In how many stages the treatment of wastewater in sewage treatment plants is carried out? 

The correct answer is 3. The treatment of wastewater in STPs (Sewage Treatment Plants) is carried out in three stages: primary, secondary, and tertiary treatment.

Primary Treatment: The first stage of wastewater treatment is called primary treatment, which involves the physical removal of solid materials and other large particles. This is usually accomplished by passing the wastewater through a series of screens and settling tanks.

Secondary Treatment: The second stage of wastewater treatment is called secondary treatment, which involves the biological decomposition of organic matter. This is achieved by using microorganisms to break down the organic matter in the wastewater.

Tertiary Treatment: The third stage of wastewater treatment is called tertiary treatment, which is an additional treatment step used to further purify the water. This stage involves the removal of any remaining contaminants, such as nutrients and dissolved solids, through processes like filtration, chlorination, and UV disinfection.