@flu: When you install the undersink costco system, you have to attach the filter’s reject water line to your under-sink plumbing. This is the water that the reverse osmosis system uses to flush its filters and it is normally run through this tube and then down your sink drain. You can detach this tube pretty easily and just point it over a bucket to collect it like I did. Or if you are fancy, you can run a longer tube and deposit it in a bigger reservoir somewhere else.
I’m pretty sure even the reject water is safe for drinking / pets / watering plants, because it’s derived from city water that’s for drinking anyway and it’s not very concentrated due to the 1:4 efficiency… more like drinking a glass of water that had evaporated for a while. Also, it tasted fine.
