A method for treating process material using a plurality of autoclaves, wherein each of the plurality of autoclaves cycles through the following steps: introducing a steam from one or more of the plurality of autoclaves to the interior of the vessel; increasing the temperature within the vessel by adding heat to the interior of the vessel using an indirect heat source; reducing the temperature and pressure within the vessel by venting a portion of the steam within the interior of the vessel to another one of the plurality vessels; increasing the temperature within the vessel by continuing to add heat to the interior of the vessel using the indirect heat source; reducing a moisture content of the process material in the interior of vessel to a predetermined value by conveying the steam to another one of the plurality of vessels.