Many people believe that coffee grounds should be poured down the sink to prevent clogging of the pipes. Some do not know what scientific explanation has this maneuver. Just they do it because someone said it or because they have read in forums and moreover it's simple to do.
Those who think they know the reason of this maneuver, commonly give two possible explanations. The first is that the coffee grounds is a very fine grain that acts as an abrasive, cleaning the pipes while passing through. The second explanation, less common, is that the coffee grounds is acid and eliminates the fat on the walls of the pipe.
Critics of the system point out that actually the coffee grounds are one of the causes of clogging and should never used to remove the obstruction. The plumbers are the greatest enemies of coffee grounds but their opinion is not usually taken into account since it is thought that by attacking the remedy what they try to do is defend their business. What would it happen if nobody calls a plumber to solve the cloggings?
The truth is that there are very conflicting opinions. Some people say they have spent years pouring the dregs of coffee without ever having to call the plumber and others just the opposite.
I took a year doing tests with two drains from the sink in my kitchen. After the tests I can safely say that the grounds do not serve to prevent or unclog the pipes, but the explanation of why for some people they seem to work fine and for others do not, it is far from simple.
I conducted the test based on two equal pipes, both fitted with PVC drains. In one of them I was neglected and poured small traces of grease and dirt, apparently easy to gobble down the sink. In the other I was careful and never poured grease or food remains, neither any coffee grounds.
After four months I started having problems with the drain where I poured the coffee grounds. It was obvious that much more water was swallowed by the "clean" pipe. I dismounted both pipes and I found that the "dirty" one had a patina of grease that had accumulated coffee powder and other solid substances so that the tube diameter was less, preventing the passage of water. In contrast, the "clean" pipe, although it had some fat, it was much less messy and diameter was hardly changed.
I cleaned the dirty pipe and the water flow smoothly again. It was obvious that the grain of coffee was too thin to be abrasive and that due to its light weight it had been trapped by the grease on the walls, amplifying the retentive effect. For the possible abrasive effect the sediment should be the size of sand grains and also be released through the tube at high speed (in fact released pressurized sand is used for polishing surfaces).
The acidity of coffee grounds, although I could not measure the PH, should not be enough. In fact I cleaned the dirty pipe pouring baking soda and vinegar, which was effective. The coffee grounds did not seem to cause the same effect, not even close.
But why are there people who says that the system work and another not? In my case, although one of the drains never received almost nothing of fat or coffee grounds, at the touch was clear it had fat. I figured that the more rough is the inside of the tube, the more fat accumulate and therefore more coffee grounds remain attached.
Thus I changed one of the drains of PVC by a copper pipe, while the other was still the original. By the copper tubing I continued pouring fat and also coffee grounds, while with the PVC one I was careful as I was before. After four months there was no problem, so I proceeded to dismantle the pipes and check the interior. Both were fairly clean and practically there was no coffee grounds stuck to the walls of the copper tubing. Logically, the roughness of the copper pipe is much lower than that of PVC pipe and that would explain why it had not obstructed.
At that time I tried an iron pipe instead of copper and in less than two months it was obstructed. The iron pipe is more rough than copper and PVC, which helps me to create the theory that the practice has confirmed.
Therefore to pour coffee grounds to the sink is not absolutely good for anything. In fact it is a serious cause of obstructions. If you have PVC piping or copper sink in the house, especially copper, it will clog very rarely, so you get the feeling that the system works. But if the pipes are made of iron, typically in the old houses, they clog very quickly and you have the certainty that the coffee grounds has not served you for nothing.
It is best to remove the sediment directly into the trash and problem solved.