During sewage treatment process, ‘activated sludge’ is

Activated sludge is a mixture of microorganisms, mainly bacteria and some fungi, that forms in the secondary settling tank of a wastewater treatment plant. It is a result of the biological treatment process used to break down organic matter present in the effluent from the primary treatment.

In the secondary treatment process, the effluent from the primary settling tank is introduced into a settling tank (aeration tank or secondary clarifier). In this tank, air is continuously pumped into the mixture to provide oxygen, promoting the growth of aerobic microorganisms. These microorganisms form clusters or flocs and actively consume the organic pollutants in the wastewater, breaking them down into simpler, less harmful compounds. After a sufficient treatment time, the activated sludge is allowed to settle, and the clarified water, known as the treated effluent, is discharged or further treated if needed. Part of the settled activated sludge is returned back to the aeration tank, where it serves as a seed for new microbial growth and maintains the biological treatment process.