Which of the following occurs in a septic tank?

• A. Oils and grease float to the top, forming a scum layer
• B. Solids settle at the bottom forming the sludge
• C. Solids and liquids are separated
• D. All of the above

Answer: (D)

The main idea here is how a septic tank uses gravity to separate waste into layers. In a functioning tank, oils and grease are lighter than water, so they float to the top and form a scum layer. Heavier, settleable solids drop to the bottom and form sludge. The liquid in the middle is relatively clarified and flows out to the drainfield as effluent. Because these three outcomes all occur inside a septic tank, the description that all of the above happens is correct. Keep in mind that solids and scum accumulate over time and the tank must be pumped periodically to maintain performance.