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Please do not put your grease down the sink.

BI Answers: Why can't I pour grease down the drain?

We've all been warned that pouring that delicious bacon grease down the drain is bad... but why is it bad?

The answer lies in the chemistry that happens after your wastewater is flushed from your pipes and delivered to the sewers: The fats in the grease and oil from your kitchen mix with the other chemicals in the sewers and form nasty conglomerations of chemicals that can build up and block the pipes that take our dirty water to the wastewater treatment plant.

According to a recent review of the subject, these fat and oil buildups caused about 47% of the up to 36,000 sewer overflows that happen annually in the U.S.

Here's how that goes down.

Grease + Sewer = Fatberg

When you pour grease into your sink it's just beginning its travels. The grease and oil head down your pipes and into the sewers where they meet up with all the other wastewater from the area. Here is where the nastiness starts.

These globs can build up in your home's pipes like this:

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But things get really nasty when these greasy globs reach the sewers and merge with everyone else's fat and oils.

The fats in the grease get broken down into their component parts — fatty acids and glycerol. These fatty acids bind calcium found in the sewers — created from biological processes including the corrosion of concrete — to create a "soap" compound.

When sewer levels rise high, these fat blobs glob onto the ceiling of the pipes, creating stalactite-type structures that are sometimes called "fatbergs." We've actually just recently been discovering how they come to be. A 2011 paper in the journal Environmental Science & Technology was the first to successfully form these deposits in the lab.

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Environmental Science & Technology

Fat, oil, and grease particles form in the presence of sewer water.

While it's called a "soap" because of its chemical composition, this isn't something you'd want to wash yourself with. These blobs of fatty compounds can become bus-sized — for example, here's a 17-ton fatberg from a British sewer:

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The Slatest

Giant fatberg in the UK.

These clogs block the sewer line and can cause disgusting and dangerous backups. While drain cleaners might clear out your pipes in your home, the greasy mess just gets washed into the sewers afterward, creating a bigger problem down the line.

In the UK, where the footage above is from, the team sent to clean up had to go down into the sewers and power-wash the sewers to dislodge and break apart the fatberg. It takes weeks to break these apart, according to the local water authority Thames Water.