A study to evaluate the effect of ozone on solids removal during the coagulation-flocculation of wastewater from Mexico City drainage (including domestic and industrial) was conducted. Results of this study show significant improvements in the effluent quality with small ozone doses. The optimum ozone dosage was found to be 3.32 mg L−1± 0.20. With this dosage it is possible to reduce the coagulant concentration from 50 mg L−1to 40 mg L−1without affecting the effluent quality. When coagulant dose is not reduced, an increase in the flocs' settling velocity due to a larger average size (with a decrease in TSS) is observed before and after filtration. In all cases, reductions in turbidity and color levels are seen when polyelectrolytes are used. The best results are achieved using a relation of ≈ 0.02 g O3/g TOC.