Researchers from Lund University in Sweden have discovered that our drinking water is to a large extent purified by millions of "good bacteria" found in water pipes and purification plants. So far, the knowledge about them has been practically non-existent, but this new research is about to change that.
http://phys.org/news/2015-12-pipes-millions-bacteria.html#jCp
Our water pipes crawl with millions of bacteria.