**4.5.2 Results and discussion**

Fig. 20 shows the effect of effective height on DO saturation rate, liquid/gas ratio and E0.

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

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

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

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

Effective height (cm) Pipe diameter (cm) Air flow rate (L/min)

1, 5, 10 1, 2, 4 12.8

under the third condition.

respectively.

evaluation criteria.

Effective heigth

**4.5.2 Results and discussion** 

**4.5 Effect of effective height on oxygen supply efficiency** 

diameter and 12.56 cm2 in cross-sectional area. Air flow rate is 12.8 L/min.

Table 4. Experimental conditions linked to various effective heights

Fig. 19. Experimental apparatus relating to various effective heights

Fig. 20 shows the effect of effective height on DO saturation rate, liquid/gas ratio and E0.

**4.5.1 Experimental conditions and methods** 

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

Improvement of Oxygen Transfer Efficiency in

**4.6.2 Results and discussion** 

sectional area (cm2) of airlift part.

0

0

Fig. 23. Effect of airlift pipe No. on the DO saturation rate

10

20

30

DO saturation rate (%)

40

50

2

4

6

Effluent water flow rate per unit

of airlift pipe number (L/min)

8

10

12

Diffused Aeration Systems Using Liquid-Film-Forming Apparatus 359

Figs. 22, 23, 24 and 25 respectively illustrate the effect of No. of airlift pipe on the effluent water flow rate per unit airlift pipe number, DO saturation rate in the effluent water, oxygen mass flow rate per unit airlift pipe number and oxygen transfer amount per unit cross-

> 3 2 1 No. of airlift pipe

Fig. 22. Effect of airlift pipe No. on the effluent water flow rate per unit airlift pipe number

No. of airlift pipe

As shown in Fig. 23, the lower the airlift pipe number is, the higher the DO saturation rate is. As revealed in Fig. 22, the effluent water flow rate per unit airlift pipe number does not change notably. Thus it appears that the aeration is evenly distributed among the airlift pipes. Namely, the air flow rate is the same through every airlift pipe. As revealed in Figs. 24 and 25, respectively, the oxygen transfer amount per unit airlift pipe number and oxygen transfer amount per unit airlift part cross-sectional area both exhibit a decreasing trend with increasing pieces of airlift pipes. If the air flow rate of every airlift pipe was same

3 2 1

The effluent flow rate displays a decreasing trend with increasing effective height. This can be explained by the fact that the higher the effective height in the atmosphere is, the larger the hydraulic head loss and the weaker the airlift effect are. DO saturation rate shows a propensity of going up of an increase in effective height. It can be easily understood that increased effective height leads to the prolonged contact time between the effluent water and air. Furthermore, in the case where the amount of the effluent water through the airlift pipes is reduced, a thinner liquid film can be formed, which can enhance the oxygen dissolution efficiency.

As a result, the oxygen transfer amount per unit air aeration volume approaches its maximum value under the experimental conditions of 1 and 5 cm in effective height, and 4 cm in pipe diameter.
