UV water purification is the most effective method of getting rid of bacteria in water. It effectively gets rid of 99.9% of waterborne microorganisms. It attacks their genetic core, enabling them from reproducing.
When water precipitates from the ocean it can purify the water somewhat but when it rains onto land the water trickles into the soil, the process of the water moving through the different minerals in the ground purify the water.
The water cycle can naturally purify water when the water vapor condenses. When Water vaporizes it leaves behind all the dirt and minerals that dissolved in the water, this process helps purify the water.