Which layer in a septic tank is formed by fats, oils, and greases?

The layer in a septic tank formed by fats, oils, and greases is known as scum. This layer floats on top of the liquid in the tank because the materials are lighter than water. Scum consists of substances such as cooking oils, soaps, and other organic matter that do not fully break down during the treatment process.

This layer can create problems if not properly managed, as it can hinder the septic system's efficiency and contribute to clogging in the drain field when scum overflows. In contrast, sludge, which is the layer found at the bottom of the tank, consists of heavier solids that settle out of the liquid waste. Other terms like crud and residual do not specifically refer to the layers formed by fats, oils, and greases in a septic tank, making scum the correct and precise answer in this context.