The septic tank is a watertight, underground container used for receiving, treating and settling human waste. The solids settle to the bottom of the tank and become sludge, while oils and other light materials float to the surface, forming a scum layer. Within the tank, anaerobic bacteria (bacteria that grow without dissolved oxygen) break down the solid wastes. When the septic tank is working properly, these bacteria can reduce solids by 50% to 60%. The liquid between the sludge on the bottom of the tank and scum on the top flows out of the tank into the dispersal area or drainfield, where the remainder of the biological breakdown and filtration occurs in the soil. The sludge and surface oils remaining in the septic tank need to be regularly pumped out and hauled away for proper disposal. Proper system design, installation, operation and maintenance will ensure that harmful biological organisms will be treated before reaching groundwater. A septic system is an important permanent part of a home or business which, when maintained and operated correctly, should last a long time.