**3.3.2 Oxygen mass transfer rate**

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 correspondingly observed.

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.

Improvement of Oxygen Transfer Efficiency in

Effective height

Aeration depth

Diffused Aeration Systems Using Liquid-Film-Forming Apparatus 349

Airlift pipe diameter

Cross-sectional area

Gas bubble diameter Air flow rate

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

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

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 digital-

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

Fig. 10. The factors affecting the liquid film formation

camera-taken gas bubbles located near the diffuser.

**4.2.1 Experimental conditions** 

Table 1.

Fig. 8. The comparison of oxygen mass transfer rate (de-oxygenated water)

Fig. 9. The comparison of oxygen mass transfer rate (activated sludge)

### **4. Effect of the structural parameters of the LFFA**

#### **4.1 Introduction**

According to the preceding pre-experiment on the LFFA, the basic factors influencing liquid film formation involve airlift pipe diameter, cross-sectional area, effective height, air flow rate, gas bubble diameter and aeration depth, etc. This section will discuss experimentally the effect of every factor on liquid film formation in detail, and the optimal design parameter of LFFA will then be identified.

Fig. 10 demonstrates a variety of factors impacting the liquid film formation. Each factor is defined as follows. Effective height refers to the length of airlift, and +/- symbols represent the lengths above/below the water surface, respectively. Aeration depth represents the distance from the diffuser to water surface, and cross-sectional area means the crosssectional area of the airlift part.

**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)

According to the preceding pre-experiment on the LFFA, the basic factors influencing liquid film formation involve airlift pipe diameter, cross-sectional area, effective height, air flow rate, gas bubble diameter and aeration depth, etc. This section will discuss experimentally the effect of every factor on liquid film formation in detail, and the optimal design

Fig. 10 demonstrates a variety of factors impacting the liquid film formation. Each factor is defined as follows. Effective height refers to the length of airlift, and +/- symbols represent the lengths above/below the water surface, respectively. Aeration depth represents the distance from the diffuser to water surface, and cross-sectional area means the cross-

Fig. 9. The comparison of oxygen mass transfer rate (activated sludge)

**4. Effect of the structural parameters of the LFFA** 

Air flow rate (L/min)

Fig. 8. The comparison of oxygen mass transfer rate (de-oxygenated water)

0

0

parameter of LFFA will then be identified.

sectional area of the airlift part.

20

40

Oxygen mass transfer rate

**4.1 Introduction** 

(mg-O2/min)

60

20

40

Oxygen mass transfer rate

(mg-O2/min)

60

Fig. 10. The factors affecting the liquid film formation
