What's the best way to dispose of the acid once I've removed the aluminum from my endmill? Should it go down the sink, onto the ground, into the neighbor's dog water bowl, into a jar, back into the 1 gallon plastic bottle?
I keep a small amount of acid in a aspirin bottle for cleaning endmills.
When it is no longer effective I pour it into a plastic cup (usually a leftover large Burger King cup), and add water to dilute it. Then I neutralize the acid with baking soda. I let it sit then pour the water off the top down the drain. Leave the cup sitting till the solids in the bottom dry and throw the cup away.

Neutralize any spills with baking soda and water immediately. Muriatic acid will eat a hole in a concrete floor.

Don't get this on your hands as it will cause burns. Gloves and safety glasses. Read the warnings on the bottle. Be careful, muriatic acid is powerful stuff. I prefer to use phosphoric acid. Phosphoric acid is one of the ingredients in Coca-Cola which is why some plumbers recommend pouring a bottle down the drain once a month to help keep the pipes clean.

Bob