UKC 2005 - ACETS

Destratification and Water Quality Improvement in Reservoirs

: Effect of Bubble Size on Destratification

Heekyung Park¹, Sunghoon Kim², Jeyoung Ahn³, and Kyungtaek Yum⁴

Professor¹, Graduate², Dept. of Civil and Environmental Engg., Korea Advanced Institute of Science and Technology (KAIST)

Engineer³ and Director General⁴, Korea Water Resources Corporation (KOWACO)

To control source water quality at reservoirs, we need to reduce waste loading rate in a watershed. Such a control requires many management activities which include regulating effluents, installing more advanced waste treatments, and consolidating and rehabilitating sewers. Besides these management options in tributaries, we can also adopt in-lake treatment technologies to directly control the quality of reservoirs. The technologies are usually known to be quicker in responses than the tributary options. As a result, the combined use of them is strongly recommended for controlling the reservoir water quality.

In Korea, reservoirs are an important water resource. Their quality is however being threatened in recent days by many reasons. One of them is thermal-induced stratification and air diffuser system is often used to break stratification and to remedy. At many of the source water reservoirs in Korea, the air diffuser systems are now being used. This is reportedly due to their effectiveness, inexpensiveness, and simplicity. Coping with its wide use, this study has been initiated to increase such a cost-effectiveness by looking at the effect of bubble size on destratification as well as other design and operating parameters of the systems.

According to previous researches, the dynamics of bubble plume and destratification efficiency   depend upon two dimensionless groups, M_h and P_N. However, the two variables include air flowrate, stratification strength, water depth, gravity, and bubble slip velocity only and do not include any information about bubble size and water volume or area in which diffusers are operated. As a result, it is thought that they are more or less inefficient to apply to design of the full-scale air diffuser systems. Thus, this study focuses on finding more details of the effect of bubble size on flow pattern and destratification efficiency and upgrading conventional design methods. For lab experiments, a 0.5m× 0.25m × 0.7m acrylic tank was used as well as hydro-dynamic simulations.

The results of this study revealed a different flow pattern from the previous researches. That is, it was found that destratification efficiency strongly depends on bubble size. Also, to properly reflect the effect in the design of air diffuser systems, we developed a modified non-dimensional plume number (P_NS) and demonstrated its practical use.