COD analysis is often carried out in environmental labs. Its wastes are considered hazardous due to the content of metals such as Cr, Ag and Hg; treating these wastes is considered complex and expensive. The experimental results of metal ion precipitation in COD wastes with affordable chemical products are reported in this work. Cr (VI) was chemically reduced by adding 200 mg.L -1 of glucose to Cr (III). Final Cr (VI) concentration was less than 0.5 mg.L -1 . Cr (III) was precipitated as a metallic hydroxide by adding NaOH and Ag was reduced to less than 0.2 mg.L -1 by adding 2 g.L -1 of NaCl. Hg was reduced to less than 0.005 mg.L -1 with 10 g.L -1 of FeS. The proposed reduction-precipitation methodology allowed minimising the liquid residue’s hazardous characteristics so that it complied with the maximum allowable values established in Chapter 6, Article 74 of Decree 1594/1984 that regulates the use of water and liquid residues.