Under a relatively low magnification of 121X, this digitally-colorized scanning electron micrograph (SEM) of an untreated water specimen extracted from a wild stream mainly used to control flooding during inclement weather, revealed the presence of unidentified organisms, which included bacteria, protozoa, and algae. In this particular view, numbers of what appeared to be rod-shaped sections of various sizes were scattered throughout the field of view, which though unconfirmed, may have been vegetative in nature, and possibly algal organisms. There were also patches of biofilm present as well.
Though many organisms found in untreated waters are harmless, there are many that are pathogenic to both humans and animals, including Giardia spp., free-living amebae, and E. coli just to name a few. During power outages, the functionality of treated water systems can be negatively affected, thereby, violating the patency of these closed systems. In these instances, advisories may be posted, encouraging inhabitants of these regions to boil their water tap water in order to kill off any pathogenic organisms that may have entered the system due to an intrasystemic drop in positive pressure.
Not only do water treatment plants remove such pathogenic organisms, but chemical that can be injurious to humans and animals are filtered from the water making it potable. Again, during power outages, or in the case of broken pipes, chemicals can contaminate tap water rendering it unsuitable to drink as well.
Thanks to the CDC's PHIL for this image.