A Fenton reaction using 150-um iron powder for treating activated sludge from livestock-raising wastewater resulted in significant decreases in chromaticity and chemical oxygen demand. The optimum weight ratio of H₂O₂/Fe(0) was 0.3, and the optimum initial pH and the optimum reaction time were 3.5 and 30 mins. Under these optimum conditions, the removal ratios of chemical oxygen demand and the chromaticity of the supernatant after polymer coagulant addition and precipitation of the sludge in a continuous miniplant experiment were 86% and 84%, respectively.