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Here's The Terrible Thing That Happens When You Pour Grease Down The Drain

Here's The Terrible Thing That Happens When You Pour Grease Down The Drain
LifeScience4 min read

bacon grease cooking pan

Flickr user icanchangethisright

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:

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.

master.img 001

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:


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.

Where is it worst?

Fatbergs are more likely to form in areas with lots of restaurants, since there is more grease heading down into the sewers to create the deposits.

People who live in old or large apartment buildings should also be careful: Their grease is competing with the grease of everyone else who has ever lived in their building and on their block - even just a tablespoon per person can really add up when it all mixes together in the sewers. This could happen at any level - within the plumbing of a home, or at the neighborhood level. It's also possible that these soapy messes can block later stages of the water treatment processes at the city level.

When these huge globs happen, they can be really terrible to deal with if not caught early. The UK fatberg seen above was only discovered when locals started complaining that they couldn't flush their toilets. It could have been much worse. "If we hadn't discovered it in time, raw sewage could have started spurting out of manholes across the whole of Kingston," Gordon Hailwood, the local sewer guy said.

county clean fatberg image


THIS is why you put your grease in the trash, not the sink.

What to do instead?

If you make a lot of grease at one time, say, frying a turkey, some areas offer fat and oil recycling to get rid of it and actually turn it into useful biofuel.

For smaller amounts of grease, let it solidify in the pan or in a jar, then throw the solid grease in the trash can. Make sure to wipe the greasy pan or dish with a paper towel to soak up the rest. Try to get as much as possible of the grease and oil into the trash instead of the drain - just the little bit that washes out with the wastewater can cause problems over time, especially in areas with high populations like cities.

"People try to discharge their oil and grease properly, but over time, you can get a fair amount of oil and grease from washing pots, pans and dishware," Ducoste told LiveScience in 2011. "The cumulative impact could be substantial. It's that long-term consistent discharge of that oil and grease, even if it's a small amount at a time, which could lead to problems."

When you do accidentally get some grease in your pipes, you can go ahead and wash it out using boiling water and a mixture of vinegar and baking soda, according ot Scott English Plumbing. This will help push it out of your pipes, though it will still be able to coagulate in the main water system.

The best plan is not to let any grease get down the drain if you don't want the sewer coming out of your faucet.

This post is part of a continuing series that answers all of your "why" questions related to science. Have your own question? Email with the subject line "Q&A"; tweet your question to @BI_Science; or post to our Facebook page.