Where does the sludge primarily come from in this process?

• A. Secondary clarifier
• B. Primary clarifier
• C. Digester
• D. Aeration basin

Answer: (B)

Sludge is the solid material separated from the wastewater as it moves through treatment. The biggest amount of sludge in a typical process comes from the primary clarifier. This is the first stage where the incoming wastewater is allowed to sit, and the heavier, settleable solids drop out quickly to form primary sludge. That sludge is concentrated and sent on for thickening and digestion. The aeration basin does produce biomass as part of the biological treatment, but the volume of sludge generated there is usually smaller compared to what’s captured in the primary clarifier. The digester then stabilizes and reduces the overall volume of sludge from both sources. So, the primary clarifier is the main source of sludge in this setup.