is pumped or cleaned out.

When grease enters the sewer system untreated, it coats the interior of pipes and can solidify downstream, forming large blockages known as fatbergs. These blockages are expensive to remove and can back up sewage into homes and businesses. Municipalities require grease traps on commercial food service establishments for exactly this reason.

In residential settings, grease traps are less common because kitchen grease volumes are lower, but some jurisdictions require them on properties with garbage disposals, high-volume cooking, or septic systems where grease loading can disrupt the septic process.

Grease traps must be cleaned regularly — typically every one to three months in commercial settings — or they lose their effectiveness. An overfull trap allows grease to pass through the outlet and into the sewer, defeating the purpose of the device.

Types

Passive hydromechanical grease interceptors (HGIs) are small units installed under sinks, relying on retention time and baffles to separate grease. Automatic grease removal units (AGRUs) use electric skimmers or heating elements to continuously remove grease and require less manual cleaning. Large underground grease interceptors serve entire commercial kitchens and are pumped by a service truck. Residential-scale units are compact and typically sit in a cabinet under the kitchen sink or nearby.

Where It Is Used

Grease traps are required in most commercial restaurant and food-service kitchen drain lines serving pot sinks, prep sinks, floor drains, and dishwashers. They are also used in institutional kitchens, cafeterias, and any facility that handles large quantities of fats and oils. Residential grease traps appear on properties with septic systems or where local code requires them.

How to Identify One

A grease trap is a rectangular or cylindrical container installed in the drain line under or near a sink. It has an inlet, an outlet, a removable lid for service access, and often an internal baffle or partition. A strong grease and food odor when the lid is opened and a visible floating layer of solidified fat are the clearest signs that the device is working — or overdue for cleaning.

Replacement

Grease traps last many years if cleaned properly, but corrosion, cracked bodies, failed baffles, or undersized capacity can require replacement. Replacement typically also involves reassessing the drain line layout to ensure the new unit is correctly sized for the volume of wastewater and grease it will handle.

Grease Trap — FAQ

How often does a grease trap need to be cleaned?

Commercial units typically need cleaning every one to three months depending on volume, while residential units under a sink may go six months between cleanings. The general rule is to clean the trap before it reaches 25 percent capacity with grease and solids, which prevents carryover into the sewer line.

Are grease traps required for residential homes?

In most jurisdictions, residential homes are not required to have grease traps unless they have a septic system or generate unusually high grease volumes. Some municipalities do require them for homes with garbage disposals connected to septic. Always check with your local building or health department.

What happens if a grease trap is not cleaned?

An overfull grease trap allows grease to bypass the device and enter the sewer line, where it can solidify and cause blockages. The trap itself may also overflow or develop odor problems. In commercial settings, failure to maintain a grease trap can result in fines from the municipality or sewer authority.

Can I pour hot water down the drain to clear a greasy trap?

Hot water temporarily liquefies grease and pushes it further into the system rather than capturing it. This practice can actually move grease past the trap and into the sewer. The correct approach is regular mechanical cleaning and removal of accumulated grease.

What is the difference between a grease trap and a grease interceptor?

The terms are often used interchangeably, but technically a grease trap typically refers to smaller, passive under-sink units while a grease interceptor refers to larger, often underground units serving an entire facility. Both work on the same principle of separating grease from wastewater by slowing flow and using baffles.