A 20 month operating experimental program was conducted at Marlborough, Massachusetts to evaluate the feasibility, engineering, and economic aspects of achieving high levels of effluent disinfection with ozone. The ozone research pilot facility was designed to operate at a constant wastewater flow rate or with diurnal flow characteristics. Two effluent qualities (filtered nitrified and filtered secondary) were evaluated to assess the impacts of water quality on disinfection performance with ozone. The absorbed ozone concentration required to achieve specific levels of disinfection was found to depend on water quality. Absorbed ozone concentrations of 15 to 20 and 35 to 40 mg/l were required to achieve high level disinfection to less than 2.2 total coliforms per 100 ml with filtered nitrified and filtered secondary effluents, respectively. Absorbed ozone concentrations of 5 to 8 and 15 to 20 mg/l were required to achieve disinfection to 70 total coliforms per 100 ml with the filtered nitrified and secondary effluents, respectively. Impacts of carrier gas ozone concentration, carrier gas flow rate, gas/liquid ratio and applied ozone dose on contactor transfer efficiencies with both effluent quality were defined. Water quality, transfer efficiency and absorbed ozone concentration are key factors affecting disinfection performance that must be evaluated for efficient design of ozone disinfection systems.