**3.2 Experimental methods**

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

Fig. 5. Single-pass LFFA apparatus

Improvement of Oxygen Transfer Efficiency in

0

0

Fig. 7. The comparison of DO saturation rate (activated sludge)

20

40

60

DO saturation rate (%)

**3.3.2 Oxygen mass transfer rate** 

correspondingly observed.

80

100

20

40

60

DO saturation rate (%)

80

100

Diffused Aeration Systems Using Liquid-Film-Forming Apparatus 347

**Liquid-film type Conventional aeration**

12.8 18.6 13.5 19.2

**Liquid-film type Conventional aeration**

12.8 18.6 13.5 19.2

Air flow rate (L/min)

Figs. 8 and 9 show the oxygen mass transfer rates for the de-oxygenated water and activated sludge, respectively. As shown in these experimental findings, whether for de-oxygenated water or activated sludge, the smaller the gas bubble diameter or the higher the aeration amount at the same gas bubble diameter is, the faster the oxygen transfer rate is. While the de-oxygenated water is in use, oxygen transfer rate of liquid-film aeration increases by 30% in regard to conventional aeration. In the case of activated sludge, an increase of 10% is

As indicated in Figs. 8 and 9, liquid-film apparatus only needs an aeration depth of a few tens of centimeters to provide adequate amount of oxygen. However, 4-5 m in aeration depth is required for the conventional aeration apparatus to supply comparable oxygen

content. Hence, liquid-film apparatus is regarded as a very energy-efficient setup.

Air flow rate (L/min)

Fig. 6. The comparison of DO saturation rate (de-oxygenated water)

A 0.08 g/L sodium sulfite solution is used to chemically de-oxygenate the tank water to around 0 mg/L at the start of each test. The activated sludge is taken from Ube Eastern Water Cleanup Center. DO concentrations are determined by YSI Model DO meter. Two types of air pumps (Iwaki APN-215CV-1 Model and Secoh DF-406 Model) are used herein. Two kinds of air diffusers are selected providing average gas bubble diameters of 3 and 6 mm, respectively. A series of the combinations of diffuser and air pump give rise to different aeration amounts, as shown in Table 1. The average bubble diameter is determined by averaging the diameters of several digital-camera-taken gas bubbles just released from the diffuser.


\* Adjusting aeration amounts contingent on different experimental conditions.

Table 1. The air flow rate as a function of diffuser and air pump types\*

The experimental results are compared in terms of DO saturation rate and oxygen transfer rate, both of which are calculated by the following Equations (1) and (2),

$$\text{DO saturation rate (\%)} = \frac{DO\_{\text{act}}}{DO\_{\text{sat}}} \times 100\% \tag{1}$$

$$\text{Oxygen mass transfer rate (mg-O}\_2/\text{min)} = \left(DO\_{\text{act}} - DO\_0\right) \times Q\_L \tag{2}$$

where DOact. is the measured DO average value in mg/L, DOsat. is the saturated DO concentration in water at 1 atm in mg/L, DOo is the initial DO concentration in mg/L, and QL is the flow rate of the effluent water in L/min.

#### **3.3 Results and discussion**

#### **3.3.1 DO saturation rate**

Fig. 6 and Fig. 7 respectively show the experimental data using de-oxygenated water and DO saturation rate using activated sludge.

As shown in Figs. 6 and 7, independently of air flow rate and aerated bubble diameter, it is suggested from both de-oxygenated water and activated sludge experiments that the effluent DO concentration associated with liquid-film aeration is always higher than that by conventional aeration. While de-oxygenated water is in service, conventional aeration yields a DO saturation rate of 80%, in contrast to up to ca. 95% by way of liquid-film aeration. Even in the case of activated sludge, the corresponding value of over 90% is achieved via liquidfilm aeration. It is found that in comparison with the conventional aeration by using continuously recirculated aeration to enhance DO concentration, for the liquid-film aeration, the DO content in the effluent can be raised up to the saturation state by only aerating once the effluent, thereby indicating strong oxygen supply capability of the latter.

A 0.08 g/L sodium sulfite solution is used to chemically de-oxygenate the tank water to around 0 mg/L at the start of each test. The activated sludge is taken from Ube Eastern Water Cleanup Center. DO concentrations are determined by YSI Model DO meter. Two types of air pumps (Iwaki APN-215CV-1 Model and Secoh DF-406 Model) are used herein. Two kinds of air diffusers are selected providing average gas bubble diameters of 3 and 6 mm, respectively. A series of the combinations of diffuser and air pump give rise to different aeration amounts, as shown in Table 1. The average bubble diameter is determined by averaging the diameters of several digital-camera-taken gas bubbles just released from the

> diffuser Air pump type Air flow rate 3 mm APN-215CV-1 12.8 L/min 3 mm DF-406 18.6 L/min 6 mm APN-215CV-1 13.5 L/min 6 mm DF-406 19.2 L/min

\* Adjusting aeration amounts contingent on different experimental conditions.

The experimental results are compared in terms of DO saturation rate and oxygen transfer

DO saturation rate (%) = *act* 100%

where DOact. is the measured DO average value in mg/L, DOsat. is the saturated DO concentration in water at 1 atm in mg/L, DOo is the initial DO concentration in mg/L, and

Fig. 6 and Fig. 7 respectively show the experimental data using de-oxygenated water and

As shown in Figs. 6 and 7, independently of air flow rate and aerated bubble diameter, it is suggested from both de-oxygenated water and activated sludge experiments that the effluent DO concentration associated with liquid-film aeration is always higher than that by conventional aeration. While de-oxygenated water is in service, conventional aeration yields a DO saturation rate of 80%, in contrast to up to ca. 95% by way of liquid-film aeration. Even in the case of activated sludge, the corresponding value of over 90% is achieved via liquidfilm aeration. It is found that in comparison with the conventional aeration by using continuously recirculated aeration to enhance DO concentration, for the liquid-film aeration, the DO content in the effluent can be raised up to the saturation state by only aerating once

the effluent, thereby indicating strong oxygen supply capability of the latter.

*sat*

*DO* <sup>×</sup> (1)

*DO*

Oxygen mass transfer rate (mg-O2/min)= (*DO DO Q act* − × <sup>0</sup> ) *<sup>L</sup>* (2)

Table 1. The air flow rate as a function of diffuser and air pump types\*

rate, both of which are calculated by the following Equations (1) and (2),

QL is the flow rate of the effluent water in L/min.

DO saturation rate using activated sludge.

**3.3 Results and discussion 3.3.1 DO saturation rate** 

diffuser.

Average bubble diameter of a

Fig. 6. The comparison of DO saturation rate (de-oxygenated water)

Fig. 7. The comparison of DO saturation rate (activated sludge)
