PURPOSE: To save fuel for an afterburner and to enhance treating efficiency by scrubbing and cooling off-gases derived from the drying and burning of sludge and leading the conditioned off-gases into the afterburner.

CONSTITUTION: Waste sludge derived from the treatment of waste water is led from the top inlet 15 of a furnace 10 and air is led from the lower part 18 of the furnace 10. Off-gas leaving the uppermost hearth 11 which is a sludge drying region is led into a scrubbing-cooling system with a venturi tube 24 and a scrubber 25 through a duct 19 and the off-gas is scrubbed with water. The off-gas freed of particulates, moisture and volatile contaminants and conditioned to 26-49°C is led into a heat exchanger 29 by a fan 28, preheated by indirect heat exchange with hot clean gas from an afterburner 31 and led into the afterburner 31 through a duct 33. The off-gas from the afterburner cooled in the heat exchanger 29 is discharged from a chimney 34 into the air.