I think you mean purifying water vs. water purification...in the Torah, there are several laws regarding impurity and how you can be purified.
Back then, there were a lot of ways of becoming impure: having leprosy from gossiping, being in the same room with a dead body, touching a dead body, touching something the dead body touched, drinking from a clay vessel that was open and in the same room that someone died in, going to a cemetery, killing someone in war, getting your period, touching someone who has their period, touching something a woman getting their period touched, etc. There are lots and lots more ways, I just can't think of them offhand.
It was important to remain pure, since if you were impure you couldn't enter the Temple and give sacrifices, which was a huge part of worship back then.
There were several ways of repurifying yourself, and most of them had to do with immersing yourself in water, called a mikveh. Nowadays, the only reason we use mikvehs is to repurify ourselves is after we get our period, since we can't have sex until we're pure again. (If you're curious, look into the laws of niddah.)
It speaks about this in the Bible in Numbers 5:1, but there are billions of other places it speaks about that I can't find.