**7. References**


**15** 

*Japan* 

**Improvement of Oxygen Transfer** 

*Division of Environmental Science and Engineering,* 

*Graduate School of Science and Engineering,* 

Tsuyoshi Imai and Hua Zhu

*Yamaguchi University* 

**Efficiency in Diffused Aeration Systems** 

Currently, aerobic bio-treatment processes, in which activated sludge system is at the center of the attention, are extensively applied in sewage treatment plants around the world. For activated sludge process, the diffused aeration has been thought to be one of the most important and indispensable operational units. However, a major concern of this operational methodology is that a large amount of compressed air has to be consumed in a diffused aeration system owing to the low oxygen transfer efficiency in water. It has been previously demonstrated that more than 40 % of the total power consumption in sewage treatment plants in Japan is related to power consumption associated with aeration alone. Therefore, the development of highly efficient aeration strategies has presently emerged as

The oxygen transfer in diffused aeration systems can be divided into two processes: bubble oxygen transfer and surface oxygen transfer. Bubble oxygen transfers into the water across the bubble-water interface as the bubbles rise from the diffuser to the water surface. Surface oxygen transfer exclusively occurs at the air-water interface situating on the water surface, originating from vigorous turbulence induced by bubble-plume motion and water circulation. Wilhelms and Martin's findings indicated that approximately one-third of the total volumetric mass transfer coefficient (kLat) is responsible for the volumetric mass transfer coefficient for surface transfer (kLas) [1]. McWhirter and Hutter determined that a representative kLas is 25-33 % of the kLat in a fine bubble diffuser system and 11-17 % of the kLat in a coarse bubble diffuser system [2]. DeMoyer *et al.* ever reported that the kLas is 59-85

Bubble transfer and surface transfer both contribute remarkably to the total oxygen transfer in the submerged aeration system. However, bubble transfer is the predominant means of oxygen transfer. So far, considerable research interests have been focused on the enhancement of the bubble transfer efficiency by developing a wide variety of new aeration techniques, including the utilization of high-purity-oxygen aeration system [4-10], deep aeration system [11-13] and fine bubble diffuser [14-17], *etc*. Nevertheless, only a little effort

has been devoted to the research on the improvement of surface transfer efficiency.

an intriguing research topic in the field of energy savings.

% of the volumetric mass transfer coefficient for bubble surface (kLab) [3].

**1. Introduction** 

**Using Liquid-Film-Forming Apparatus** 

