Learn & Understand

Fatbergs: Why Grease Is the Sewer's Worst Enemy

The grease trap calculator sizes the device that intercepts fats, oils, and grease from a commercial kitchen before they reach the sewer. The reason such a device is legally required, and why undersizing it is so costly, comes down to what grease does once it escapes into the pipes: it congeals, accumulates, and, in the worst cases, forms monstrous blockages that have become infamous in modern cities. Grease is, quietly, one of the great enemies of the sewer system.

The FOG Problem

Plumbers and utilities have a name for the culprit: FOG, for fats, oils, and grease. Poured hot down a drain, grease is liquid and seems harmless. But as it travels into the cooler pipes and sewers, it cools and solidifies, clinging to pipe walls and building up over time. Unlike food scraps that a system can carry away, congealed grease is sticky, persistent, and steadily narrows the pipe, catching more debris as it grows. A commercial kitchen produces vast amounts of it, which is why grease management is a regulated necessity for food businesses.

The Rise of the Fatberg

In large sewer systems, accumulated grease combines with other debris flushed down drains to form enormous congealed masses, dubbed "fatbergs", that can grow to astonishing size and weight, clogging major sewers and requiring costly, unpleasant removal. These masses form as grease binds with other materials into a hardened, rock-like conglomerate. The fatberg is the extreme, city-scale consequence of grease entering the sewer, and it dramatizes exactly why keeping grease out at the source matters so much.

How a Grease Trap Works

The grease trap exploits a simple physical fact: grease is lighter than water and floats. Kitchen wastewater enters a chamber where it slows down and is held for a time, and during that pause the grease rises to the top while the cleaner water flows out from below. The accumulated grease is trapped at the surface, to be periodically removed, while only relatively grease-free water continues to the sewer. The trap is a settling chamber that separates FOG by flotation before it can escape and congeal downstream.

Why Size Is Everything

The trap only works if the wastewater stays inside long enough for the grease to rise and separate, which is exactly why the calculator sizes the trap to the kitchen's flow. Too small a trap, and water rushes through too quickly, carrying grease straight past and out to the sewer, defeating the whole purpose. An undersized trap therefore fails silently at first, then loudly, as grease escapes to clog pipes and, ultimately, contribute to the fatbergs the system fears. Sizing the trap to give the grease time to float is the entire game, and getting it wrong is a costly mistake the calculator exists to prevent.

For the drain lines carrying the trap's effluent onward, use the Drain Slope Calculator; for the venting that keeps those drains flowing, the Drain Vent Pipe Size Calculator.