Effects of bubble-plume mixing and side-stream oxygenation on water quality in eutrophic reservoirs

Water-quality problems, including eutrophication, hypoxia, and algal blooms, continue to affect lakes and reservoirs in many regions of the world. Water-quality management systems such as bubble-plume mixing systems and side-stream oxygenation systems are employed to deal with water-quality problems, but have resulted in both positive and negative outcomes. To examine the effects of bubble-plume mixing and side-stream oxygenation, a coupled three-dimensional (3-D) hydrodynamic model is adopted in conjunction with whole-reservoir in-situ mixing experiments in a drinking-water reservoir. This reservoir has a side-stream supersaturation (SSS) hypolimnetic oxygenation system and a bubble-plume epilimnetic mixing (EM) system installed to reduce hypolimnetic hypoxia and algal blooms. The present study aims to improve the understanding of the effects of the water-quality management systems in natural water bodies through a series of integrated field and numerical experiments. To predict the effects of bubble-plume mixing and side-stream oxygenation systems, a water-jet model coupled with the 3-D hydrodynamic model was first developed based on a jet flow theory. The water-jet model simulated the mixing and oxygen addition during operation of the SSS system. Using a 3-D approach both experimentally and numerically, the subsequent investigation in the present study visualized the thermal structure and analysed the constituent transport under the influence of bubble-plume mixing and side-stream oxygenation in the relatively small and shallow water body. Finally, the response of phytoplankton to bubble-plume induced mixing was investigated using a whole-reservoir field experiment. The validated 3-D hydrodynamic model was coupled with the Aquatic EcoDynamics (AED) model through the framework for aquatic biogeochemical modelling (FABM)