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(private septic systems and municipal facilities) typically form a floating layer in their primary settling tank, consisting of low-density liquids such as oils, buoyant solids, and soap foam. This is referred to as scum and is slowly decomposed by microorganisms, eventually falling to the bottom of the settling tank as part of the sludge. When private septic tanks are emptied of solids, the tank is typically vacuumed empty and the incompletely digested scum is added to the incompletely digested sludge, further adding to its aroma and bioactivity.

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