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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.

Sklivvz
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MetaEd
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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? – MetaEd Nov 23 '11 at 22:42
  • 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 01:21
  • 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 02:23
  • Am I the only one wondering if it is a good idea from a cost perspective? Is it less environmentally and economically damaging to have the waste carried by truck to landfill, or pumped to a waste-water filtration pump (where presumably most of it is filtered out into landfill)? – Oddthinking Nov 24 '11 at 03:02
  • Restored original scope of question in title (not limited to the garbage disposal). – MetaEd Nov 24 '11 at 08:28
  • @MetaEd: No I did not intend to change the scope of your question. Sklivvz has reverted that part of my change. Sorry for the confusion. – Flimzy Nov 24 '11 at 15:41
  • I used to do this (= dispose coffee grounds in sink) and *did* get a drain blockage. The content of the blockage was undeniably coffee dregs. Now, it could have been that there were special confounding factors that aided this blockage but this is at least one case where blockage with coffee grounds occurred. – Konrad Rudolph Nov 26 '11 at 21:11
  • Affirmative. I found your question while researching a way to avoid a plumber bill. I did not know this could happen but my sink is hopelessly clogged after dumping the grounds. Nothing dissolves coffee that I can find therefore many articles advise against putting the grounds into your water pipes. –  Feb 02 '12 at 18:14
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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 03 '12 at 01:28
  • It wouldnt be difficult to actually test this myth would it? Who is willing to block their drains for science? – Tjaart Jun 28 '12 at 20:11
  • @Tjaart Quoting from the FAQ: "Due to the nature of Skeptics, the community needs to enforce the idea of no original research to encourage healthy voting. Users are required to reference all significant claims they make in their answers." In other words, Wikipedia rules. Answers should reference respected sources which use peer review. So yes, by all means test the myth, but get the results published in a respected journal before referencing it here. – MetaEd Jun 28 '12 at 23:01
  • I would not know this for coffee ground, but I do know for Tea. Do not rinse tea leaves down the sink. It will get clogged fairly easely. – Lyrion Jun 17 '13 at 07:59
  • @Oddthinking I don't know whether Vienna is special, but we have separate garbage collection for biodegradables, which are turned into compost. Coffee grounds definitely belong in there, not in the normal garbage or down the drain (the latter is actually pretty much unheard of in Austria). – Sebastian Redl Feb 26 '16 at 15:31
  • @MετάEd It's not quite Wikipedia rules, though, because Skeptics encourages references to primary sources, while Wikipedia prefers secondary sources. – Sebastian Redl Feb 26 '16 at 15:31
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    According to a disposal maker, coffee grounds don't cause issues with disposals or drains, but can cause problems on unclean plumbing [Insinkerator Blog](https://blog.insinkerator.com/what-not-put-garbage-disposal/). Several plumbing sites mention that coffee grounds are a form of sediment, & settle to the bottom of septic systems instead of breaking down [TheToiletZone](https://thetoiletzone.com/are-coffee-grounds-safe-septic/). The EPA says don't put coffee grounds(or other food waste) in a septic system [How Healthy is Your Septic System](https://cfpub.epa.gov/npstbx/files/RISWSeptic.pdf) – aVeRTRAC Aug 17 '19 at 22:45
  • Related questions [Can coffee grounds clean pipes when disposed in the sink?](https://skeptics.stackexchange.com/questions/37703/can-coffee-grounds-clean-pipes-when-disposed-in-the-sink/44728#44728) – aVeRTRAC Aug 17 '19 at 22:57

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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.

Oddthinking
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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. – Sparky Apr 01 '14 at 12:56
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    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. – Sparky Apr 01 '14 at 12:56
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    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 01 '14 at 23:34
  • The last sentence of the first quote strongly suggests the grounds came direct from the machine. – Oddthinking Apr 01 '14 at 23:36
  • I can support this, weakly, by saying that when our garbage disposal broke the plumber dispatched by the home insurance company blamed it on coffee grounds clogging the disposal, and said that it was therefore abuse and not covered by the insurance. On the other hand, these guys seem to have significant incentive to blame everything on abuse so their employer (the insurance) continues to send them out, so I take it with a grain of salt – iayork Feb 25 '16 at 18:25
  • This answer is both incorrect, but also poorly sourced. I can't post a better answer here because the question is protected, but will post it as a comment under the original. – aVeRTRAC Aug 17 '19 at 22:29