What is the purpose of the final clarifier?

• A. To aerate the mixed liquor
• B. To settle MLSS
• C. To digest solids
• D. To disinfect effluent

Answer: (B)

In an activated sludge system, the final clarifier is where gravity does the work of separating solids from liquid. The mixed liquor that exits the aeration basin contains suspended solids (biomass and particulates). The clarifier allows these solids to settle, producing clarified effluent and a sludge layer. The settled solids can be returned to the aeration basin (return activated sludge) to maintain the biomass, while excess is wasted. This settling action reduces the solids content in the effluent, or MLSS, before any downstream treatment like disinfection. The other steps—aeration, digestion, and disinfection—happen in different parts of the treatment train, not in the final clarifier.