Secondary treatment is an essential step in the process of wastewater treatment. It focuses on removing the remaining pollutants and impurities from the water that are not completely eliminated during primary treatment. This stage involves the use of biological processes to break down organic matter and remove harmful substances.

Working of Secondary Treatment:
1. Sedimentation: After the primary treatment, the wastewater flows into a secondary treatment tank, where it undergoes sedimentation. In this process, the flow of water slows down, allowing the solid particles to settle at the bottom of the tank. This forms sludge, which is removed and further treated separately.

2. Aeration: In the next step, aeration takes place. The wastewater is mixed with air to provide oxygen for the growth of microorganisms. The microorganisms, such as bacteria and fungi, consume the organic matter present in the water as their food source. This biological decomposition of organic matter is known as biodegradation.

3. Activated Sludge Process: The most common method used for secondary treatment is the activated sludge process. In this method, the wastewater is mixed with a large population of microorganisms in aeration tanks. The microorganisms form a floc-like substance called activated sludge, which absorbs and breaks down the organic matter present in the water.

4. Settling Tanks: After the aeration process, the mixture of wastewater and activated sludge is transferred to settling tanks. Here, the activated sludge settles at the bottom, forming sludge, while the treated water moves to the next stage. The settled sludge is pumped back into the aeration tank to maintain a continuous supply of microorganisms.

5. Disinfection: The treated water from the settling tanks undergoes disinfection to eliminate any remaining pathogens or disease-causing organisms. Common methods of disinfection include chlorination, ultraviolet (UV) radiation, or ozonation. These processes kill or inactivate the harmful microorganisms, making the water safe for reuse or discharge into the environment.

6. Final Effluent: The water that has undergone secondary treatment is referred to as the final effluent. It is significantly cleaner and free from most pollutants compared to the raw wastewater. The final effluent can be further treated through advanced processes if required, depending on the intended use of the water.

Secondary treatment plays a crucial role in wastewater treatment as it effectively removes a large portion of the organic matter and harmful substances present in the water. This helps in protecting the environment and preventing the contamination of water bodies when the treated water is discharged.