Most surface waters contain bacteria, fungi, and algae.Culturing and counting the colony forming units per milliliter(CFU/ml) is bacteria are measured.
Municipal water treatment facilities typically add chlorine to kill microorganisms. In most water purification systems the chlorine is removed in the first step, which simply allows bacteria to multiply in the system.
Ultrapure water systems must have a 0.2 micron filter or a smaller absolute filter on the outlet. This prevents bacteria from contaminating the ultrapure water product needed. Also, all water pathways in the system being used should be sanitized regularly.