There are accumulations of remained chemical additives and contaminants in the process water of semi-closed linerboard mill. High temperature of the process water aggravates the anaerobic digestion of contaminated process water and causes the generation of hazardous gases, which are from the biological reaction of varied additives and contaminants. The hydrogen sulfide in the gases easily combine with moisture in the air, and become sulfuric acid, which causes corrosion of paper machinery. This hydrogen sulfide is from the reduction of sulfate ions in the process water, and the sulfate ions are mostly from the alum. We changed the alum to PAC (Poly Aluminum Chloride). The results were preventing generation of hydrogen sulfide, and equivalent sizing effect by the use of PAC.