A process for treating a waste feedstock using a gasifier and the gasifier for same. Hot exhaust from an engine travels through a series of hollow heating plates stacked vertically within a gasifier reactor with spaces between each set of successive heating plates forming reaction zones. Each reaction zone is divided into an upper treatment area and a lower treatment area by a rotating disk. Waste material travels from an outer feed spot along the top surface of the rotating disk radially inwardly to a drop area located at the radially innermost portion where it drops to the top surface of the hollow heating plate below. The waste material is then conveyed radially outward to a chute to the next reaction zone or once fully processed to an exit from the reactor. Vapors from the waste material are drawn off each reaction zone through an outlet for further processing.