The septic tank is the place where the first stage of wastewater treatment occurs. The conditions in the tanks are ideal for bacterial growth. The bacteria utilize the organic waste as food. After the bacteria digest the food, it settles to the bottom of the tank and is identified as sludge. Very little of the soaps, fats and grease are eaten by the bacteria and because they are lighter than water; these continue to float on top of the water. This build up is known as scum. The area between the sludge and scum is known as the clear zone. This is the zone of treated wastewater that moves out of the septic tank to the next component of the disposal system. Gases generated during liquefaction are normally vented through the buildings plumbing vents.