After completing the tertiary settling stage, the wastewater is almost ready to be returned back into the environment. Up to this point, the main thrust of treatment has been with removing pollutants like solids, ammonia, and oxygen demand that can cause harm to the aquatic life downstream.

Since the treatment process uses naturally occurring bacteria to aid in the treatment there have been no steps taken to eliminate those bacteria. The disinfection process is the final treatment stage whose sole purpose is to eliminate the bacteria that we have depended upon so far.