Abstract: Urban waste is dried and burned in a mechanical grate furnace in a reducing atmosphere to reduce the formation of NO.sub.x. Consumption of the clinker is completed in a well at the base of which a device such as a plasma torch is provided for vitrifying the clinker. The combustion gases are burned in the presence of secondary air in a circulating fluidized bed furnace. Most of the fly ash is removed from the flue gases in a filter before the gases are passed through a boiler in which they are cooled. The flue gases then pass through a conventional cleansing area from which they are evacuated by a stack. There are very low concentrations of harmful products in the flue gases and in the clinker. The fly ash collected on the exit side of the fluidized bed is recycled with the clinker to be vitrified. Heavy metals evaporated in the furnaces are recovered via a purge.