Bio Sand FIlters are a alternative filtration method that cleans the water.
Layers of Gravel to Course Sand with at top coat of fine sand are placed in a vessel. In the top 1cm layer of the sand good bacteria developes after two weeks of being installled.
When bad water is added to the mix lots of the bad bacteria are eaten by the good bacteria. The water flows thru the sand with bad elements being removed as it goes.
Authough this method does entirely remove all the bad bacteria it has been known to remove about 95% of the bad stuff from the water.