**4.7.2 Results and discussion**

360 Mass Transfer - Advanced Aspects

(6 L/min), the same result is supposed to be obtained. On the contrary, the experimental data from Figs. 24 and 25 both tend to decay. This is due to the fact that for 1 piece of airlift pipe, the effluent can completely overflow through the periphery of the pipe. However, for 2 or 3 pieces of airlift pipes, a fraction of effluent overflowing from a certain airlift pipe can flow back into another airlift pipe, or 2 streams of effluents overflowing from 2 airlift pipes can hinder with each other, and thus flow back into the respective airlift pipes again, thereby causing the reduced effluent flow rate. Otherwise, with increasing airlift pipe number, the coverage area of the capture part is also increased. However, since only one diffuser is serving, it is impossible for aeration to uniformly distribute towards every airlift pipe. As a consequence, when designing the system, every airlift pipe should be separately installed. In the meanwhile, the design of the capture part should take into consideration that the gas bubbles can be well captured, and subsequently uniformly distributed to every airlift pipe.

> 3 2 1 No. of airlift pipe

Fig. 24. Effect of airlift pipe No. on the oxygen mass flow rate per unit airlift pipe number

 3 2 1 No. of airlift pipe

Fig. 25. Effect of airlift pipe No. on the oxygen transfer amount per unit airlift part cross-

0

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

10

20

30

Oxygen mass flow rate per unit of

Oxygen transfer amount per unit of airlift

sectional area (cm2)

part cross-sectional area (mg - O /Lcm )

 2

2

airlift pipe number (mg-O /min)

2

40

50

Figs. 26, 27 and 28 respectively present the effects of air flow rate on the liquid/gas ratio, DO saturation rate and oxygen transfer amount normalized to 1 L of air (E0).

As indicated in Fig. 26, at all air flow rates except for that of 6 L/min, any liquid/gas ratio does not change too much. It is thus deduced that the energy loss is very low in the airlift part. The findings of DO saturation rate are shown in Fig. 27. Below 12 L/min, it does not make a difference. In contrast, beyond 14 L/min, DO saturation rate exhibits an attenuating tendency. Thus, the excess air flow rate brings about the energy waste. As revealed in Fig. 28, oxygen transfer amount normalized to 1 L of aeration air is gradually increasing in the 6- 12 L/min range. However, below 14 L/min, it tends to decrease gradually. Owing to nearly unchanged liquid/gas ratio shown in Fig. 26, the dominant factor affecting oxygen transfer rate is DO concentration. The reason lies in that with air flow rate increasing, it will lead to increasing effluent flow rate, rendering too much effluent water flowing through the airlift part in relation to gas bubble number, thereby inhibiting the liquid-film formation. In summary, under the present experimental conditions, the optimal air flow rate normalized to 1 cm2 of cross-sectional area is 1.1 L/min.

Fig. 26. Effect of air flow rate on the liquid/gas ratio

Improvement of Oxygen Transfer Efficiency in

conventional aeration system

**5.3 Calculation methods** 

study. The experimental apparatus is shown in Fig. 29.

Diffused Aeration Systems Using Liquid-Film-Forming Apparatus 363

by applying air flow rates of 6, 8, 12 and 12.8 L/min. As a control, under the otherwise identical experimental conditions, the conventional aeration test is also investigated in this

Liquid-film-type aeration experiment Conventional aeration experiment Fig. 29. The experimental apparatus diagrams of liquid-film aeration system and

> *L L ka kaT* γ

*DO k a V <sup>E</sup>*

flow rate at 20 °C and 1 atm, *V* (m3) is effective capacity of water tank,

1) represents the concentration of sodium sulfite solution.

*A*

efficiency, which is calculated based upon Equation (6) [19],

The calculation method for total volumetric mass transfer coefficient (kLa) follows the ASCE Standard for Measurement of Oxygen Transfer in Clean Water [18]. Then the obtained kLat is calibrated to a standard reference temperature of 20 °C by using Equation (5) [16, 18],

( ) ( ) (<sup>20</sup> ) 20 1 1.024 *<sup>T</sup>*

where kLa (20) and kLa (T) (hr-1) are kLa at 20 °C and the actual water temperature of T °C, respectively, γ is the activity coefficient of salt concentration and γ = 8.8×10-6×Cz+1, Cz (mg l-

The performance of oxygen mass transfer is assessed in terms of oxygen mass transfer

<sup>3</sup> (20) (20) 10 (20) <sup>100</sup> (20) *S L*

<sup>−</sup> <sup>×</sup> <sup>=</sup> <sup>×</sup> i i

*G O*ρ

where EA (20) refers to oxygen transfer efficiency at 20 °C, (20) *Lk a* (l/hr) is kLa at 20 °C, (20) *DOS* (mg/L) is liquid-phase saturated DO concentration at 20 °C, (20) *GS* (m3/hr) is air

and 1 atm (ρ = 1.204 kg/m3), and Ow (-) is oxygen content in air (Ow = 0.233 O2-kg/air-kg).

*S w*

<sup>−</sup> =× × (5)

i i (6)

ρ

is air density at 20°C

Fig. 27. DO saturation rate as a function of air flow rate

Fig. 28. E0 as a function of air flow rate
