line 13 towards condenser 8 this re use of the wash waters is in no way obligatory it is however useful if only for the economy that is achieved after describing the treatment of the solids resulting from the thermolysis operation the treatment of the gases coming from and flowing towards reactor 4 will now be described in connection with fig1 as has already been mentioned the heating gases have a cocurrent circulation with respect to the waste that is they circulate upwards in reactor 4 these heating gases are accompanied by gases generated by the thermolysis so that the weight flow of outgoing gas is higher than the weight flow of incoming gas a fan 16 or any other means for setting the gaseous mixture in motion may be placed at the outlet of reheating means 15 so as to provide the circulating of said mixture however this stirring is not obligatory for the implementing of the invention according to this first embodiment of the invention part of the thermolysis gases is then sent via a line 251 towards a gas gas heat exchanger 17 so as to be reheated again before entering reactor 4 via line 18 at the outlet of.