The industrial and municipal waste waters are treated in Effluent Treatment Plant (ETP) prior to disposal in water bodies.
Generally the under-mentioned treatments are given in ETP:
(i) Primary Treatment:
This is a physical process, when the separation of large debris is done, which is followed by sedimentation in tanks or clarifiers of improved type.
(ii) Secondary Treatment:
This is a biological process and carried out by the activity of microorganisms. In this treatment, the waste water is pumped in shallow stabilisation or oxidation ponds, where the microorganisms (microbes) oxidise its organic matter.
This process results in release of carbon dioxide and formation of sludge also known as bio-solid. The sludge is continuously aerated to accelerate its oxidation.
Algae grown in the upper lighted zone of the wastewater provide aeration by generating oxygen.
(iii) Tertiary Treatment:
This is physicochemical process, which removes turbidity in wastewater caused by nutrients, such as nitrogen, phosphorus, etc., dissolved organic matter, heavy metals or pathogens.
This step involves chemical oxidation of wastewater by strong oxidising agents like chlorine gas, perchlorate salts, ozone gas and ultra-violet (UV) radiation.
After tertiary treatment, the wastewater is discharged into natural waters and may be used for irrigation.