An underground storage tank for wastes from homes not connected to a sewer line. Waste goes directly from the home to the tank.Septic tanks are generally constructed from fiberglass or concrete materials with both an outlet and inlet pipe. The process of the septic tank is that the wastewater is treated organically by holding the water in the tank for a long enough time for the liquids and solids to separate. Septic tanks have a basic design and have a rectangular or round shape.