Wastewater disinfection by ozone was investigated at pilot and full scale on different wastewater effluents and two types of ozone reactors. It was demonstrated that water quality and, in particular, suspended solids and organic content strongly influence the required ozone dose for a given level of disinfection. The increase in contact time and residual ozone concentration did not improve the log removal of viruses and bacteria. However, the ‘Ct’ approach, commonly applied in drinking water treatment can be used for wastewater ozonation, if a sufficient ozone dose can be transferred to the effluent resulting in an ozone residual which can be measured. These considerations should be taken into account for the improved design of ozonation facilities. It should be underlined that short contact times are only possible if fast balanced distribution of the ozone dose is achieved as rapidly as possible, in order to satisfy fast chemical reactions (colloidal matter destabilisation, zeta potential, etc.) and enable a uniform distributed ozone residual for the slower reactions (disinfection, oxidation of micropollutants, etc.).