**6.1 Introduction**

364 Mass Transfer - Advanced Aspects

As shown in Table 7 and Fig. 30, (20) *Lk a* of liquid film aeration system displays an evolving trend same as that of conventional aeration system. That is, in the air flow rate range of 6-12 L/min, it gradually increases with increasing air flow rate. Beyond the upper limit, it will decrease slowly. However, as shown in Table 7 and Fig. 31. The (20) *EA* values of both liquid film aeration system and conventional aeration system decrease with increasing air

> Conventional aeration system

6 9.6 8.4 6.64 5.81 8 12.4 11.2 6.39 5.78 12 14.9 14.0 5.12 4.81 12.8 12.2 13.2 4.02 4.33

4 6 8 10 12 14

Fig. 30. The comparative data of (20) *Lk a* of liquid film and conventional aeration systems As compared with (20) *Lk a* and (20) *EA* of both aeration systems, at the air flow rate ranging from 6 to 12 L/min, the aeration efficiency of liquid film aeration system increases by 6.3- 14.3%. Particularly at the air flow rate of 6 L/min, (20) *EA* is still up to 6.64% even utilizing a very shallow aeration depth of 0.5 m. The efficiency of liquid-film aeration is enhanced by

14.3% relative to the conventional aeration system under the identical conditions.

Air flow rate (L/min)

Table 7. Comparison of liquid film aeration system with conventional aeration system

(20) *Lk a* .(l/hr) (20) *EA* .

Liquid-film-type aeration Conventional aeration

Liquid film aeration system

(%)

Conventional aeration system

**5.4 Results and discussion** 

flow rate.

Air flow rate(L/min)

0

4

k

La(20) (1/hr)

8

12

16

Liquid film aeration system

> In order to evaluate the oxygen transfer performance of the LFFA, in this section, the overall oxygen transfer process is divided into two-step oxygen transfer. That is, in the conventional aeration system, oxygen transport process is split into bubble transfer and surface transfer. In contrast, in the liquid film aeration system, it is divided into bubble transfer and liquid film transfer. Oxygen transfer efficiency of every step is separately derived from the experiment. The performance of the LFFA is thus evaluated.
