TYPE OF TANK is a broad term that can refer to a variety of tanks used for different purposes. A SEPTIC tank is a watertight chamber made of concrete, fiberglass, or plastic, used to store sewage and wastewater. GAS tanks are containers designed to store flammable gases such as propane or natural GAS. ISOLATION tanks are tanks that are used to contain hazardous materials, such as chemicals, in order to prevent them from entering the environment.