What is the name of the layer of scum that forms at the top of the septic tank?

The layer of scum that forms at the top of the septic tank is commonly referred to as the scum layer. This layer consists primarily of lighter materials such as fats, oils, and greases that rise to the surface due to their lower density compared to the liquid waste in the tank. The purpose of the scum layer is to trap gases produced during the decomposition of waste as well as to prevent those gases from escaping into the environment. The scum layer is an important component of the septic tank system, as it helps to maintain the effectiveness of the tank by allowing for anaerobic bacteria to break down organic matter within the tank.

The other terms provided, such as the sludge layer, effluent layer, and solid waste layer, refer to different aspects of the septic tank's function. The sludge layer typically forms at the bottom of the tank and consists of heavier solids that settle out of the wastewater, while the effluent layer is the treated liquid that exits the tank and moves to the drain field. Solid waste layer is a non-standard term that does not accurately describe a specific layer in the context of septic tank operations.