**2. Novel idea: Liquid-film aeration system (LFAS)**

Liquid-film aeration system (LFAS) which means that LFFA developed in this study is installed on the water surface of existing aeration tanks. Under no extra energy consumption circumstances, water body is wholly membranized into a liquid film (i.e., a thin water film located at the water surface-air interface) by LFFA, thus realizing the oxygen supply towards liquid-film-formed water body simultaneously from the interior and exterior of the liquid film. The efficiency of atmospheric oxygen enrichment is improved as a consequence of dual-fold oxygen provisions. Namely, in addition to the existing oxygen supply efficiency of an aeration tank, the oxygen supply efficiency of LFAS is increased by one more term from the LFFA. As a result, the oxygen supply efficiency of the entire aeration system is enhanced.

Schematic drawings of the LFFA are illustrated in Fig. 1. The LFFA, made of plastics here, has a simple configuration constituting three sections: (1) trumpet-shaped capture part as a gas collector for converging the released gas bubbles; (2) airlift part, mainly made up of some pipes orienting parallel to each other, aiming to facilitate the relevant liquid film formation near the water surface and (3) effluent part which alternately reserves and discharges the treated water involving higher-concentration dissolved oxygen (DO).

Optionally, the LFFA can be compatibly installed in an existing aeration system. The whole set of equipment is denoted as liquid-film aeration system (Fig. 2). The joint interface between the capture and airlift parts of this LFFA is flush with the water surface with the former just submerged in water and the latter completely exposed to air. During the operating process, the gas bubbles released from a diffuser first rise to the airlift part through the capture part and then self-assemble into a macroscopic aggregated system of gas bubbles over the airlift part. This macroscopic ensemble of gas bubbles floating in the atmosphere in close proximity to the water surface are microscopically comprised of the combination of numerous agglomerated gas bubbles and a fraction of the bulk water (serving as liquid-film boundary layer surrounding each individual bubble herein) entrained by the ascending bubbles. In this case, air fills both the exterior and interior of these bubbles as well as the thin liquid film, as described above. Hence, oxygen transfer can take place both across the inner interface (gas bubble and liquid film) and the outer interface (atmosphere and liquid film) of the thin liquid film, which means that the available interfacial contact area between oxygen and water can be greatly multiplied, thereby promoting effectively oxygen transfer efficiency. Subsequently, oxygenic water is

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.

Liquid-film aeration system (LFAS) which means that LFFA developed in this study is installed on the water surface of existing aeration tanks. Under no extra energy consumption circumstances, water body is wholly membranized into a liquid film (i.e., a thin water film located at the water surface-air interface) by LFFA, thus realizing the oxygen supply towards liquid-film-formed water body simultaneously from the interior and exterior of the liquid film. The efficiency of atmospheric oxygen enrichment is improved as a consequence of dual-fold oxygen provisions. Namely, in addition to the existing oxygen supply efficiency of an aeration tank, the oxygen supply efficiency of LFAS is increased by one more term from the LFFA. As a result, the oxygen supply efficiency of the entire aeration system is

Schematic drawings of the LFFA are illustrated in Fig. 1. The LFFA, made of plastics here, has a simple configuration constituting three sections: (1) trumpet-shaped capture part as a gas collector for converging the released gas bubbles; (2) airlift part, mainly made up of some pipes orienting parallel to each other, aiming to facilitate the relevant liquid film formation near the water surface and (3) effluent part which alternately reserves and

Optionally, the LFFA can be compatibly installed in an existing aeration system. The whole set of equipment is denoted as liquid-film aeration system (Fig. 2). The joint interface between the capture and airlift parts of this LFFA is flush with the water surface with the former just submerged in water and the latter completely exposed to air. During the operating process, the gas bubbles released from a diffuser first rise to the airlift part through the capture part and then self-assemble into a macroscopic aggregated system of gas bubbles over the airlift part. This macroscopic ensemble of gas bubbles floating in the atmosphere in close proximity to the water surface are microscopically comprised of the combination of numerous agglomerated gas bubbles and a fraction of the bulk water (serving as liquid-film boundary layer surrounding each individual bubble herein) entrained by the ascending bubbles. In this case, air fills both the exterior and interior of these bubbles as well as the thin liquid film, as described above. Hence, oxygen transfer can take place both across the inner interface (gas bubble and liquid film) and the outer interface (atmosphere and liquid film) of the thin liquid film, which means that the available interfacial contact area between oxygen and water can be greatly multiplied, thereby promoting effectively oxygen transfer efficiency. Subsequently, oxygenic water is

discharges the treated water involving higher-concentration dissolved oxygen (DO).

**2. Novel idea: Liquid-film aeration system (LFAS)** 

enhanced.

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 connected to the original water tank itself.

Fig. 1. Schematic diagram of the LFFA

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

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

Improvement of Oxygen Transfer Efficiency in

**3. Pre-experiment on the LFFA** 

**3.2 Experimental methods** 

recycle-type society.

**3.1 Introduction** 

Effective height

Aeration depth

Fig. 5. Single-pass LFFA apparatus

Diffused Aeration Systems Using Liquid-Film-Forming Apparatus 345

LFFA has a very simple structure, and its production cost is very low. Optionally, it can be made from recycled plastics. As another advantage, this apparatus itself does not consume any power, and can be easily installed on the water surface of existing aeration tank without large-scale retrofitting. Thus this very cheap setup is well suited for the application in the

In order to evaluate the oxygen transfer performance of the LFFA, under the experimental conditions of different bubble diameters and different aeration amounts, comparative experiments are conducted on a liquid-film aeration system and conventional aeration

The diffuser is set in a 28.5 cm deep and 15 L capacity cylindrical water tank with a surface area of 526.3 cm2. Its location is at the middle of the experimental water tank bottom with an aeration depth of 26 cm. The liquid-film apparatus with a pipe diameter of 1 cm, effective height of 10 cm and cross-sectional area of 12.56 cm2 is mounted on the surface of a water tank. During the operational period, single-pass fashion is employed for this apparatus. After measurement of DO concentration and temperature in the initially de-oxygenated water, a given amount of aeration is provided and the aeration lasts for 4 min. The DO concentration in the discharged water from the effluent part is periodically measured at a sampling interval of 30 s. The amount of the effluent water is calculated before the aeration is suspended. To prevent the water surface in the water tank from going downwards, the volume of the de-oxygenated water equivalents to that of the treated water is periodically poured into the water tank to maintain the water surface's balance throughout the experiment. As a control, the conventional aeration experiment is also conducted at the same water disposal volume, aeration depth and aeration duration as the liquid-film

Air diffuser Air pump

Airlift pipe diameter

Cross-sectional area

system by respectively using de-oxygenated water and activated sludge.

aeration experiment. The experimental setup is demonstrated in Fig. 5.

Fig. 4. The picture of forming liquid film

LFAS possesses the following characteristics.

Due to the superior oxygen supply efficiency, even if the aeration depth is as low as less than 1 m, a sufficient amount of oxygen can still be provided. In contrast, the conventional aeration tank necessitates a depth of 4-5 m to achieve a commensurate oxygen supply. Therefore, LFAS is a very energy-saving aeration system.

Although some energy is consumed as a consequence of the friction between the airlift tube wall of LFFA and surrounding water body, we suggest that this portion of energy consumption is far below the wasted energy in the conventional aeration system. Moreover, the energy wasted in the traditional aeration system can be re-used for the oxygen supply. Therefore, this novel LFAS-based method definitely opens an energy-efficient pathway to improve the oxygen transfer efficiency.

LFFA has a very simple structure, and its production cost is very low. Optionally, it can be made from recycled plastics. As another advantage, this apparatus itself does not consume any power, and can be easily installed on the water surface of existing aeration tank without large-scale retrofitting. Thus this very cheap setup is well suited for the application in the recycle-type society.
