Microorganisms
Surface water includes a variety of microorganisms, including bacteria, protozoa and algae.  Municipal water plants remove most of these organisms, but bacteria enter the feedwater in an unprotected water system through breaks in the system or contamination of the dispenser. Once in the system, the bacteria excrete a substance that adheres them to all available surfaces, and are difficult to remove from hard to clean surfaces.

To detect and count bacteria, it is usually necessary to run the water through a 0.45 micron filter and then culture the resulting waste material on a suitable medium for several days. 

Removing microorganism contamination from water.
Reverse osmosis, microporous filtration, ultrafiltration and ultraviolet oxidation are all excellent ways of removing bacterial and microorganism contamination from water.