Are coffee grounds harmful for a septic tank?
You will often hear someone say they have put coffee grounds into their drain for years without problems, but the general consensus is that it is not a good idea. Here’s why: coffee grounds do not decompose well in the septic system, and tend to build up as solids that have to be manually pumped out.
A better use for coffee grounds is to add them to a compost pile. Coffee grounds are acidic, and may be too acidic for direct application as mulch around acid-loving plants such as azaleas, but are fine for composting with plenty of other organic materials. Over the years I have heard of all types of ideas around how to keep the septic tank operating at a high level and some have suggested many other types of ideas such as salts, orange peels, and other somewhat unusual and weird notions. My advice is to put nothing in your septic tank and have a professional septic tank system company manage any maintenance required as issues present themselves. I have heard that if you have a skunk coffee grounds can be helpful as well as orange or lemon peals. So save those treatments for ridding yourself of such animals, but I certainly think you are barking up the wrong tree in employing coffee grounds in any plumbing situation.
I once got a call from a guy who was having some serious issues with his line work leading out from the septic tank into his yard. He thought it required great investigation and commenced to start digging some starter holes to take a look at whether the lines were working properly. Of course I told him it was not the approach he wanted to take and that he would more likely compromise the integrity of the septic system and that he should stop and bring in a company to evaluate the system if had issues. But he was having none of that and insisted his approach would net results and when I came out to check on his progress he had dug 8 to 10 holes looking for who knows what. Fortunately he stopped his efforts and got some professional help for his septic problems.