A wastewater treatment plant is basically a bug factory.

You are growing bacteria to clean the water.
90% of all the work that goes on in the secondary portion - i.e. the biological stage is the growth of a biomass to degrade organics and remove pollution. The bacteria are really what you want to look at. The higher life forms are nice, they indicate the age and health of the biomass, but the bacteria are the actual workhorses of the wastewater treatment system.
In this section, we will look at the basics of the typical bacteria found in your wastewater treatment plant.