Flow measurements in pipelines using slug tracer have proved highly accurate for turbulent flow. This study experimentally investigates the effectiveness of using a passive rotor on the accuracy of discharge measurements in sewer pipes based on a saline slug tracer technique. For this purpose, a saline injector stack was developed to help inject saline at selected injection points. A passive axial flow rotor was also proposed and encased in the injector stack to enhance the mixing of injected tracer with the transmitted downstream flow and to decrease the required minimum mixing length. It was found that adding the passive rotor significantly increased the accuracy of the flow measurements. Two tracer flow formulas were developed: one based on the dimensional analysis approach and the other based on a semi-empirical formula obtained from the mass conservation approach. The resultant formulas compared favourably with flow metering, especially when utilizing the passive fan unit.