**4.2 Effect of altering gas bubble diameter on oxygen supply efficiency 4.2.1 Experimental conditions**

In this trial, gas bubble diameter is varied by changing the type of diffuser. Diffuser can offer 2 kinds of gas bubbles with a diameter of 3 and 6 mm, respectively. The various combinations of diffuser and air pump as well as air flow rate are listed in Table 1. The average gas bubble diameter is determined by averaging the diameters of many digitalcamera-taken gas bubbles located near the diffuser.

The parameter of the single-pass LFFA apparatus is listed as follows: 4, 6 and 10 mm in pipe diameter, 10 cm in effective height, and 12.56 cm2 in cross-sectional area. The rig of experimental apparatus and experimental conditions are respectively shown in Fig. 11 and Table 1.

Fig. 11. Experimental apparatus relating to the variation in gas bubble diameter

Improvement of Oxygen Transfer Efficiency in

thereby enhancing the oxygen transport ability.

0

0

applied, it is very hard to make a credible judgement.

Fig. 14. Effect of gas bubble diameter on oxygen mass transfer rate

20

40

(mg-O2/min)

Oxygen mass transfer rate

60

80

Fig. 13. Effect of gas bubble diameter on the effluent flow rate

3

Effluent flow rate (L/min)

6

9

Diffused Aeration Systems Using Liquid-Film-Forming Apparatus 351

As indicated in Fig. 12, DO saturation rates always maintain higher than 80% in all experiments, suggesting the superior oxygen transport ability of this novel setup. In Fig. 12, when the same air pump is used, the aeration amount of the diffuser providing a gas bubble diameter of 3 mm is less than that of the diffuser with a bubble diameter of 6 mm. However, it is found from the experiment results of 4 mm pipe diameter that though the aeration amount is comparatively small for the 3 mm diameter bubbles, DO saturation rate is still higher than that of 6 mm diameter bubbles. Likewise, the same trend is observed for the 6 and 10 mm diameter pipes. This is attributed to the fact that the finely divided gas bubbles allow for the enlarged contact area between water and gas bubbles in the aeration system,

Gas bubble diameter 3 mm 6 mm

Gas bubble diameter 3 mm 6 mm

4 6 10 4 6 10

Airlift pipe diameter (mm)

APN-215 DF-406

Air pump type

4 6 10 4 6 10

Airlift pipe diameter (mm)

APN-215 DF-406

As shown in Figs. 13 and 14, both effluent flow rate and oxygen transfer rate exhibit a tendency same as DO saturation rate. However, as a result of the different aeration amounts

Air pump type

### **4.2.2 Experimental methods**

The LFFA operating in a single-pass manner is set in a 28.5 cm deep and 15 L cylindrical tank with a surface area of 526.3 cm2. After measurement of DO concentration and temperature in the initially deoxygenated water, a given amount of aeration is provided and lasts for 4 min. The DO concentration in the discharged water from the effluent part is periodically measured at an interval of 30 s. The amount of the effluent water is calculated before aeration is stopped. To prevent the water surface in the water tank from going downwards, the volume of the de-oxygenated water equivalent to that of the treated water is periodically poured into the water tank to maintain the water surface's balance throughout the experiment.

DO concentrations are determined with YSI Model DO meter. Air pump includes Iwaki APN-215CV-1 Model and Secoh DF-406 Model.

#### **4.2.3 Calculation methods**

The parameters such as DO saturation rate, oxygen transfer rate and oxygen transfer mass per unit air aeration volume are used for evaluating the experimental results in this experiment. The calculation is based on Equations (1), (2) and (3).

$$E\_O = \frac{\left(DO\_{act} - DO\_0\right) \times Q\_L}{Q\_G} \tag{3}$$

Herein, E0 stands for oxygen transfer mass per unit air aeration volume in mg-O2/L-air, DOact. is the actually measured average DO concentration in mg/L, DOo is the initial DO concentration in mg/L, QL is the effluent flow rate in L/min, QG is air flow rate in L/min.

#### **4.2.4 Results and discussion**

The experimental results of the DO saturation rate, effluent flow rate, oxygen mass transfer rate and oxygen transfer mass per unit air aeration volume are respectively shown in Figs. 12, 13, 14 and 15.

Fig. 12. Effect of gas bubble diameter on the DO saturation rate

The LFFA operating in a single-pass manner is set in a 28.5 cm deep and 15 L cylindrical tank with a surface area of 526.3 cm2. After measurement of DO concentration and temperature in the initially deoxygenated water, a given amount of aeration is provided and lasts for 4 min. The DO concentration in the discharged water from the effluent part is periodically measured at an interval of 30 s. The amount of the effluent water is calculated before aeration is stopped. To prevent the water surface in the water tank from going downwards, the volume of the de-oxygenated water equivalent to that of the treated water is periodically poured into the water tank to maintain the water surface's balance

DO concentrations are determined with YSI Model DO meter. Air pump includes Iwaki

The parameters such as DO saturation rate, oxygen transfer rate and oxygen transfer mass per unit air aeration volume are used for evaluating the experimental results in this

( *act* <sup>0</sup> ) *<sup>L</sup>*

*DO DO Q <sup>E</sup> Q*

Herein, E0 stands for oxygen transfer mass per unit air aeration volume in mg-O2/L-air, DOact. is the actually measured average DO concentration in mg/L, DOo is the initial DO concentration in mg/L, QL is the effluent flow rate in L/min, QG is air flow rate in L/min.

The experimental results of the DO saturation rate, effluent flow rate, oxygen mass transfer rate and oxygen transfer mass per unit air aeration volume are respectively shown in Figs.

Gas bubble diameter 3 mm 6 mm

4 6 10 4 6 10

Airlift pipe diameter (mm)

APN-215 DF-406

Air pump type

*G*

− × <sup>=</sup> (3)

**4.2.2 Experimental methods** 

throughout the experiment.

**4.2.3 Calculation methods** 

**4.2.4 Results and discussion** 

0

Fig. 12. Effect of gas bubble diameter on the DO saturation rate

20

40

DO saturation rate (%)

60

80

100

12, 13, 14 and 15.

APN-215CV-1 Model and Secoh DF-406 Model.

experiment. The calculation is based on Equations (1), (2) and (3).

*O*

As indicated in Fig. 12, DO saturation rates always maintain higher than 80% in all experiments, suggesting the superior oxygen transport ability of this novel setup. In Fig. 12, when the same air pump is used, the aeration amount of the diffuser providing a gas bubble diameter of 3 mm is less than that of the diffuser with a bubble diameter of 6 mm. However, it is found from the experiment results of 4 mm pipe diameter that though the aeration amount is comparatively small for the 3 mm diameter bubbles, DO saturation rate is still higher than that of 6 mm diameter bubbles. Likewise, the same trend is observed for the 6 and 10 mm diameter pipes. This is attributed to the fact that the finely divided gas bubbles allow for the enlarged contact area between water and gas bubbles in the aeration system, thereby enhancing the oxygen transport ability.

Fig. 13. Effect of gas bubble diameter on the effluent flow rate

Fig. 14. Effect of gas bubble diameter on oxygen mass transfer rate

As shown in Figs. 13 and 14, both effluent flow rate and oxygen transfer rate exhibit a tendency same as DO saturation rate. However, as a result of the different aeration amounts applied, it is very hard to make a credible judgement.

Improvement of Oxygen Transfer Efficiency in

Aeration depth

Aeration depth (cm)

Table 3 and Fig. 17.

Pipe diameter (cm)

Diffused Aeration Systems Using Liquid-Film-Forming Apparatus 353

Air diffuser Air pump

26 5.36 83.7 37.4 63 5.60 98.5 44.1

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

Experimental conditions and experimental apparatus diagram are respectively shown in

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

increased when conducting this series of experiments involving 5 cm diameter pipes.

Cross-sectional area (cm2)

DO saturation rate (%)

> Aeration flux\* [L/(min・cm2)]

Oxygen mass transfer rate (mg-O2/min)

> Effective height (cm)

> > 1, 5, 10

Fig. 16. Experimental apparatus relevant to the change in aeration depth

Table 2. Experimental results pertaining to the variation in aeration depth

Effluent flow rate (L/min)

**4.4 Effect of pipe diameter on oxygen transfer efficiency** 

**4.4.1 Experimental conditions and methods** 

Air flow rate (L/min)

0.6, 1, 2, 4 12.8 12.5 1.0 5 20.0 19.6 1.0

Table 3. Experimental conditions relating to various pipe diameters

Therefore, the parameter E0 (DO amount per unit oxygen supply amount) is introduced herein. Its experimental result is shown in Fig. 15. As shown in this figure, when the pipe diameters are 4, 6 and 10 mm, the same trends are all manifested. That is, E0 of 3 mm diameter gas bubble is greater than that of 6 mm.

Fig. 15. Effect of gas bubble diameter on E0

Similarly, while the other air pump is used, the similar evolving trends are observed for DO saturation rate, effluent flow rate, oxygen transfer rate and oxygen transfer efficiency. Namely, the smaller the gas bubble diameter is, the higher the result is. It is shown that the LFFA is likewise subjected to the effect of the interfacial contact area between air and water.

#### **4.3 Effect of aeration depth on oxygen supply efficiency 4.3.1 Experimental conditions and methods**

At an air flow rate of 13.5 L/min, aeration depth is respectively set at 26 and 63 cm. LFFA adopts a single-pass apparatus with a pipe diameter of 6 mm, effective height of 10 cm and cross-sectional area of 12.56 cm2. The diffuser in connection with an Iwaki APN-215CV-1 Model air pump can release gas bubbles with an average diameter of 3 mm. The experimental apparatus relating to the differing aeration depths is shown in Fig. 16. Experimental methods refer to Section 4.2.2.

The experimental results are evaluated in terms of DO saturation rate (*cf.* Equation 1) and oxygen transfer rate (*cf.* Equation 2).

### **4.3.2 Results and discussion**

The experimental results are shown in Table 2. The larger the aeration depth is, the higher the DO saturation rate, effluent flow rate and oxygen transfer rate are. Thus, the oxygen supply ability of LFFA also obeys the oxygen diffusion regime at the gas-liquid interface.

At an aeration depth of 63 cm, the DO concentration in the effluent stream approximately approaches the saturation value, thereby determining the feasible aeration depth located at roughly 60 cm for the LFAS.

Therefore, the parameter E0 (DO amount per unit oxygen supply amount) is introduced herein. Its experimental result is shown in Fig. 15. As shown in this figure, when the pipe diameters are 4, 6 and 10 mm, the same trends are all manifested. That is, E0 of 3 mm

Gas bubble diameter 3 mm 6 mm

4 6 10 4 6 10

Airlift pipe diameter (mm)

APN-215 DF-406

Similarly, while the other air pump is used, the similar evolving trends are observed for DO saturation rate, effluent flow rate, oxygen transfer rate and oxygen transfer efficiency. Namely, the smaller the gas bubble diameter is, the higher the result is. It is shown that the LFFA is likewise subjected to the effect of the interfacial contact area between air and water.

At an air flow rate of 13.5 L/min, aeration depth is respectively set at 26 and 63 cm. LFFA adopts a single-pass apparatus with a pipe diameter of 6 mm, effective height of 10 cm and cross-sectional area of 12.56 cm2. The diffuser in connection with an Iwaki APN-215CV-1 Model air pump can release gas bubbles with an average diameter of 3 mm. The

The experimental results are evaluated in terms of DO saturation rate (*cf.* Equation 1) and

The experimental results are shown in Table 2. The larger the aeration depth is, the higher the DO saturation rate, effluent flow rate and oxygen transfer rate are. Thus, the oxygen supply ability of LFFA also obeys the oxygen diffusion regime at the gas-liquid interface. At an aeration depth of 63 cm, the DO concentration in the effluent stream approximately approaches the saturation value, thereby determining the feasible aeration depth located at

experimental apparatus relating to the differing aeration depths is shown in Fig. 16.

Air pump type

diameter gas bubble is greater than that of 6 mm.

0

Fig. 15. Effect of gas bubble diameter on E0

**4.3.1 Experimental conditions and methods** 

Experimental methods refer to Section 4.2.2.

oxygen transfer rate (*cf.* Equation 2).

**4.3.2 Results and discussion** 

roughly 60 cm for the LFAS.

**4.3 Effect of aeration depth on oxygen supply efficiency** 

1

2

Eo (mg-O2/L-Air)

3

4

5

Fig. 16. Experimental apparatus relevant to the change in aeration depth


Table 2. Experimental results pertaining to the variation in aeration depth
