Large eddy simulation (LES) is conducted to characterize oxygen dissolution in the draft tube of a pre-designed and optimized modular pump-turbine system. The air injection is applied over the peripheral surface on the draft tube. Mixture multiphase model is used to predict the spatial and temporal distribution of the air and the water. The simulation is conducted at the best efficiency point with a Reynolds number of 1.06×10 7 which is based on the rotation speed and the reference diameter of the runner. The vorticity is suppressed inside the draft tube, and the standard deviation of the power generation is decreased roughly 70% after aeration. The power generation is reduced by approximately 4-5% with peripheral aeration. Almost uniform dissolved oxygen concentration of 2-3 mg/l is observed in the radial direction near the outlet which is more than sufficient for wastewater treatment process.