Pilot plant studies were conducted in Brampton, Ontario, Canada to establish the optimum criteria for the ozone disinfection of effluents under various conditions of wastewater treatment plant operations. The effects of ozonation on the physical and chemical characteristics of the wastewater effluents were examined as well. More than 99‰ reductions of Total Coliform Fecal Coliform and Fecal Streptococci counts were achieved and some upgrading of the effluent in terms of COD and color were also attained.