Theory:

Chemical Treatment of Wastewater
Sometimes the water may contain harmful chemicals even after treating it physically and biologically.
 
These chemicals are disinfected from the water by using disinfectants such as chlorine and Ozone. Hence, Chlorine gas or ozone gas is often introduced in this water to purify it chemically.
Schematic representation of wastewater treatment plant (WWTP)
2018123-175356914-3232-wwtp.png
 
Role of eucalyptus plant in cleaning water
The eucalyptus plants can absorb the water at a faster rate and release pure water vapour into the atmosphere. Thus they act as a natural purifier. Due to this it is suggested to  plant Eucalyptus trees along the sewage ponds.
 
eucalyptus-trees-239509_1280.jpg   eucalyptus-2711285_1280.jpg
Reference:
https://www.askiitians.com/revision-notes/class-7-science/wastewater-story/#