Distinguish return activated sludge (RAS) from waste activated sludge (WAS) and explain their roles in process control.

• A. WAS is returned to the aeration basin.
• B. RAS is wasted to control solids.
• C. RAS is settled biomass returned to the aeration basin to maintain MLSS/SRT; WAS is excess sludge wasted to control solids buildup and maintain SRT.
• D. RAS and WAS are both discharged to off-site.

Answer: (C)

In an activated sludge system, controlling how long solids stay in the mix and how much biomass remains active is essential for stable treatment. Return activated sludge is the portion of settled biomass drawn from the secondary clarifier and returned to the aeration basin. This keeps the mixed liquor suspended solids (MLSS) at the design level and maintains the desired solids retention time (SRT) by recycling active microorganisms back into the basin. Without this return, biomass would be lost with the effluent, and treatment efficiency would fall as the microbial population ages or drops.

Waste activated sludge, on the other hand, is the excess biomass removed from the system and sent to wasting. This prevents solids buildup and ensures the SRT stays within the target range. If too little sludge is wasted, solids accumulate, raising MLSS and changing the microbial community in ways that can reduce treatment performance and settleability. If too much is wasted, there isn’t enough biomass to meet organic and nutrient removal targets.

By adjusting the rates of returning and wasting sludge, operators control MLSS and SRT to meet process goals. RAS maintains the active population and reactor capacity, while WAS trims excess solids to prevent overgrowth and maintain proper reactor balance.