The five main components of the activated sludge process include Aeration Tank, Secondary Clarifier, WAS, and which two components?

• A. Air supply and mixing
• B. Air supply and RAS
• C. Return Activated Sludge and aeration
• D. RAS and WAS

Answer: (B)

In the activated sludge process, keeping a healthy population of aerobic microbes and a steady oxygen supply is essential to keep treatment going. The air we pump into the aeration tank provides the oxygen that these microbes need to break down the organic matter in the wastewater. Without sufficient air, the microbes slow down or stop producing the desired breakdown.

The other missing piece is returning some of the settled solids back to the aeration tank. This is done with return activated sludge (RAS). Recycling part of the biomass from the secondary clarifier helps maintain the right concentration of microorganisms and the proper sludge age in the aeration tank, which keeps the system stable and effective. It also prevents too much biomass from being removed with the waste sludge.

So the two components that complete the set are the air supply for oxygenation and the return activated sludge to recycle biomass. This pairing ensures continuous treatment: oxygen supports the microbes, and returning sludge maintains the active population and treatment capacity.