A greasy scummy layer of deep tan or dark brown foam on an aerator tank is most likely due to:

• A. Young mixed liquor solids
• B. Old mixed liquor solids
• C. Plant received toxic load
• D. Brown color indicates everything is dying

Answer: (B)

When the mixed liquor in the aeration tank has aged, the biomass tends to produce more surface-active substances and extracellular polymers. These substances lower surface tension and help stabilize foams, so fats, oils, greases, and other organics can rise to the surface and form a greasy, tan to dark brown scum layer. In older mixed liquor solids, the community skew toward organisms that generate these stickier byproducts, making such a scummy layer more likely to appear. In contrast, young mixed liquor solids are more actively digesting organics and less prone to sustaining that greasy surface. Toxic load might cause stress signs, but it doesn’t specifically explain a persistent greasy scum layer, and brown color alone isn’t a definitive indicator that everything is dying.