If you want my honest opinion on this: don't waste the water. Just because it's there doesn't mean you should abuse it, you could ruin it for everyone. They might not charge now, but if you go using mega-litres on a weekly basis then they might reconsider. There are tonnes of countries and areas that would kill for any amount of water. Besides I doubt the pressure would be there to run anything of the likes of a water powered generator, it's not just the quantity but the force that powers traditional hydroelectricity.

I have a more resourceful idea. Go and buy a few hundred empty 1L water bottles if you're determined to use unneeded amounts of water on a large scale and then fill them with water and then give them out to people or if you really want too, find a way to send them to those in need (although could be expensive for transport). Think of a way to benefit someone else instead of pouring it down the sink.