What settles at the bottom of a septic tank as the biological action of bacteria proceeds?

In the context of a septic tank, the correct answer is sludge, which refers to the solid waste material that settles at the bottom of the tank as a result of the biological action of bacteria.

When wastewater enters the septic tank, the heavier solids are subjected to gravitational forces and settle at the bottom. This is where the anaerobic bacteria perform their work, breaking down these solids into simpler substances over time. The layer of solid waste that accumulates at the bottom consists primarily of decomposed organic materials and is referred to as sludge.

Scum, which is another term used in septic systems, actually refers to the lighter materials that rise to the top of the tank, such as grease and oils. While both sludge and scum are important components in the functioning of a septic system, sludge is specifically the material that settles at the bottom.

Therefore, understanding the roles of sludge and scum in septic systems is crucial for effective maintenance and operation.