**2.2.1 Single orifice: bubble train**

To test the possibility of measuring bubble size with the use of an electrochemical probe in a bubble column, a train of calibrated bubbles is generated in a tube in which gas is injected under closely controlled conditions. To obtain different bubble sizes, three tubes, T1, T2, and

Electrochemical Probe for Frictional Force and Bubble Measurements

homogeneous bubbling regime in any conducting medium.

following expression (Essadki et al., 1997):

**3. Innovative electrochemical reactors** 

removal efficiency and energy consumption.

these problems, while being also economically attractive.

**3.1 Electrocoagulation-electroflotation theories** 

**2.2.2 Gas injector** 

and Innovative Electrochemical Reactors for Electrocoagulation/Electroflotation 53

In the case of a gas injector consisting of two concentric circular tubes with regularly spaced holes, the average frequency is used (<f>) to evaluate the bubble diameter in the bubble column for the homogeneous bubbly flow. The bubble diameter is estimated by the

1.5 *<sup>g</sup>*

The possibility of measuring bubble size by electrochemical probe is possible for the

This part describes the electrocoagulation and electroflotation as electrochemical methods to treat waste water. The conventional reactors used and the different disposition of electrodes are pointed out. It is also explained how innovative reactors can improve the process of waste water treatment. Thus, specific energy and electrode consumptions are even smaller without the need for mechanical agitation, pumping requirements and air injection, which

An application is then presented to show the efficiency of electrocoagulation/ electroflotation in removing colour from synthetic and real textile wastewater by using aluminum and iron electrodes in an external-loop airlift reactor. The defluoridation is also showed. The time, pH, conductivity and current density are the most parameters for the

According to many authors, electrochemical technique such as electrocoagulation, electroflotation, electrodecantation have a lot of advantages comparatively to other techniques. Biological treatments are cheaper than other methods, but for example for the decolorization of dye wastewater, dye toxicity usually inhibits bacterial growth and limits therefore the efficiency of the decolorization. Physico-chemical methods include adsorption (e.g. on active carbon), coagulation–flocculation (using inorganic salts or polymers), chemical oxidation (chlorination, ozonisation, etc.) and photodegradation (UV/H2O2, UV/TiO2, etc.). However, these technologies usually need additional chemicals which sometimes produce a secondary pollution and a huge volume of sludge. Water treatments based on the electrocoagulation technique have been recently proved to circumvent most of

This technique is based on the in situ formation of coagulant as the sacrificial anode (usually aluminum or iron) corrodes due to an applied current (figure 7). Aluminum and iron materials are usually used as anodes, the dissolution of which produces hydroxides, oxyhydroxides and polymeric hydroxides. In EC, settling is the most common option, while

flotation may be achieved by H2 (electroflotation) or assisted by air injection.

could not be achieved in other kinds of conventional gas-liquid contacting devices.

*U*

*<sup>d</sup> <sup>f</sup>* (10)

*b*

T4, with inner diameters of 1, 2, and 4 mm, respectively, are placed 5 cm below the probe. The probe is placed 30 cm above the liquid distributor and in the column center.

At low gas volumetric flow rates, the effect of the regular passage of bubbles close to the electrochemical probe on the diffusion limit current (or the velocity gradient) is studied. The stability of the bubble frequency is shown on the signal by regularly spaced peaks (fig.5). Signals similar to this are obtained for all positions of the front of the spherical probe (active surface) exposed to bubbles (0° < < 120°). The frequency spectrum clearly shows a single peak representing the bubble frequency, fb (fig.6).

Fig. 5. History records of the velocity gradient at the wall: Effect of the regular passage of bubbles near the electrochemical probe.

Knowing the volumetric flow rate, Qg, we can obtain the bubble volume, Vb by the relation:

$$V\_{\ b} = \frac{\mathcal{Q}\_{\mathcal{g}}}{f\_{\ b}} \tag{9}$$

Assuming spherical bubbles, we can deduce the bubble diameter: db = (6Vb/)1/3.

A comparison at low gas flow rates is achieved by video recording when the bubble emission frequency and volume are obtained. Excellent agreement is observed, proving that the two methods give exactly the same frequency and the same average bubble diameter within 3%.

Fig. 6. Power spectral density function (Ps/s2): Effect of the regular passage of bubbles near the electrochemical probe.
