Oxygen dissolution via pump-turbine – Application to wastewater treatment

Large eddy simulations of a ventilated pump-turbine were conducted using LES-mixture model. Peripheral and central aeration were applied to study the effectiveness of oxygen dissolution and the pump-turbine performance. The peripheral draft tube aeration was performed by air injection through (1) a continuous orifice and (2) a series of discrete orifices along the surface of the draft tube. The simulations were performed when the system is operating in the turbine mode. Peripheral aeration was more effective for the achieved dissolved oxygen level and the oxygen dissolution efficiency. The mean dissolved oxygen concentration and the dissolution efficiency inside the draft tube were predicted to be 1.8 mg/l and 80% using the continuous or discrete peripheral aeration and 1.4 mg/l and 25% using the central aeration. A mean value ranging from 1.0 mg/l to 2.0 mg/l was considered to be sufficient for aerobic bacteria to treat the wastewater. Aeration resulted in a minor penalty on the power generation, while it provided a significant reduction in the flow-induced vibration. The central aeration was more effective in reducing the amplitude of pressure fluctuations and yielded more stable turbine operating conditions.