Activated sludge is a highly effective biological process used in wastewater treatment plants to remove organic matter and nutrients from wastewater. It involves the use of a mixed culture of microorganisms, primarily bacteria, within a suspended mass of floc. The floc consists of bacterial cells, organic matter, and other substances.

The process of activated sludge treatment can be divided into four main stages: aeration, settling, waste sludge removal, and sludge recycling.

1. Aeration:
During the aeration stage, wastewater is mixed with the activated sludge in an aeration tank. Oxygen is supplied to the tank through diffusers or mechanical agitation, which promotes the growth and activity of aerobic bacteria. These bacteria consume the organic matter present in the wastewater as their food source, thereby breaking it down into simpler forms.

2. Settling:
After the aeration stage, the mixture of wastewater and activated sludge flows into a settling tank, also known as a clarifier or sedimentation tank. Here, the wastewater is allowed to settle, and the activated sludge forms a layer at the bottom of the tank. The settling process removes the solid particles, floc, and excess activated sludge from the treated wastewater.

3. Waste sludge removal:
Once the wastewater has settled, the clarified effluent is removed from the top of the settling tank and discharged. The excess activated sludge, which is rich in microorganisms, is removed from the bottom of the tank. This waste sludge is either sent for further treatment or returned to the aeration tank to maintain the desired microbial population.

4. Sludge recycling:
A portion of the waste sludge is recycled back to the aeration tank, known as the return activated sludge (RAS). This recycling helps maintain a high concentration of microorganisms in the system, promoting efficient treatment. The remaining waste sludge, known as the waste activated sludge (WAS), is further treated to reduce its volume and stabilize its organic content before disposal.

Overall, activated sludge treatment is an effective and widely used biological process for wastewater treatment. It harnesses the metabolic activities of microorganisms, primarily bacteria, to remove organic matter and nutrients from wastewater, resulting in cleaner and safer water that can be safely discharged into the environment.