Will emptying used coffee grounds into the sink drain damage my pipes? I've been using a French press and emptying the grounds into the sink afterwards The main reason for doing this is that I read somewhere that coffee grounds are a good way to deter pests like cockroaches. I figured washing the used grounds down the drain would keep them from setting up shop in the sink. It only recently occurred to me that this might not be such a good idea and I wanted to know from the more experienced coffee drinkers if I should stop doing this.
You don‘t need the advice of coffee drinkers, you need advice from a plumber (which I am admittedly not, but I have enough contacts in the industry) and a gardener to chime in.

In short: Don’t do it.  

Coffee grounds may smell nice and there is that persistent old wives’ tale that will claim that it has [some random positive effect]. It’s not true. Any plumber will tell you that it’s best to pour nothing except liquids down the drains. Coffee grounds are particles that may


clump together 
collect like sediment in traps
bind to other fatty residue
provide food to microorganisms that form that slimy and stinky biofilm in the pipes.


And yes, most plumbers will love you for the habit of washing down your grounds, because it keeps them in business. 

Coffee grounds are lovely additions for compost, so either simply add them to your garden compost or treat them like you would any other plant-based kitchen waste.