The worldwide rising demand for water has made its wise utilization mandatory, especially for the industrial sector. Over the last few decades, a major focus has been given to upgrading industrial wastewater treatment reactors. However, preexisting wastewater treatment reactors are often found to underperform due to faulty design, imperfect mixing that highly affects their treatment efficiencies. The residence time distribution (RTD) study has proved to be effective to identify anomalies present in a wastewater treatment reactor. The RTD experiment is carried out using a suitable tracer and hence often termed as the tracer technique. This review discusses the application of the tracer technique in various wastewater treatment stations and assesses the results obtained from recent experimental and simulation studies available in the literature. The paper also discusses common modeling techniques used to analyze the performance of these reactors which is the key for reactor design and process optimization.