00KITCHEN SCIENCE, UNDER THE STREET

Where the pan
fat goes.

A kitchen sink with a steel strainer in the plughole beside a frying pan resting on the draining board, in soft daylight — AI-generated image, not a photograph of a real person or place.

Fat that pours easily in the pan sets hard in the pipes.

Fats, oils and grease cool and cling to pipes on the way down. In the sewer they mix with wet wipes and react into a hard, soap-like mass. Water companies call the result a fatberg, and some weigh more than a hundred tonnes.

COOL ITthen bin itUpdated 19 October 2026

Check the sink

01Principles

Let it cool
Scrape into the bin
Wipe the pan first
Keep fat out of the drain

02THE SHORT ANSWER

Cool it,
bin it.

Hot water and washing-up liquid don't fix it.
Fat still clings once the water cools.

IN THE PIPE

It sets as it cools

Thames Water says water doesn't stay hot as it travels down the drain. When greasy water reaches cooler pipes, the fat hardens and sticks.

Check the sink

IN THE SEWER

Fat turns to soap

Fatbergs are not just cooled fat. The fats undergo saponification, a soap-making reaction that needs calcium, fatty acids, grease and water.

See the numbers

AT HOME

Jar, bin or food caddy

Water companies say to let fat cool, scrape solid fat into the bin or food waste, and pour liquid oil into a sealed jar or bottle for the bin.

What to do

03A QUICK SINK CHECK

What's going
down the sink?

Pick something you might do after cooking. The bar shows how much it adds to the risk of a blockage, according to water company guidance.

Based on Thames Water and Wessex Water guidance on fats, oils and grease. General information about drains.

Sink check

Choose an actionOil poured into a jarPan wiped with kitchen rollGrease with lots of hot waterGrease with washing-up liquidCooled oil straight down

Oil poured into a jarRight idea

Pour used oil into a sealable container such as an old jar or bottle and put it in the bin. Some councils also collect cooking oil for recycling.

Pan wiped with kitchen rollRight idea

Wiping pans and plates with paper towels before washing soaks up the grease, and the paper goes in the bin.

Grease with lots of hot waterMyth

Hot water doesn't melt grease for long. It cools further down the drain, and the fat hardens and sticks to the pipe.

Grease with washing-up liquidMyth

Washing-up liquid doesn't break it down. Thames Water says the fat still cools, clumps and clings to pipes.

Cooled oil straight downStill sticks

Even at room temperature, cooking oils don't dissolve. They float on water and cling inside pipes, building up just like solid fat.

Source: Thames Water, Cooking fats, oils and grease; Wessex Water, How should I dispose of cooking fat and oil?

04TWO NUMBERS WORTH KNOWING

Small spoonfuls,
giant blockages.

One number for a single fatberg,
one for a year of blockages.

120 tonnes

Weight of one fatberg in a trunk sewer

Stretching over a kilometre through a sewer 20 metres underground, it took two weeks to clear. Thames Water compared it to the size of a blue whale.

OXFORD, 2025 · THAMES WATER

75,000

Sewer blockages cleared by one company

Thames Water says it clears about 75,000 blockages a year at a cost of £18 million. Between January and early August 2025, 11,104 were fat, oil and grease blockages.

EVERY YEAR · THAMES WATER

In the same period Thames Water cleared 23,556 wet wipe blockages. Wipes and fat together are what bind a fatberg.

05FOUR HABITS FROM THE GUIDANCE

Keep the fat
out of the pipe.

None of these needs special kit. They are what UK water companies suggest for household kitchens.

Let it cool in the pan
Leave fat from roasting or frying to cool and set. Once solid, scrape it into the bin or your food waste caddy.
Jar the liquid oil
Pour used liquid oil into a sealable container, like an old jar or bottle, then put it in with your household waste.
Wipe before you wash
Use kitchen roll to soak up grease from pans and plates before they go in the sink, and bin the paper.
Fit a sink strainer
A strainer in the plughole catches food scraps that could build up with fat into a blockage. Check whether your council recycles cooking oil.

The inside of an old brick sewer tunnel lit by a torch beam, with a pale greasy crust along the walls — AI-generated image, not a photograph of a real person or place.

06A FATBERG IN A MUSEUM

The fatberg that
went on display.

A small glass jar of used golden cooking oil with a lid, standing on a kitchen worktop next to a frying pan — AI-generated image, not a photograph of a real person or place.

In September 2017 a fatberg about 250 metres long and weighing over 130 tonnes was found under Whitechapel in London.

Two pieces were cut off, dried for several weeks and shown at the Museum of London in 2018. Some of the rest was turned into biodiesel.

07Questions

Questions people ask

What is a fatberg?

A hard mass in a sewer made of congealed fat, oil and grease bound together with wet wipes and other items that don't break down when flushed.

Does pouring hot water after grease help?

No. Thames Water says the water doesn't stay hot down the drain, and when it reaches cooler pipes the grease hardens and sticks.

Is liquid cooking oil safe down the sink?

Thames Water says no: even at room temperature, oils don't dissolve. They float on water and cling to pipes, building up like solid fat.

Why are fatbergs so hard?

The fat undergoes saponification, turning into a solid, soap-like substance. It needs calcium, free fatty acids, fats, oils and grease, and water.

What should I do with leftover fat?

Water companies say to let it cool and scrape it into the bin or food waste, or pour liquid oil into a sealed container for the bin. Some councils collect cooking oil.

08Sources

Where the pan
fat goes.