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My wife firmly believes coffee grounds must not be disposed of down the sink drain because they will cause a blockage in the waste pipes. She has it from family and from accounts on the world-wide web.

I am suspicious that this is an urban myth. Ground coffee beans might be no different from any other ground food waste which is disposed of down the sink drain, such as food waste which is ground in the under-sink garbage disposal. As with any food waste, surely they should be rinsed down with water so as not to choke the pipe, but I suspect coffee grounds pose no unique problem.

I know that at my place of employment, we rinse 1–2 cups of coffee grounds down the sink drain at least once daily, have done so five days a week for the eight years I have been employed there, and have not had a blockage caused by coffee grounds during that time. But this is only an anecdote.

Homeowners with septic tanks have to be careful what they can dispose of in the drains, and I have read opinions that coffee grounds can be incompatible with septic tanks, but never accompanied by supporting evidence.

Having performed a Google search for [ coffee grounds plug drain ] I see this is a widely held belief that generates much speculation and many anecdotes, and also that there are widely held counter-beliefs and counter-anecdotes e.g., coffee grounds help scour built-up fats out of the pipes or help eliminate odors (other than coffee odors, presumably). I conclude that factual answers to this question would be widely useful.

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Thank you for the edit, Flimzy. Note that the myth is not limited to the use of the garbage disposal; did you mean to change the scope of the question? –  MετάEd Nov 23 '11 at 22:42
    
I hope not. I've been at it for a year and a half in this apartment. 'Course I have a compost heap now, so I've quit, but... –  dmckee Nov 23 '11 at 22:53
    
Drinking many cups of coffee every day would cause me to be more concerned about the health of my digestive system and internal organs than a possibly clogged drain (which can quickly and easily be fixed). =O +1 for an interesting question. –  Randolf Richardson Nov 24 '11 at 1:21
    
Not an answer, but every time I've had to plunge the disposal it was because somebody put rice down it and didn't run enough water with it. I take the same precaution with coffee grounds. A little butter or bacon fat also helps to solidify it. –  Mike Dunlavey Nov 24 '11 at 2:23
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@Amma: I heard, au contraire, that the coffee can rub of particles of fat, which has a real ability to block the sink. The hot and liquid oily fat cools down in the sink, and sticks to the rand. It can attract coffee, which, for its color, will dominate your impression, if you clean the sink, but coffee alone can't block the sink. Use boiling water, to loosen the fat. –  user unknown Feb 3 '12 at 1:28
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1 Answer

There have been reported cases of coffee grounds blocking drains:

Morrisons cafe shut as coffee grounds block drain, By Claire French, April 12, 2012

A DRAIN blocked up with coffee grounds led to the closure of a supermarket cafe in Reigate for a day while the problem was sorted out.

The cafe at Morrisons in Bell Street was voluntarily shut by the store on Monday, April 2 following a visit by an environmental health officer from Reigate and Banstead Borough Council.

A drain at the cafe became blocked because of a build-up of coffee grounds and water in a pipe from the back of the coffee machines.

This is supported by opinion from professional drain cleaners:

As the grease builds up in the drain, other substances can get stuck in it. One of the most common of these substances is coffee grounds, but any other food stuff can get trapped.

These aren't the most definitive sources, but this doesn't seem like the sort of question that will be studied in a lab; my search for scientific sources was fruitless.

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The Morrisons café is an example at an industrial level, who knows the amount of coffee they drain within their pipes? Moreover, I'm not an expert in this but, disposal of their coffee grounds may not be performed automatically by their machines, in which case they probably have processes on how to manually carry out such task and they may not follow them properly. –  user853568 Apr 1 at 12:56
    
The professional drain cleaners don't look too professional to me. They have a website and so what? Their slogan is "We fix what your husband repairs", which is a bad obvious stereotype targeting a female population that would have even less of a clue than their DIYer husbands; therefore, they can pretty much say whatever they like and also throw in that old chestnut about coffee grounds. –  user853568 Apr 1 at 12:56
    
Their casual sexism isn't a reflection on their expertise - that is an ad hominem attack. I confess to an appeal to authority here. I call it is an "opinion" and emphasize that it isn't a definitive source. I am open about the weakness in the argument. Can anyone do better? –  Oddthinking Apr 1 at 23:34
    
The last sentence of the first quote strongly suggests the grounds came direct from the machine. –  Oddthinking Apr 1 at 23:36
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