We have a large (100,000 ltr) underground cistern, and after the spring rain, as soon as it warms up, it gets a bit whiffy. To address the problem, I put about 5 kg of what's called Calce Vergine (rock lime?) in a bucket and lower it into the water on a rope. It bubbles away for a minute or two before being what I think is hydrated lime. I then shake/agitate the bucket in the water every couple on minutes until the white mush has disolved in the cistern water.
It's a technique that people have used here for hundreds of years, and works a treat.
Of course, I'm talking about stored water rather than transient water.
Phew! First post over.