The activated sludge removed from the secondary treatment process (excluding biomass) is called which of the following?

• A. Mixed Liquor Suspended Solids (MLSS)
• B. Waste Activated Sludge (WAS)
• C. Return Activated Sludge (RAS)
• D. Mixed Liquor Volatile Suspended Solids (MLVSS)

Answer: (B)

Waste Activated Sludge is the excess sludge that is deliberately removed from the activated sludge process to keep the mixed-liquor solids at the desired level. In an aeration tank, solids include biomass and inert material, collectively called mixed liquor suspended solids (MLSS). To maintain proper treatment performance, a portion of the settled sludge is not returned to the reactor but wasted; that wasted portion is the waste activated sludge.

Understanding the other terms helps place this in context: MLSS is the total suspended solids in the mixed liquor, including both biomass and inert solids. MLVSS represents the volatile portion of MLSS, which corresponds to the organic, biomass part. Return activated sludge is the portion of settled solids that is returned to the aeration tank to maintain MLSS. So the sludge that's removed from the secondary treatment process (the excess biomass) is specifically called Waste Activated Sludge.