GUIDE

What Is a Fatberg, and How Does FOG Build Up in a Sewer?

A fatberg is a solid mass of congealed fats, oils and grease, known as FOG, fused together with wipes, rags and other debris inside a sewer pipe or a lift station wet well. FOG arrives from restaurant kitchens, food processing lines, dairy wash-down water and ordinary household drains, and it stays liquid only as long as the water around it stays warm. Once it cools, it congeals and reacts with calcium naturally present in wastewater to form a soap-like solid that clings to pipe walls and thickens with every new slug of warm grease that passes through. Left alone in anaerobic wastewater, wastewater without usable oxygen, that buildup does not break down on its own, and combined with debris it can grow into the kind of mass that makes headlines. This guide covers what a fatberg is, the widely reported Whitechapel fatberg found in London's sewers, how FOG gets into a collection system in the first place, why the buildup keeps growing once it starts, what utilities do to control it, and where biological treatment fits alongside those measures.

Congealed fat and grease harden into a mass a crew has to cut out by hand once it goes far enough.

A fatberg is a mass of congealed fats, oils and grease bound together with wipes, rags and other debris inside a sewer pipe.
Fats, oils and grease enter sewer systems from kitchens, restaurants, food processing operations and wash-down water.
Grease congeals as it cools and reacts with calcium present in wastewater, forming a soap-like solid that adheres to pipe walls.
A grease cap and the solids trapped beneath it consume dissolved oxygen in the surrounding wastewater.
Anaerobic wastewater, wastewater without usable oxygen, leaves fats, oils and grease largely undisturbed rather than breaking them down.
Grease interceptors, FOG discharge ordinances and routine jetting are standard measures collection systems use to control buildup.
Aerobic bacteria, bacteria that use oxygen, digest fats, oils, grease and organic solids when usable oxygen is available to them.

Published September 24, 2026

What is a fatberg?

A fatberg is a mass of congealed fats, oils and grease, FOG for short, hardened inside a sewer pipe and fused with wipes, rags, dental floss and other debris that should never enter a drain. FOG alone would only build a soft layer on a pipe wall; wipes and other material that will not break down give that layer something to grab onto, and the combination hardens into a mass that can feel as solid as concrete. A fatberg can form in a gravity sewer, a force main or a lift station wet well, anywhere flow is slow and debris collects long enough. Small FOG buildups are common almost everywhere; a true fatberg is the extreme, headline-making version of the same process.

The Whitechapel fatberg: how large can one get?

The best-documented example is the fatberg found in the Whitechapel sewer in London in 2017, widely reported at roughly 250 metres long and about 130 tonnes, a solid mass of grease, wipes and debris packed into a Victorian-era line. Crews needed about nine weeks of manual cutting to break it apart, working the whole time in a confined, hazardous space. It is an extreme case, not a typical one, but it shows what ordinary FOG buildup can become when it goes unmanaged long enough in a pipe with slow flow and heavy debris. Most sewer systems never grow anything close to that size, because routine maintenance interrupts the buildup first.

How does FOG get into a sewer in the first place?

FOG enters a collection system from ordinary sources: restaurant fryers and dish pits, food processing lines, dairy parlors and equipment being washed down, and household kitchen drains. It leaves the point of use as a warm liquid and stays a liquid as long as the surrounding water stays warm. Once it cools below body temperature, it congeals into a soft solid and reacts with calcium naturally present in wastewater, forming a soap-like solid that clings to whatever surface it meets first, usually a pipe crown or wall or the inside of a wet well. FOG entering the system is normal; what happens to it once it cools is what determines the problem.

Why does a fatberg keep growing once it starts?

Anaerobic wastewater, wastewater that has run out of usable oxygen, leaves fats, oils and grease largely undisturbed, because the bacteria that could digest that material have no oxygen to work with. The grease cap and the organic solids settled beneath it consume what dissolved oxygen remains, and once that is gone the bacteria still present strip oxygen from sulfate instead, and the sulfide left behind becomes hydrogen sulfide (H2S) gas, the source of the rotten-egg smell and of corrosion in concrete and steel. A pipe with a persistent grease problem is often the same pipe with the persistent odor complaint: both start with bacteria starved of oxygen, and an anaerobic pipe interrupts neither process on its own.

What do utilities do about FOG buildup?

Most utilities manage FOG with a layered set of controls, all of them necessary regardless of anything else running in the system. FOG discharge ordinances set limits on what restaurants and food processors can send into the collection system. Grease interceptors and traps installed at those sources capture grease before it reaches the sewer and need regular pumping and inspection to keep working. Utilities also inspect known trouble spots on a schedule, since certain pipe stretches and lift stations predictably collect grease faster than others. When a line does build up or block, jetting and mechanical cutting remove what has already solidified, the only way to clear material hardened past the point anything else moves it.

Where does biological treatment fit?

Aerobic bacteria, the bacteria that need usable oxygen to work, digest fats, oils, grease and organic sludge when that oxygen is available to them. In an anaerobic collection system those same bacteria are present but starved, so FOG entering the system simply accumulates. Supplying usable oxygen as far upstream as water and power allow gives a collection system the ability to digest FOG on the way to the plant instead of storing it. This runs alongside the controls above, not in place of them: an existing blockage or fatberg still has to be jetted or cut out first, since biological treatment only works on grease still soft enough for bacteria to reach. More on how that program targets FOG is on the FOG removal page.

FOG control measures compared

The table below sets the standard controls a utility already runs against an upstream oxygen program.

Option	What it does	Where it fits best	Limits
Grease interceptors and traps	Capture FOG at the source before it enters the collection system	Restaurants, food processing plants and other high-grease dischargers	Need regular pumping and inspection, and do nothing for grease already downstream
FOG discharge ordinances and inspections	Set discharge limits and hold sources accountable for interceptor upkeep	Utilities managing many grease sources across a service area	Depend on enforcement, and cannot remove grease already built up in the pipe
Jetting and mechanical cutting	Physically clears grease and fatbergs already solidified in a pipe	A pipe or wet well already blocked or nearly blocked	Addresses the blockage that exists, not the buildup still arriving from upstream
Upstream biological oxygen program	Supplies usable oxygen to bacteria already in the wastewater so they digest FOG along the way	A system where recurring buildup, not a single blockage, is the pattern	An existing blockage still needs jetting first, and it runs as a continuous service, not a one-time fix

Interceptors, ordinances and jetting manage FOG at its source and clear what has already solidified; none of that changes once a system also runs an upstream oxygen program. The program’s job is narrower: keep grease that gets past those controls from becoming the next blockage.

Summary

A fatberg is congealed FOG fused with wipes and debris into a mass hard enough to require manual cutting, as the widely reported Whitechapel case in London showed.
FOG builds up because anaerobic wastewater leaves grease undisturbed, and the same oxygen-starved conditions that let grease accumulate are what let hydrogen sulfide form.
Interceptors, ordinances, inspections and jetting remain necessary controls; an upstream biological oxygen program adds ongoing digestion of FOG in the pipe, alongside those controls rather than instead of them.

COMMON QUESTIONS

Straight answers.

What is a fatberg?+

A fatberg is a large, solid mass of congealed fats, oils and grease fused together with wipes, rags and other debris inside a sewer pipe or a lift station wet well. It forms when fat and grease from kitchens and wash-down water cool inside the pipe, congeal and bind to whatever solid debris passes through with them. Left long enough, a fatberg can narrow or block a pipe entirely, and cutting one out is typically a manual job.

How do fatbergs form?+

Fatbergs form when fats, oils and grease entering a sewer cool below body temperature, congeal and react with calcium naturally present in wastewater to create a soap-like solid. That solid sticks to pipe walls and thickens with every new slug of warm grease, trapping wipes and other debris as it grows. Anaerobic wastewater, wastewater without usable oxygen, leaves the grease undisturbed, so nothing in the pipe works against the buildup on its own.

What causes FOG buildup in sewer pipes?+

FOG buildup in sewer pipes is caused by fats, oils and grease from kitchens, restaurants, food processing operations and wash-down water cooling and congealing once they enter the collection system. The cooled grease sticks to pipe walls instead of continuing to flow, and each new slug of warm grease adds another layer on top of the last. Anaerobic conditions inside most sewer lines do nothing to break the layer apart, since the bacteria that could digest it have no usable oxygen to work with.

Why is grease in sewer lines also linked to odor and corrosion?+

Grease in sewer lines and hydrogen sulfide (H2S) odor share a root cause rather than one directly causing the other. A grease cap and the solids settled beneath it consume dissolved oxygen, and bacteria in that oxygen-starved wastewater strip oxygen from sulfate instead, which is how H2S forms. A pipe with a persistent grease problem is frequently the same pipe with the persistent odor and corrosion complaint.

How is a fatberg removed?+

Fatberg removal is typically a manual job: crews cut the mass apart by hand or with specialized equipment and haul it out in pieces, since a fatberg is usually too solid and too tangled with debris for jetting alone to clear it. Smaller, earlier-stage FOG buildup can often be jetted or mechanically cut before it reaches that stage. Preventing new buildup from forming is a separate matter from clearing what has already solidified.

Can biological treatment prevent fatbergs from forming?+

Biological treatment cannot dissolve a fatberg that has already solidified and trapped debris; a blockage that solid still needs to be jetted or cut out. What biological treatment can do is give aerobic bacteria usable oxygen so they digest fats, oils and grease while it is still soft, before it has the chance to build into that kind of mass. Supplying that oxygen as far upstream as possible is how a collection system digests FOG along the way instead of storing it.

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