**5.3 Calculation methods**

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],

$$k\_L a(20) = 1/\gamma \times k\_L a(T) \times 1.024^{(20-T)}\tag{5}$$

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-1) represents the concentration of sodium sulfite solution.

The performance of oxygen mass transfer is assessed in terms of oxygen mass transfer efficiency, which is calculated based upon Equation (6) [19],

$$E\_A \text{(20)} = \frac{DO\_S \text{(20)} \cdot k\_L a \text{(20)} \cdot V \times 10^{-3}}{G\_S \text{(20)} \cdot \rho \cdot O\_w} \times 100 \tag{6}$$

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 flow rate at 20 °C and 1 atm, *V* (m3) is effective capacity of water tank, ρ is air density at 20°C and 1 atm (ρ = 1.204 kg/m3), and Ow (-) is oxygen content in air (Ow = 0.233 O2-kg/air-kg).

Improvement of Oxygen Transfer Efficiency in

0

**6.1 Introduction** 

Fig. 32.

3

E

A(20) (%)

6

9

exterior of the resulting liquid film with the air.

**6. Analysis of the performance of LFFA** 

**6.2 Experimental conditions and methods** 

experiment. The performance of the LFFA is thus evaluated.

Diffused Aeration Systems Using Liquid-Film-Forming Apparatus 365

As indicated above, water body can form a liquid film in the atmosphere. The LFFA can remarkably improve oxygen supply efficiency by simultaneously contacting the interior and

Liquid-film-type aeration Conventional aeration

4 6 8 10 12 14

Fig. 31. The comparison data of (20) *EA* of liquid film and conventional aeration systems

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

By means of a recycling liquid film apparatus (4 cm in each individual airlift pipe diameter, 1 cm in effective height, 12.56 cm2 in cross-sectional area of the airlift part), liquid film aeration tests in a 53 cm deep, 80 L water tank with a surface area of 1510 cm2 are carried out applying an air flow rate of 6 L/min. As a control, under the otherwise identical experimental conditions, the conventional aeration test with air and diffused test with nitrogen are both investigated in this study. Their experimental apparatuses are shown in

Air flow rate (L/min)

## **5.4 Results and discussion**

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 flow rate.


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

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.

As indicated above, water body can form a liquid film in the atmosphere. The LFFA can remarkably improve oxygen supply efficiency by simultaneously contacting the interior and exterior of the resulting liquid film with the air.

Air flow rate (L/min)

Fig. 31. The comparison data of (20) *EA* of liquid film and conventional aeration systems
