Design of Air Diffusers for Mixing of Stratified Reservoirs

Successful operation of an existing reservoir requires maintaining good water quality for domestic use with the consideration of other environmental aspects such as aquatic life. Thermal stratification in the water column, where warm and cold water form layers, may result in poor mixing of the water column during the summer. As a result of the incomplete mixing of the water column and lack of light for the photosynthesis at the hypolimnion, water column can become anoxic. As oxygen depletion progresses to anoxia, ammonia, sulphides, and oxidized metals such as iron and manganese of bottom sediments are released to water. Iron and manganese are reduced to soluble forms which are toxic to aquatic life. Thus, thermal stratification of reservoirs eventually leads to poor downstream quality.

This study was motivated by the degradation of water quality in summer due to thermal stratification as observed in many reservoirs and lakes around the world. Artificial destratification has been widely used to improve water quality associated with eutrophic conditions in thermally stratified lakes and reservoirs. Various forms of hypolimnetic aeration/oxygenation systems have been installed with varying degrees of success in the past. This paper summarizes the pervious efforts related to the topic. The objective of this study is to investigate the methods of artificial destratification on the structure of the thermal stratification, and its effect on dissolved oxygen concentrations through numerical modelling. Ansys Fluent model was used to investigate mixing hydrodynamics in a thermally stratified water colum when air diffuser with different configurations was implemented to the model. For the same air flow rate, the model was utilized to simulate the air flow from different configuration and number of diffusers and the effects of the diffuser configurations on mixing process of the reservoir were discussed. The results showed that for the same air flow rate, using smaller diameter, but higher number of diffusers resulted in a faster mixing of the water column. The study also investigated the effect of air bubble size and the effect of air density in the bubble plume.