As the Sound Company at many festivals, including Alan Surtees' lovely Shrewsbury of course!!, I can tell you that when we clear the stage after each concert, there are always loads of half drunk bottles of water left by the artists. I have often thought, "What a waste this is".

One solution I saw a couple of years back (at Ely I think it was) was the provision of one of those office water coolers just off the back of the stage, with a supply of plastic cups. Artists could freely fill up when they wanted, and none got wasted.
The water came in a huge container - it was bought in I think, so pure, but could just as easily have been filled up from a handy tap. You could even wash up and re-use the plastic cups if you really wanted to conserve plastic. And no plastic bottles to be thrown away, and no half full bottles of expensive water to be thrown away either!

Worth a thought I would think.

G