BACKGROUND: The ozonation of a mixture of contaminants commonly found in secondary effluents has been carried out in an artificially contaminated secondary effluent. Competitive ozonation experiments in heterogeneous and homogeneous mode have also been carried out to determine the second-order direct ozonation rate constants. RESULTS: Inlet ozone concentration, alkaline conditions and addition of 10(-3) mol L(-1) of H(2)O(2) positively affected the degree of mineralization and the disappearance of chemical oxygen demand of the mixture. Reaction rates depend on pH; at pH 7 the following direct ozone rate constants were obtained: 2.7 x 10(5), 2.5 x 10(3), 2.5 x 10(4), 6.2 x 10(5), 3.2 x 10(5), 3.4 x 10(5), 8.0 x 10(5) and 4.6 x 10(5) mol(-1) L(1) s(-1) for acetaminophen, metoprolol, caffeine, antipyrine, norfloxacin, ketorolac, doxycycline, hydroxybiphenyl, and diclofenac, respectively. CONCLUSIONS: The positive effect on TOC and COD removal that ozone dosage exerts is not applicable to individual contaminants. An optimum ozone concentration can be found with no further improvement of the depletion rate of organics as the ozone inlet concentration is increased. Carbonates affect the oxidation of recalcitrant compounds like atrazine. Carbonate concentration must be considered when dealing with real effluents. Addition of hydrogen peroxide can increase the mineralization level obtained, however the increase in complexity and costs does not justify its addition. (C) 2011 Society of Chemical Industry