**4.4 Effect of pipe diameter on oxygen transfer efficiency 4.4.1 Experimental conditions and methods**

Three series of experiments are carried out with pipe diameters of 0.6, 1, 2, 4 and 5 cm, and effective heights of 1, 5 and 10 cm. At the pipe diameter of 0.6, 1, 2 and 4 cm, the diameter of the airlift part is set at 4 cm to attain the same cross-sectional area. By contrast, at a pipe diameter of 5 cm, setting the airlift part diameter at 5 cm leads to the altered cross-sectional area. In order to maintain the same aeration flux, the air flow rate is correspondingly increased when conducting this series of experiments involving 5 cm diameter pipes.

Experimental conditions and experimental apparatus diagram are respectively shown in Table 3 and Fig. 17.


\* aeration flux:air flow rate per unit cross-sectional area.

Table 3. Experimental conditions relating to various pipe diameters

Improvement of Oxygen Transfer Efficiency in

0.0

4

0

Fig. 18. Effect of airlift pipe diameter

1

2

EA (%)

E0 (mg-O2/L-Air)

3

0.5

1.0

1.5

液/気比

DO saturation rate (%) Liquid/gas ratio

2.0

2.5

0

20

40

60

DO飽和率 (%)

80

100

Airlift pipe diameter

Diffused Aeration Systems Using Liquid-Film-Forming Apparatus 355

0.6 1 2 4 5

1 5 10

1 510 有効高さ (cm)

Effective height (cm)

1 510

Fig. 17. Experimental apparatus relevant to various pipe diameters

The evaluation criteria include DO saturation rate (*cf.* Equation 1), liquid/gas ratio and E0 (*cf.* Equation 3). The calculation of liquid/gas ratio is given by Equation 4.

$$\text{Liquid/gas ratio} = Q\_L / Q\_G \tag{4}$$

where QL refers to the effluent flow rate (L/min) and QG is air flow rate (L/min).

### **4.4.2 Results and discussion**

The effect of pipe diameter on DO saturation rate, liquid/gas ratio, and E0 is shown in Fig. 18.

Irrespectively of effective height, DO saturation rate, liquid/gas ratio and E0 all exhibit similar trends. DO saturation rate tends to become large with decreasing pipe diameter. This is due to the fact that when the gas bubbles are passing through the LFFA setup, they are finely split, thereby enlarging dramatically the surface area of liquid film and consequently accelerating oxygen transport speed.

Liquid/gas ratio slowly increases with increasing pipe diameter and reaches the maximum value at 4 cm in pipe diameter. Because finely splitting of gas bubbles in the airlift part consumes some energy, the larger the pipe diameter, the smaller the hydraulic head loss in the airlift part, which thus increases the water flow rate there. However, if pipe diameter is over-sized, the airlift effect will be weakened, resulting in the reduced water flow rate through the airlift part. A direct outcome of interacting the 2 factors above with each other is the occurrence of an optimal parameter under a certain condition.

As shown by Equation 3, both DO concentration and liquid/gas ratio in the effluent water have an effect on oxygen transfer amount per unit air aeration volume, and 3 optimal conditions arise. They are listed as follows: (1) 4 cm in pipe diameter and 1 cm in effective height; (2) 4 cm in pipe diameter and 5 cm in effective height, and (3) 0.6 in pipe diameter

The evaluation criteria include DO saturation rate (*cf.* Equation 1), liquid/gas ratio and E0

Cross-sectional area

Airlift pipe diameter

Liquid/gas ratio=*Q QL G* (4)

The effect of pipe diameter on DO saturation rate, liquid/gas ratio, and E0 is shown in Fig.

Irrespectively of effective height, DO saturation rate, liquid/gas ratio and E0 all exhibit similar trends. DO saturation rate tends to become large with decreasing pipe diameter. This is due to the fact that when the gas bubbles are passing through the LFFA setup, they are finely split, thereby enlarging dramatically the surface area of liquid film and consequently

Liquid/gas ratio slowly increases with increasing pipe diameter and reaches the maximum value at 4 cm in pipe diameter. Because finely splitting of gas bubbles in the airlift part consumes some energy, the larger the pipe diameter, the smaller the hydraulic head loss in the airlift part, which thus increases the water flow rate there. However, if pipe diameter is over-sized, the airlift effect will be weakened, resulting in the reduced water flow rate through the airlift part. A direct outcome of interacting the 2 factors above with each other is

As shown by Equation 3, both DO concentration and liquid/gas ratio in the effluent water have an effect on oxygen transfer amount per unit air aeration volume, and 3 optimal conditions arise. They are listed as follows: (1) 4 cm in pipe diameter and 1 cm in effective height; (2) 4 cm in pipe diameter and 5 cm in effective height, and (3) 0.6 in pipe diameter

Air diffuser Air pump

**4.4.2 Results and discussion** 

accelerating oxygen transport speed.

18.

Fig. 17. Experimental apparatus relevant to various pipe diameters

the occurrence of an optimal parameter under a certain condition.

(*cf.* Equation 3). The calculation of liquid/gas ratio is given by Equation 4.

where QL refers to the effluent flow rate (L/min) and QG is air flow rate (L/min).

Fig. 18. Effect of airlift pipe diameter

Improvement of Oxygen Transfer Efficiency in

0

0

0.0

Fig. 20. Experimental findings as a function of effective height

0.5

1.0

1.5

2.0

EA (%)

g-O2/L-Air)

E0 (m 2.5

3.0

3.5

0.5

1

1.5

液/気比

2

2.5

20

40

DO飽和率(%)

DO saturation rate (%) Liquid/gas ratio

60

80

100

Effective height (cm)

Diffused Aeration Systems Using Liquid-Film-Forming Apparatus 357

1 5 10

124

124

124 エアリフト径(cm)

Airlift pipe diameter (cm)

and 1 cm in effective height. Meanwhile, as shown in Fig. 18, under the first condition, DO concentration relatively significantly affects oxygen transfer amount per unit air aeration volume. In contrast, liquid/gas ratio in the effluent plays a comparatively pronounced role under the third condition.

### **4.5 Effect of effective height on oxygen supply efficiency 4.5.1 Experimental conditions and methods**

Effective height stands for airlift height. In this trial, the effective height is set at 1, 5 and 10 cm. Herein, the other structural parameters of the single-pass LFFA are 1, 2 and 4 cm in pipe diameter and 12.56 cm2 in cross-sectional area. Air flow rate is 12.8 L/min.

Experimental conditions and experimental data are shown in Table 4 and Fig. 19, respectively.

DO saturation rate, liquid/gas ratio (*cf.* Equation 4) and E0 (*cf.* Equation 3) are used as the evaluation criteria.


Table 4. Experimental conditions linked to various effective heights

Fig. 19. Experimental apparatus relating to various effective heights
