A study was conducted to determine the effects of ozonation on pH, heavy metals, and COD of four sludges, i.e., return activated sludge, thickened sludge, anae‐robically digested sludge, and bottom sediments of lake as a control sludge. It was found that heavy metals were released from the sludges after ozonation. Total COD decreased considerably and dissolved COD increased as a result of ozonation. Slight changes in pH values were observed during ozonation, and odor was completely eliminated from all sludges.