Abstract
Kuhn, W.L., Sabil, S.N., al-Khayat, O. & Dahlhaug, O.G. 2025. DeGas project report. Summary of experimental and numerical studies on mitigating total dissolved gas supersaturation downstream hydropower plants using technical methods. NINA Report 2703. Norwegian Institute for Nature Research.
This report summarizes the findings of the DeGas project, a comprehensive research initiative aimed at mitigating the environmental effects of total dissolved gas (TDG) supersaturation resulting from hydropower operations. Hydropower, an important renewable energy source, can generate TDG saturation levels that pose a significant ecological risk, primarily through the induction of gas bubble disease (GBD) in fish and other aquatic organisms. This project combined controlled laboratory experiments with advanced numerical modeling to develop and evaluate viable technical mitigation methods. The experimental component, conducted at NTNU, systematically assessed the efficiency of aeration (gas bubbling) via microbubble diffusers and ultrasonic degassing, both as standalone and combined systems. The numerical modeling component focused on the Otra and the Skibotn River, which are high-risk sites downstream of hydropower plants, to characterize the persistence of TDG saturation levels and develop several predictive transport models. Principal findings from the experimental studies demonstrate that while aeration offers modest reduction of TDG supersaturation, acoustic degassing is highly effective, achieving substantial reductions in both static and flowing-water environments. The numerical studies identified a 2D model based on the finite difference method as an accurate and reliable tool for site-specific risk assessment. Summarizing, the DeGas project provides a robust scientific and engineering foundation for developing practical, retrofittable solutions that enable a balance between sustainable hydropower generation and the conservation of aquatic ecosystems.