If you plated 1 ml of milk that contained 30,000 microbes per ml then 30,000 colonies would emerge on the plate and that's far too many to count. Thus the 30,000 microbes is a large number of microbes, but none are pathogens. Be sure to count all colonies on the plates. You can use the colony counters to get a statistical average of the number of colonies. The colony counter has an illuminated grid divided into 1 sq.
Municipal water is purified in several steps.
The Giardia cyst is not killed by chlorination and that's why filtration is performed, to remove the Giardia cyst. Ground water can be harvested from springs and wells. Soil behaves as a filter. Some soils are better filters than others. Fine particle clay soils are the most efficient at removing microbes while course gravel soils are less efficient.