For potable water treatment, the intent of disinfection is to provide absolute safety in terms of pathogenic microorganisms or viruses. In the case of waste water treatment, the main goal of disinfection is to reduce the concentration of germs. The pratical conditions of ozone application can be changed, so that the necessary applied ozone dose is reduced. Certain polyelectrolytes have an effect on the mass transfer of ozone. That effect seems to be present both on bubble–water mass transfer and on disinfection. So the addition of polyelectrolyte before ozonation is able to reduce the applied ozone dose required ; that is to increase the efficiency of disinfection by ozone.