This study investigates the impact of equipment geometry on residence time distribution (RTD) using computational fluid dynamics (CFD) methods in a wastewater treatment tank with different configurations of static mixer elements. With growing environmental concerns, optimizing wastewater treatment processes is crucial. Proper mixing in these units can be achieved by optimal placement of static mixer elements such as baffle walls to create circulation zones and increase residence time of the fluid within the control volume. A CFD model of a wastewater treatment tank was developed and validated using experimental RTD data under three distinct mixer configurations.The experimentally validated model was subsequently enhanced by investigating the degradation of methylene blue (MB) during ozonation in the system. The results of the model allowed for the analysis of how tank geometry—specifically, the number and placement of baffles—affects the flow field and MB conversion. RTD was characterized using expectancy and standard deviation of residence time, revealing a link between RTD and MB degradation efficiency. Results showed that constructional parameters significantly influence residence time and mixing efficiency, with a potential 60% increase in expectancy. The model demonstrated high predictive accuracy, ranging from 75% in the worst case to nearly 90% in the best case.