The preliminary treatments (grit removal, screening, sand removal) are intended to remove coarse suspended matters such as grit, sand and rags, pieces of fabric, fibres, yarns. The biological compartment consists of a conventional activated sludge plant composed of a predenitrification stage, followed by biological oxidation and secondary settling. Ozone is dosed on part of the recycle from the secondary settler to the biological basin in order to reduce sludge production and to solve some operational problems due to filamentous bacteria growth and consequent biological foam (sludge bulking problems and hindered sedimentation). Then, tertiary treatments (coagulation-flocculation,clarification and ozonation) are required to remove suspended solids, recalcitrant compounds and colour.