Ans. (D) All of these.
Sewage treatment is defined as the process that removes contaminants from the municipal wastewater, having industrial and household wastewater, including sludge.
The secondary treatment of wastewater involves removal of colloidal and dissolve organic compounds. These compounds require oxygen for their breakdown processes. Thus, removal of these compounds decrease the BOD (biological oxygen demand) of water.
the tertiary treatment includes processes that improves the quality of treated water before releasing it to environment. It involves removal of excess nutrients present in wastewater, such as phosphorus and nitrogen as these nutrients enhance growth of algae and cause algal bloom.
Tertiary treatment also includes addition of disinfectants to remove microorganisms present in it.
Thus, the correct answer is option (D).
