**3.5.3 Some results**

64 Electrochemical Cells – New Advances in Fundamental Researches and Applications

Fig. 14. Influence of the axial position of the electrodes (*H1*) and current density (*j*) on the

max 5.8 (cm/s) *<sup>D</sup>*

in which *hDmax* is the maximum dispersion height corresponding to *H1*=7cm. This equation confirms the key role of the axial position of the electrodes on reactor hydrodynamics and

The corresponding turbidity data based on A450 without filtration is reported in figure 15 for an electrolysis time of 10 min and two axial positions of the electrodes. In all cases, flocs occupy nearly 1 cm thickness at the free surface of the disengagement section and no settling is reported in the junction and in the separator. Figure 15 shows however that turbidity rose when H1 =7cm for current density higher than 15 mA/cm2. This corresponds to ULd values in the range 9–10 cm/s in figure 15. Conversely, complete flotation is always observed for H1 = 47 cm, as A450 remains low, about 0.06. As a result, electrode position must be chosen in order to maintain ULd always lower than 9 cm/s, regardless of current density in the range of *j* studied (Fig. 15). Such a condition is achieved for H1 = 47 cm, as shown in figure 15, which corresponds nearly to mid-height in the riser. However, H1 = 47 cm is only a coarse approximation of the optimum electrode height, but the simple way to optimize mixing conditions does not consist in adjusting precisely H1 because it is easier from a practical point of view to adjust the clear liquid height *h*. Indeed, *h* affects simultaneously *h*D and *h*Dmax in Eq. (17), but its range (2 cm < *h* < 14 cm in this work) is

*D <sup>h</sup> U j <sup>h</sup>* 

0.20

Two-parameter model is used to fit the data. The results are given in equation (17):

(17)

=2.4 mS/cm).

overall liquid recirculation *ULd* (*h*=14 cm; initial pH: 8.3; initial conductivity:

*Ld*

mixing properties, and the weaker influence of current.

usually far smaller than H1 (between 7 and 77 cm).

The efficiency of electrocoagulation/electroflotation in removing colour from synthetic textile wastewater by using aluminum and iron electrodes in an external-loop airlift reactor is presented. Disperse, reactive and the mixture are used to determine the optimized parameters.

The real textile wastewater is then used using the optimized parameters. Three effluents were also used: disperse, reactive and the mixture. Energy is determined for each kind of dye.

#### **3.5.3.1 Synthetic dye**

For this synthetic dye, chemical oxygen demand (COD) is measured using the standard closed reflux colorimetric method. Initial COD was about 2500 mg O2/L. Initial pH is varied between 5 and 10 using minute addition of 0.1 M H2SO4 or NaOH solutions. The conductivity (i.e. the ionic strength) of dye solutions is adjusted by sodium chloride addition in the range 1.0–29 mS/cm, which covers the range usually explored in the literature.

COD, color removal and turbidity efficiencies (*YCOD*, *YCOL* and *YABS*) are expressed as percentage and defined as:

$$Y\_{\text{COD}} = \frac{\text{COD \(t=0\)} - \text{COD \(t\)}}{\text{COD \(t=0\)}} \tag{18a}$$

$$\mathbf{Y}\_{\text{COL}} = \frac{\mathbf{A}\_{450}\left(\mathbf{t} = \mathbf{0}\right) - \mathbf{A}\_{450}\left(\mathbf{t}\right)}{\mathbf{A}\_{450}\left(\mathbf{t} = \mathbf{0}\right)}\tag{18b}$$

*YCOL* and *YABS* are obtained using the same equation (Eq.18b), but *YCOL* was based on absorbance measurements at 450 nm after filtration, while *YABS* is measured without

Electrochemical Probe for Frictional Force and Bubble Measurements

dye for disperse and 50 kWh/kg dye for the mixture.

**Edye (kWh/kg dye)**

Fig. 18. Influence of conductivity

**3.5.3.2 Real textile dye** 

synthetic dye.

dye.

after EC treatment.

and operation time (initial pH: 8.3).

and Innovative Electrochemical Reactors for Electrocoagulation/Electroflotation 67

The real textile wastewater is then used. Three effluents are also used: disperse, reactive and the mixture. The color efficiency is between 70 and 90% and COD efficiency reached 78%. The specific electrical energy consumption per kg dye removed (Edye) in optimal conditions for real effluent is calculated. 170 kWh/kg dye is required for a reactive dye, 120 kWh/kg

0 5 10 15 20 25 30 **Conductivity (mS/cm)**

For disperse dye, the removal efficiency is better using aluminium electrodes, whereas, the iron electrodes show more efficiency for removing colour for reactive dye and mixed

Figure 19 shows a photo representing a real effluent before EC treatment and 10 minutes

Before EC treatment 10 minutes after EC treatment.

Fig. 19. Photo showing decolourization by EC in external loop airlift reactor of real textile

on energy consumption *Edye* at constant current density *j*

filtration/decantation. *YABS* is used to analyze qualitatively the evolution of turbidity over time. This parameter shows whether the flocs flotate or are destroyed and driven by the liquid flow. The influence of the initial pH on COD and turbidity removals is illustrated in Figure 16 at constant current density and initial conductivity. An optimum is found for the initial pH, which is between 7.0 and 8.0, although it differs slightly between COD and color removal yields.

Fig. 16. Influence of the initial pH on COD removal and decolorization after 8 minutes operation (conductivity: =2.4 mS/cm, current density: *j*=28.5 mA/cm²).

The influence of conductivity is illustrated in Figure 17 shows an increase of *YCOD* and *YCOL* with for the red dye between 2 and 28 mS/cm. *YCOL* enhancement becomes however slight when is higher than 15 mS/cm.

The decrease in specific energy consumption *Edye* due to the increase of conductivity is illustrated by Figure 18. This figure indicates that *Edye* can be divided roughly by a factor 13 when conductivity is multiplied by a factor 12.

Fig. 17. Influence of conductivity on COD and color removal efficiencies (initial pH: 8.3; current density: *j*=28.6 mS/cm).

Fig. 18. Influence of conductivity on energy consumption *Edye* at constant current density *j* and operation time (initial pH: 8.3).

### **3.5.3.2 Real textile dye**

66 Electrochemical Cells – New Advances in Fundamental Researches and Applications

filtration/decantation. *YABS* is used to analyze qualitatively the evolution of turbidity over time. This parameter shows whether the flocs flotate or are destroyed and driven by the liquid flow. The influence of the initial pH on COD and turbidity removals is illustrated in Figure 16 at constant current density and initial conductivity. An optimum is found for the initial pH, which is between 7.0 and 8.0, although it differs slightly between COD and color

> 456789 10 11 **initial pH**

Fig. 16. Influence of the initial pH on COD removal and decolorization after 8 minutes

=2.4 mS/cm, current density: *j*=28.5 mA/cm²).

for the red dye between 2 and 28 mS/cm. *YCOL* enhancement becomes however slight

0 5 10 15 20 25 30 **Conductivity (mS/cm)**

on COD and color removal efficiencies (initial pH: 8.3;

YCOL YCOD

The influence of conductivity is illustrated in Figure 17 shows an increase of *YCOD* and *YCOL*

The decrease in specific energy consumption *Edye* due to the increase of conductivity is illustrated by Figure 18. This figure indicates that *Edye* can be divided roughly by a factor 13

COD Turbidity

removal yields.

operation (conductivity:

with 

when 

is higher than 15 mS/cm.

Fig. 17. Influence of conductivity

current density: *j*=28.6 mS/cm).

when conductivity is multiplied by a factor 12.

**Removal efficiency Y (%)**

**Removal efficiency Y (%)** 

The real textile wastewater is then used. Three effluents are also used: disperse, reactive and the mixture. The color efficiency is between 70 and 90% and COD efficiency reached 78%. The specific electrical energy consumption per kg dye removed (Edye) in optimal conditions for real effluent is calculated. 170 kWh/kg dye is required for a reactive dye, 120 kWh/kg dye for disperse and 50 kWh/kg dye for the mixture.

For disperse dye, the removal efficiency is better using aluminium electrodes, whereas, the iron electrodes show more efficiency for removing colour for reactive dye and mixed synthetic dye.

Figure 19 shows a photo representing a real effluent before EC treatment and 10 minutes after EC treatment.

Before EC treatment 10 minutes after EC treatment.

Fig. 19. Photo showing decolourization by EC in external loop airlift reactor of real textile dye.

Electrochemical Probe for Frictional Force and Bubble Measurements

**3.5.4 External loop airlift reactor as a continuous reactor** 

QL varied between 0.1 and 2 L/min.

practical a -pulse of tracer at the inlet.

current intensity and to decrease the inlet flow.

flow increases (0 < QL < 2 L/min).

Interesting results are also obtained:

and Innovative Electrochemical Reactors for Electrocoagulation/Electroflotation 69

The reactor is operated in a continuous flow of liquid. The inlet volumetric liquid flow-rate

A study of the residence time distribution (RTD) analysis of liquid phase has been performed. The liquid RTD is determined by means of the tracer response technique. An approximated δ-Dirac pulse of tracer solution (NaCl) is injected into the reactors at a certain time (t = 0) and the outlet signal is detected by conductivity probe and recorded by the acquisition system. The tracer is injected as quickly as possible to obtain as closely as

The syringe (S1) was introduced in the drain that is open to the flow and representing the inlet of the whole reactor. The probe conductivity is placed at the exit position of the reactor (Fig.12). Examples of RTD measurements are shown in in figure 21. Two kinds of signal are observed in this figure. One showed two peaks for the case of QL = 0.36 L/min, I = 6 A, H1 = 7cm and the other showed one peak for the case of QL = 0.73 L/min, I = 1 A, H1 = 47 cm.

The main flow (QL) is divided into two flows: one exit directly the rector by crossing the junction and the other crosses the riser, the separator zone and the downcomer to exit. The percentage of flow that quit the reactor without reacting increased when the main flow increased and the current intensity decreased. The experiments confirm also that the liquid

So to support the reaction during the electrocoagulation, it is necessary to amplify the


E(t) H1 = 7cm, I = 6A, QL = 0.36 L/min

H1 = 7cm, I = 1A, QL = 0.73

Fig. 21. E-curve as a global RTD in External-loop airlift reactor for (QL = 0.36 L/min, I = 6 A,

0 200 400 600 800 1000 Time (s)

H1 = 7cm: 2 peaks) and (QL = 0.73 L/min , H1 = 7 cm, I = 1 A, 47 cm: 1 peak).

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07

crosses the reactor without achieving loops in the case of the continuous flow.

#### **3.5.3.3 Defluoridation of drinking water**

An excess amount of fluoride anions in drinking water has been known to cause adverse effects on human health. To prevent these harmful consequences, especially problems resulting from fluorosis, the World Health Organization (WHO) fixed the maximum acceptable concentration of fluoride anions in drinking water to 1.5 mg/L (Essadki et al., 2009).

The defluoridation of drinking water by EC is studied in the same reactor. Current density values j between 2.8 and 17 mA/cm2 are investigated, which corresponded to current (I=jS) in the range 0.5–3 A. Samples are filtered and the concentrations of the remaining fluoride content are determined in the solution by means of a combined selective fluoride electrode ISEC301F and a PhM240 ion-meter (Radiometer Analytical, France), using the addition of a TISAB II buffer solution to prevent interference from other ions.

Experiments are carried using an initial fluoride concentration [F-]0 between 10-20 mg/L by adding sodium fluoride NaF (Carlo Erba Réactifs, France). The efficiency of fluoride removal can be calculated as follows:

$$Y(\%) = 100 \times \frac{\left[\overline{F}^-\right]\_0 \left[\overline{F}^-\right]}{\left[\overline{F}^-\right]\_0} \tag{19}$$

[F- ]0 : the initial fluoride concentration .

[F- ] : the remaining concentration of fluoride .

Figure 20 shows the effect of the current intensity on the evolution of the fluoride concentration. For *I* = 0.5A corresponding to a current density of 2.85mA/cm2, fluoride concentration reaches only 4.5 mg/L for an electrolysis time of 30 min. Conversely, for exceeding 1A, i.e. for a current density higher than 5.7mA/cm2, one converges towards a concentration of 1mg/L and more rapidly as current density is increased. This confirms that defluoridation can be achieved at low current density. The relatively low efficiency observed at 0.5A can be attributed to the weak charge loading produced in this case, 0.47 F/m3. As expected, the efficiency of EC depends on the amount of coagulant produced in situ.

Fig. 20. Evolution of fluoride concentration during EC in the STR: influence of current intensity (initial pH 7.4 and *C*0 = 15mg/L).
