**Improvement of Oxygen Transfer Efficiency in Diffused Aeration Systems Using Liquid-Film-Forming Apparatus**

Tsuyoshi Imai and Hua Zhu

*Division of Environmental Science and Engineering, Graduate School of Science and Engineering, Yamaguchi University Japan* 

### **1. Introduction**

340 Mass Transfer - Advanced Aspects

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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 an intriguing research topic in the field of energy savings.

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

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.

Improvement of Oxygen Transfer Efficiency in

connected to the original water tank itself.

Fig. 1. Schematic diagram of the LFFA

Fig. 2. Liquid-film aeration system (LFAS)

a liquid film.

Diffused Aeration Systems Using Liquid-Film-Forming Apparatus 343

discharged downstream from the effluent part. Furthermore, LFAS can supply oxygen either in a once-through mode in the case of which the effluent part is directly connected to another water reservoir, or in a recycle mode in the case of which the effluent part is directly

The picture of the actual experimental apparatus is shown in Fig. 3. Fig. 4 reveals the form of

The improvement of oxygen transfer capability across water surface is pursued in this study. The objective is fulfilled with a liquid-film-forming apparatus (LFFA) by pre-forming an aggregative entity in the atmosphere near water surface. This entity is constructed of a large quantity of air-filled gas bubbles with the periphery of each gas bubble surrounded by an ultrathin layer of liquid film, thereby enlarging notably the effective interfacial contact area between air and water. Consequently, energy consumption problem can be solved by reducing the aeration depth down to 1 m or less without trading off the ideal aeration efficacy in a diffused aeration system. Based on the concept above, lab-scale experimental apparatus is designed in this study and the efficiency of oxygen transfer in this novel apparatus is determined either numerically or experimentally. Furthermore, a number of factors affecting the efficiency of oxygen transfer are also examined in detail, and the feasibility is preliminarily explored for the application in wastewater treatment plants.
