The inspector will measure the scum and sludge layers with special tools.

Measuring the scum and sludge layers. The inspector will measure the scum and sludge layers with special tools inserted through the inspection port. A proper inspection will also include a visual observation of the scum and sludge layers. (The sludge layer is the heavier solids that have settled down to the bottom of the tank. The scum layer is made up of grease and light solids that float near the top of the tank.)