the tangentially directed nozzles 73 for supplying the mixing air jets by which the temperature conditions within the individual hearths can be controlled in response solely to the temperature therein while the apparatus will normally operate in the above described mode there will of course be times when for one reason or another the apparatus operates at extreme conditions outside the range shown in fig5 and the waste material becomes rather dry or very wet as described above when the temperature in the after burner 42 begins to rise the sludge feed control 67 controls the sludge feed divider so as to feed a greater proportion of the sludge to the upper feed drying hearth 48 when the sludge has a normal moisture content this results in reducing the temperature of the gas due to evaporation of moisture into the gas and the temperature in the afterburner hearth will fall however if the sludge is too dry insufficient moisture will be evaporated in the upper feed drying hearth 48 and the temperature will continue to rise this will cause the sludge feed control 67 to control the sludge feed divider to feed still more sludge to the upper feed drying hearth 48 until eventually all of the sludge is being fed to the upper feed drying hearth 48 and practically no sludge is being fed to the lower feed burning hearth at this point the temperature in the after burner hearth will still not have been reduced and accordingly some measure must be taken to.