The present invention relates to a process to treat municipal solid waste comprising the steps of, feeding waste material into a pressure vessel (6,6), closing the vessel and optionally evacuating the contents of air, agitating the vessel while having steam pressure in the vessel at about 2.5 bar (abs) or higher, and about 4.8 bar (abs) or lower, and a temperature of about 127 °C or higher and about 148 °C or lower for at least about 15 min, reducing the pressure in the vessel, optionally evacuating the contents of the vessel to 0.6 bar abs or lower, discharging the vessel, classifying the treated waste, wherein steam at about 165 °C or more and a pressure of about 5 bar (abs) or more, is injected into the vessel after closing to raise the pressure within the vessel to 2.5-4.8 bar (abs) in a condensing atmosphere, and, for a suitable period preferably continue to inject steam at a reduced flowrate to allow heating of the waste material by steam condensation, and thereby maintain the waste treatment conditions. The improvement allows more efficient, faster process, while using less energy and water, reducing the effluent burden.
