How a Grease Trap Actually Works

A grease trap is a holding box installed between your kitchen drains and the city sewer line. Its only job is to slow down the water long enough for grease to float to the top and food solids to sink to the bottom — so that neither one makes it into the public sewer system. That's it. The science is straightforward; the problems start when it fills up and nobody cleans it.

Every time water drains from your sinks, pots, and dishwasher, it enters the trap as a warm mixture of grease, food particles, and water. Inside the trap those three materials separate by density:

• Top layer — FOG (Fats, Oils, Grease): Grease is lighter than water, so it rises and stays near the surface. As it cools it solidifies into a semi-solid cap.

• Middle layer — Effluent water: The relatively clean water in the middle slowly passes through and continues to the sewer outlet.

• Bottom layer — Food solids: Heavier particles — rice, meat scraps, breading — sink and accumulate as a sludge layer on the floor of the trap.

Between the two main compartments sits a baffle wall (a divider with an opening below the waterline). Water has to travel under the baffle to exit, which forces it away from the grease cap on top.

There are two common configurations in Los Angeles kitchens:

Under-sink (interior) grease traps — These are compact units, typically 10 to 50 gallons, installed directly under a prep sink or dishwasher. They fill fast. A busy taqueria or sandwich shop might need service every one to two weeks. They're easier to access but easier to ignore, which is where most violations start.

Outdoor grease interceptors — These are larger in-ground tanks, commonly 500 to 2,000+ gallons, installed outside in a parking lot or alley. They're mandated by LA County for most full-service restaurants and high-volume operations. Because they hold more, they tolerate longer service intervals — but they also cause much bigger problems when they overflow.

Not sure which one you have or the difference between a trap and an interceptor? Read our comprehensive comparison.

Every shift, a small amount of FOG gets added on top of the existing cap. Food solids keep piling on the bottom. The middle layer of clean water gets narrower and narrower. Once the FOG layer reaches roughly 25% of the trap's total depth, the separation process starts to break down — grease starts riding out with the effluent water and entering the sewer line.

This is where the LA County Health Department's "25% rule" comes from. It's not an arbitrary threshold — it's the point where the trap stops doing its job and starts becoming a liability.

A full grease trap doesn't explode or make noise. The warning signs are subtle at first: drains slow down, a faint sulfur smell develops (hydrogen sulfide from anaerobic bacteria digesting the sludge), and eventually you start seeing greasy film in your sinks. By the time the smell is noticeable to customers, you're already overdue.

Worse, if the trap overflows or FOG bypasses the unit, it enters the city sewer and begins solidifying on pipe walls downstream. That's your problem now — and potentially your liability, since LA Sanitation can trace FOG blockages back to the source establishment.

When we service a grease trap, we pump out all three layers — not just the top FOG cap. That means vacuuming the sludge from the bottom, scraping the baffle walls, and doing a visual inspection of the baffles and lid gaskets before we close it back up. After service, the trap is back to near-full capacity and doing its job again. We document the gallons extracted and issue a waste manifest — both required by LACDPH for compliance.