The basic principle of the secondary clarifier is to separate what from what?

• A. Gases from liquids
• B. Solids from liquids
• C. Solids from gases
• D. Liquids from gases

Answer: (B)

The basic principle is gravity settling of suspended solids from the liquid. In a secondary clarifier, the mixture that comes from the biological treatment zone is kept nearly still so the heavier solids—biomass and other suspended particles—can settle to the bottom, forming sludge. The clearer liquid then overflows the weir at the top, leaving most of the solids behind. This separation of solids from the liquid is what the clarifier is designed to do.

Context helps: these settled solids (sludge) are typically pumped away for further processing, while the clarified effluent moves on for disinfection or discharge. The effectiveness depends on how well the solids can form and settle as flocs, the flow distribution across the tank, and keeping solids from being carried out with the effluent.

The other options don’t describe what happens in a secondary clarifier—the process isn’t about separating gases from liquids, solids from gases, or liquids from gases. The key action is the removal of suspended solids from the liquid.