such as bacteria and protozoa can use the small particles and dissolved
organic matter, not removed in primary treatment, as food. Secondary
or biological treatment is performed in a tank containing a "soup"
of starved microbes called activated sludge. Like us, these microbes
require air to live (they are aerobic
organisms) and thus air is pumped into the tank. Microorganisms
in this aeration tank
use the dissolved and particulate organic matter as food, producing
more microorganisms. Thus the waste materials that left the primary
clarifier are changed into microorganisms which can be collected
and separated from the water in the next step.
aeration tank showing air diffusers