PURPOSE:To evaporate and dry efficiently the waste liquid, and to reduce the corrosion, etc. due to halogen by regulating the amt. of a waste liquid from a wet dust collector which is injected into the upstream of a dry dust collector with the concn. of halogen or suspended matter in the waste liquid. CONSTITUTION:The waste gas 2 from a coal-fired boiler 1 is introduced into a dry dust collector 3, and the dust and soot 4 are discharged to the outside of the system. The waste gas 5 therefrom is introduced into a gas heater 33, and heat-exchanged with a purified gas 9 from an absorption tower 8. The heat-exchanged gas 9 is introduced into a cooling tower 6, and almost all the dust and soot and halogen compds. are removed. The gas is then introduced into the tower 8 to remove SO2. A part of the circulating liquid of the cooling tower wherein the dust and soot and halogen compds. are accumulated is supplied 13 into a neutralization tank 25. In this case, the flow rate of the liquid discharged from the cooling tower is controlled 41 by detecting 39 the Cl concn., and converting the detected signal into the external signal of a regulator 40 so that the Cl concn. of the slurry in the neutralization tank may be regulated to 10,000ppm, and a part of the circulating liquid is supplied 13 into the tank 25. The sludge 36 is separated from the slurry sent into a sludge separator 35 from the tank 25, and the total amt. of the filtrate 37 is supplied into an evaporator 37 provided at the upstream of the apparatus 3 and dried.