I always assumed the same… Turns out, a wastewater plant and treatment plant for drinking water look a lot more alike than I thought.

I’m pretty sure you could drink it, you’d just want to do it far enough away that you don’t have to think about where it came from.

It sounds like you’re probably good.

The slope itself shouldn’t matter as long as the drain hole at the bottom of the sink is still higher than the drain hole in the wall. You can also angle the p-trap itself a little bit, but it still needs to be able to hold enough water in the bottom to block the pipe.