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. Sewage treatment is also performed in several steps that closely parallel water purification procedures. Primary involves collecting the sewage at the treatment plant and the removal of solids by sedimentation and precipitation. Secondary treatment involves reducing the concentration of the dissolved chemicals. This is a microbiological process.