**2. Experimental setup to study the oxygen transfer of aerators**

The setup [7] consists of a rectangular tank (**Figure 1**) with *L* = 0.3 m, *h* = 0.88 m, and hydrostatic load *H* = 0.8 m, filled with 79.2 l of water. In the tank (1) the aerator (2) equipped with interchangeable metallic plates (MPs) is immersed and tested. Upstream of it a flow-meter (3) is connected for measuring the air flow rate through the aerator and a differential manometer (5) for measuring the pressure drop across the aerator. The experimental setup is sized so that the walls of the tank do not affect the mass transfer of the air bubble column to the water (*L* = 6·*D*). With the peristaltic pump (6) the water is sampled in the middle of the tank, 2 cm from the wall, avoiding the air bubbles in the DO measuring cell. The water passes through the oximeter (7) and is then reintroduced into the system.

In **Table 1** the geometric characteristics of MPs, series 1 and 2, are presented. The last column shows, in bold, the theoretical air-water interface area at the outlet of the bubble from the injection aperture. The design of MP in the second series increases the air-water interfacial

**Figure 1.** Experimental setup (1–tank, 2–aerator, 3–flow-meter, 4–air compressor, 5–manometer, 6–peristaltic pump, 7–

Experimental Study of Standard Aeration Efficiency in a Bubble Column

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351

immersed at a depth *H* is calculated from the balance of the Archimedean and superficial ten-

\_\_3 <sup>4</sup> <sup>⋅</sup> \_\_\_ *<sup>d</sup> g*) 1/3

) of an air bubble at its detachment from the hole of an aerated system

(1)

area (*a*) at the bubbles outlet by up to 2.5 times (**Table 1**).

sion forces, neglecting the weight of the air in the bubble [8].

*R*<sup>0</sup> = (

The initial radius (*R*<sup>0</sup>

**Figure 2.** Tested metallic plates in operation.

oximeter, MP–interchangeable metallic plate).

Five interchangeable perforated metallic plates with holes of *d* = 0.2, 0.3, 0.5, 0.9, 1.6, and 2.4 mm (**Figures 2** and **3**) are tested. To avoid bubble coalescence, the holes are located 10 *d* apart, and to negate any influence of the contraction coefficient of the hole, its length is 5 *d*. To increase the active emission area, and implicitly the interface area, the measurements are repeated for a second series of MPs with orifices placed 7 *d* apart. For both series of tests, the diameter of the MP is *D* = 44.8 mm. The aerator intake has a conical shape to ensure uniform air repartition at the intakes of the aeration holes.

**Figure 1.** Experimental setup (1–tank, 2–aerator, 3–flow-meter, 4–air compressor, 5–manometer, 6–peristaltic pump, 7– oximeter, MP–interchangeable metallic plate).

**Figure 2.** Tested metallic plates in operation.

hydraulic turbines, in which the design of the aeration systems (orifice diameter, configuration, operating conditions, and location) may significantly improve the quality of turbine aeration [1, 2]. The aerator design parameters are used to determine the best balance between oxygen transfer (volume of air injected and size and shape of generated bubbles)

Aerators (spargers) can be made from porous ceramic or metallic materials, fritted (sintered) glass, or plastic, each having specific features in emitting bubbles to increase the contact sur-

In literature, the volumetric mass transfer coefficient Kla was also obtained in situ, [3–6], for different configurations. As the configuration and operating conditions are far from this

The main objective of this study is to optimize an aeration device, that is, to achieve the best dissolved oxygen (DO) transfer versus minimized energy consumption for injection. The classical experiment of an ascending bubble column was used to compare many injection devices. The aeration devices are mainly perforated metallic plates (MPs). Two parameters are studied: orifice size and arrangement. As a control, the active admission area is kept constant for all configurations. The injection air flow rate is controlled and the main aeration parameters (Kla and standard oxygen transfer rates [SOTR]) measured. Comparison of standard aeration efficiency (SAE) is also recorded. The efficiency results are compared with two other aerators, that is, a ceramic plate (CP) and a glass plate (GP). Finally, the optimized configuration in terms of SAE (best compromise between dissolved oxygen transfer and energy consumption

**2. Experimental setup to study the oxygen transfer of aerators**

the oximeter (7) and is then reintroduced into the system.

air repartition at the intakes of the aeration holes.

The setup [7] consists of a rectangular tank (**Figure 1**) with *L* = 0.3 m, *h* = 0.88 m, and hydrostatic load *H* = 0.8 m, filled with 79.2 l of water. In the tank (1) the aerator (2) equipped with interchangeable metallic plates (MPs) is immersed and tested. Upstream of it a flow-meter (3) is connected for measuring the air flow rate through the aerator and a differential manometer (5) for measuring the pressure drop across the aerator. The experimental setup is sized so that the walls of the tank do not affect the mass transfer of the air bubble column to the water (*L* = 6·*D*). With the peristaltic pump (6) the water is sampled in the middle of the tank, 2 cm from the wall, avoiding the air bubbles in the DO measuring cell. The water passes through

Five interchangeable perforated metallic plates with holes of *d* = 0.2, 0.3, 0.5, 0.9, 1.6, and 2.4 mm (**Figures 2** and **3**) are tested. To avoid bubble coalescence, the holes are located 10 *d* apart, and to negate any influence of the contraction coefficient of the hole, its length is 5 *d*. To increase the active emission area, and implicitly the interface area, the measurements are repeated for a second series of MPs with orifices placed 7 *d* apart. For both series of tests, the diameter of the MP is *D* = 44.8 mm. The aerator intake has a conical shape to ensure uniform

and energy consumption.

350 Desalination and Water Treatment

face between the gas and the liquid.

for the air injection) is chosen.

experiment, a direct comparison cannot be performed.

In **Table 1** the geometric characteristics of MPs, series 1 and 2, are presented. The last column shows, in bold, the theoretical air-water interface area at the outlet of the bubble from the injection aperture. The design of MP in the second series increases the air-water interfacial area (*a*) at the bubbles outlet by up to 2.5 times (**Table 1**).

The initial radius (*R*<sup>0</sup> ) of an air bubble at its detachment from the hole of an aerated system immersed at a depth *H* is calculated from the balance of the Archimedean and superficial tension forces, neglecting the weight of the air in the bubble [8].

$$R\_0 = \left(\frac{3}{4} \cdot \frac{d\sigma}{\rho \mathcal{G}}\right)^{1/3} \tag{1}$$

**Figure 3.** Layout of holes for 7*d* arrangement for 0.1 and 0.5 mm hole diameter.

The interface area (*a*) of the first swarm of bubbles is considered spherical at the time of detachment and is calculated with the relationship shown in Eq. (2). The active area of the intake of the air into water is calculated with the relationship (3).

$$
\mathfrak{a} = \mathfrak{n} \cdot \pi \mathfrak{R}\_0^2. \tag{2}
$$

For the theoretical evaluation of the interfacial area (*a***<sup>i</sup>**

**Table 2.** Void fraction for the second series of plates at Q = 0.1 l/s.

**Table 1.** Emission performance of the two series of metallic plates S1 and S2.

*MP Vvoid* **(l)** *nb*

*ai* <sup>=</sup> *nb* <sup>⋅</sup> *Ab* <sup>=</sup> \_\_\_\_

*<sup>ε</sup>* <sup>=</sup> *<sup>V</sup>*\_\_\_\_ *void*

The average global void fraction is calculated using relationship (5)

relationship (4)

*d* **(mm)** *N R***<sup>0</sup>**

**(mm)**

*s* **(mm2 )**

2.4 — 4 — 2.38 — 18.1 — 1.1 — 285 — — 1.6 1.6 6 14 2.08 2.08 12.1 28.1 0.8 1.8 326 761 **2.33** 0.9 0.9 21 43 1.72 1.72 13.4 27.4 0.9 1.7 781 1599 **2.05** 0.5 0.5 61 151 1.41 1.41 12.0 29.6 0.8 1.9 1524 3772 **2.47** 0.3 0.3 177 414 1.19 1.19 12.5 29.3 0.8 1.9 3150 7367 **2.34** 0.2 0.2 385 951 1.04 1.04 12.1 29.9 0.8 1.9 5233 12,926 **2.47**

MP 1.6 0.25 2953 0.347 MP 0.9 0.26 4601 0.361 MP 0.5 0.27 8449 0.375 MP 0.3 0.28 15,905 0.389 MP 0.2 0.29 27,403 0.403 MP 0.1 0.3 42,468 0.417

**S1 S2 S1 S2 S1 S2 S1 S2 S1 S2 S1 S2**

*s'* **(%)** *a* **(mm2 )**

Experimental Study of Standard Aeration Efficiency in a Bubble Column

 **(−) ε (%)**

*a \_\_\_S***2** *aS***1**

353

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next chapter.

air flow rate *Q* = 0.1 l/s is injected in water. By measuring the contact time of air bubbles in water (*T*) and the sudden shutdown of the air supply, the void volume from the system is obtained: *Vvoid* = *T · Q*. The interfacial area of the all bubbles in the system is calculated using

The experimental results of the mass transfer of the two sets of plates are presented in the

3*TQ R*0

) of all the bubbles in the system an

. (4)

*<sup>V</sup>* 100. (5)

$$s \equiv n \cdot \pi d^2 / 4 \tag{3}$$

The interface area (*a*) increases in the second series by up to 2.8 times (**Table 2**).


**Table 1.** Emission performance of the two series of metallic plates S1 and S2.


**Table 2.** Void fraction for the second series of plates at Q = 0.1 l/s.

The interface area (*a*) of the first swarm of bubbles is considered spherical at the time of detachment and is calculated with the relationship shown in Eq. (2). The active area of the

*s* = *n* ⋅ *πd*<sup>2</sup> /4 (3)

The interface area (*a*) increases in the second series by up to 2.8 times (**Table 2**).

2

. (2)

intake of the air into water is calculated with the relationship (3).

**Figure 3.** Layout of holes for 7*d* arrangement for 0.1 and 0.5 mm hole diameter.

*a* = *n* ⋅ *πR*<sup>0</sup>

352 Desalination and Water Treatment

For the theoretical evaluation of the interfacial area (*a***<sup>i</sup>** ) of all the bubbles in the system an air flow rate *Q* = 0.1 l/s is injected in water. By measuring the contact time of air bubbles in water (*T*) and the sudden shutdown of the air supply, the void volume from the system is obtained: *Vvoid* = *T · Q*. The interfacial area of the all bubbles in the system is calculated using relationship (4)

$$a\_i = m\_b \cdot A\_b = \frac{3TQ}{R\_b}.\tag{4}$$

The average global void fraction is calculated using relationship (5)

$$
\varepsilon = \frac{V\_{mid}}{V} 100.\tag{5}
$$

The experimental results of the mass transfer of the two sets of plates are presented in the next chapter.
