PURPOSE: To effectively recover even low temperature-waste gas by providing a regenerative rotary type air preheater having a soot blower on its air side pathway side on the downstream side of an electric dust collector.

CONSTITUTION: Pulp wastewater B previously concentrated to a high concentration is sent out under pressure by a positive blower 1 in a boiler 3, heated by a steam type air preheater 2, and then burned with combustion air A supplied. The combustion gas G is heat-exchanged with supplied water W in a fuel economizer 5 and discharged through an electric dust collector 6 and an inductive blower 7 to the open air through a stack 8. In this case, a gas type air preheater 10 is provided on the downstream side of the inductive blower 7, by which the temperature of high-temperature waste gas at the outlet of fuel economizer 5 is cooled to about 120°C for heat recovery. As the preheater 10, a regenerative rotary type air preheater is used, having soot blowers 101a and 101b in its air side pathway to enable steam or compressed air to be blown out.