In a septic tank, what happens to solids?

• A. They dissolve in the liquid
• B. They remain suspended evenly
• C. They settle at the bottom forming the sludge
• D. They rise to the top

Answer: (C)

Heavy solids are pulled down by gravity in a septic tank, so they settle to the bottom and form a sludge layer. Lighter materials like fats and oils rise to the top, creating a scum layer, while the middle liquid layer remains relatively clear and is where microbial digestion happens. This separation keeps solids from passing into the drain field and gives time for some decomposition to occur. Over time, the bottom layer accumulates and must be pumped out to maintain system function. The other ideas aren’t correct because solids don’t dissolve in the liquid, they don’t stay suspended evenly, and they don’t rise to the top.