How to Avoid Total Dissolved Gas Supersaturation in Water from Hydropower Plants by Employing Ultrasound

In Norway, more than 10 hydropower plants are known to have caused biologically relevant levels of total dissolved gas supersaturation in the rivers downstream power plants. This phenomenon is causing fish kills due to gas bubble disease and have large impacts on the biodiversity. The gas supersaturation is often caused by undersized or blocked brook intakes creating turbulent flows and resulting in large amounts of air dragged into the tunnel. One possible solution to this problem is employing power ultrasound (20 kHz - 1 MHz) for degassing water prior to releasing it back into the river system. Acoustic cavitation is known to have a positive effect on the degassing mechanism, and this paper is investigating whether ultrasound can be applied to create acoustic cavitation and avoid biologically relevant levels of total dissolved gas supersaturation from hydropower plants. The objective is to develop background knowledge for constructing an experimental setup in the Waterpower Laboratory at NTNU to investigate whether gas supersaturation can be decreased from power plants by application of ultrasound. Preliminary experiments, carried out at the NTNU Hydrogen Energy and Sonochemistry Laboratory, exploring the behaviour of the degassing process at different ultrasonic frequencies and amplitudes conclude that the most effective degassing occur at high acoustic amplitude and a frequency of 24 kHz.