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“I am a plumber. All my espresso pucks go down my sink drain. I just flush them down with plenty of water. Been doing it for many many years. NOTE: I'm on city sewer, not septic, so my advice is for that setup only. I do not know how coffee grounds behave in septic systems, but I suspect the bacteria in the tank would break them down, especially in modern septics with air injection and field batch-dosing systems.”
https://www.reddit.com/r/Coffee/comments/4zn4ef/pouring_your_grounds_down_the_sink/