capacities range from 25-150 tons per hour. The soil may be incorporated into the asphalt as a recycling process or the treated soil may be used for other purposes. Asphalt rotary dryers are normally constructed of carbon steel and have a soil discharge temperature of 300 °F to 600 °F. Typically, asphalt plant aggregate dryers are identical to the countercurrent rotary desorbers described above and are effective on the same types of contaminants. The primary difference is that an afterburner is not required for incorporation of clean aggregate into the asphalt mix. In some areas, asphalt plants that use petroleum-contaminated soil for aggregate may be required to be equipped with an afterburner. Thermal screw A thermal screw desorber typically consists of a series of 1-4 augers. The auger system conveys, mixes, and heats contaminated soils to volatilize moisture and organic contaminants into a purge gas stream. Augers can be arranged in series to increase the soil residence time, or they can be configured in parallel to increase throughput capacity. Most thermal screw systems circulate a hot heat-transfer oil through the hollow flights of the auger and return the hot oil through the shaft to the heat transfer fluid heating system. The heated oil is also circulated through the jacketed trough in which each auger rotates. Thermal screws can also be steam-heated. Systems heated with oil can achieve soil temperatures of up to 500 °F, and steam-heated systems can heat soil to approximately 350 °F. Most of the gas generated during heating of the heat-transfer oil does not come into contact the waste material and can be discharged directly to the atmosphere without emission controls. The remainder of the flue gas maintains the thermal screw purge gas exit temperature above 300 degrees F. This ensures that volatilized organics and moisture do not condense. In addition, the recycled flue gas has a low oxygen content (less than 2% by volume) which minimizes oxidation of the organics and reduces the explosion hazard. If pretreatment analytical data indicates a high organic content (greater than 4 percent), use of a thermal screw is recommended. After the treated soil exits the thermal screw, water is sprayed on the soil for cooling and dust control. Thermal screws are available with soil treatment capacities ranging from 3-15 tons per hour. Since thermal screws are indirectly heated, the volume of purge gas from the primary thermal treatment unit is