Here's Why You Should Never Put Coffee Grounds Down The Drain
There seems to be something of a split when you talk to people about whether coffee grounds are safe to put down the drain or not. Half of the population promotes the idea, positing that the grittiness of the grounds actually helps to clear out the pipes and remove some of the food debris that is stuck in the plumbing system of the kitchen. Some also suggest that coffee grounds help in deodorizing some of the smells that result in bits of food collecting in the drain.
The other half of the population, though, avoids dumping coffee grounds down the sink like the plague.
The second half of the population is definitely onto something because coffee is just as likely to clog the pipes as any other food debris (via Delish). In fact, unlike the majority of foods, coffee grounds actually tend to clump together rather than dissolve in water, which ultimately makes the problem worse as it creates clogs and wrecks your day as you try to unplug your sink (via Mr. Rooter).
Alternatives to throwing away your coffee grounds
Given the fact that replacing the drain pipes under your sink can be quite pricey, anywhere from $100 to $350, it might well pay to take care of coffee grounds properly (via Home Guide).
While some coffee preparation methods make it easy to dispose of the ground (making coffee with a paper filter allows you to just toss the filter grounds straight into the trash), others can be more tricky. If you're using a pour-over filter, for example, you may have to whack the filter against the trash can to get all of the grounds into the can.
If you have a compost or yard waste bin, coffee grounds are a welcome addition. Even if you don't have such a system, you can simply use coffee ground in your garden as a fertilizer. Coffee grounds will help the microorganism environment in the soil, will improve drainage, and it also helps the soil retain water (via Gardening Know How).